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How long does it take for light from the Sun to reach Earth?

The answer is either 8 minutes 20 seconds or thousands of years, depending on how you look at it!

Russell Deeks

It takes light from the Sun 8 minutes and 20 seconds to reach us on Earth – on average.

That’s the simple, straightforward answer – but whatever the razor-toting Mr Occam may tell you, the simple, straightforward answer seldom tells you the whole truth.

In this case, there are two factors that complicate the picture.

Firstly, the answers that it takes light from the Sun 8m 20s to reach Earth is derived by dividing the distance from the Sun to Earth – around 150 million kilometres – by the speed of light , which is around 300,000 km/s (kilometres per second).

That’ll give you a journey time of some 500 seconds, or eight minutes 20 seconds.

light travel to earth from sun

However, as Earth’s orbit around the Sun is slightly elliptical rather than perfectly circular, the actual distance between our planet and the big fiery globule of burning gas that gives Earth light and heat can be anywhere from 147 to 152 million kilometres.

So light's actual journey time from the Sun to Earth can be anywhere from 490 to 507 seconds, or 8m 10s to 8m 27s.

Secondly, the answer above is how long it takes for any given photon of light to journey from the Sun’s surface to that of Earth – but that’s not the whole story.

light travel to earth from sun

Photons of light, you see, aren’t formed at the Sun’s surface: they’re ‘made’ deep inside the star and because the Sun is, to quote Douglas Adams, "vastly, hugely, mind-bogglingly big", it can take them thousands or even tens of thousands of years to reach the surface of the Sun, before they embark on their 8m 20s journey to Earth.

This means that while the photons of light coming your computer or screen that enable you to read this article are just nanoseconds old, those that hit your eyes when you step outside on a sunny day actually began their journey several thousand years ago.

Which is a bit longer than 8m 20s.

light travel to earth from sun

And which, as a side note, is why shadows are inherently funny.

Imagine being born inside a star, spending thousands of years fighting your way to its surface, bravely traversing millions of miles of interplanetary space and battling through Earth’s atmosphere all the way down to the ground… only, after all that effort, to have your plans thwarted by a lamp post or wheelie bin!

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April 15, 2013

How long does it take sunlight to reach the Earth?

by Fraser Cain, Universe Today

How long does it take sunlight to reach the Earth?

Here's a question… how long does it take sunlight to reach Earth? This sounds like a strange question, but think about it. Sunlight travels at the speed of light. Photons emitted from the surface of the Sun need to travel across the vacuum of space to reach our eyes.

The short answer is that it takes sunlight an average of 8 minutes and 20 seconds to travel from the Sun to the Earth.

If the Sun suddenly disappeared from the Universe (not that this could actually happen, don't panic), it would take a little more than 8 minutes before you realized it was time to put on a sweater.

Here's the math. We orbit the Sun at a distance of about 150 million km. Light moves at 300,000 kilometers/second. Divide these and you get 500 seconds, or 8 minutes and 20 seconds.

This is an average number. Remember, the Earth follows an elliptical orbit around the Sun, ranging from 147 million to 152 million km. At its closest point, sunlight only takes 490 seconds to reach Earth. And then at the most distant point, it takes 507 seconds for sunlight to make the journey.

But the story of light gets even more interesting, when you think about the journey light needs to make inside the Sun.

You probably know that photons are created by fusion reactions inside the Sun's core. They start off as gamma radiation and then are emitted and absorbed countless times in the Sun's radiative zone, wandering around inside the massive star before they finally reach the surface .

What you probably don't know, is that these photons striking your eyeballs were ACTUALLY created tens of thousands of years ago and it took that long for them to be emitted by the sun.

Once they escaped the surface, it was only a short 8 minutes for those photons to cross the vast distance from the Sun to the Earth

As you look outward into space, you're actually looking backwards in time.

The light you see from your computer is nanoseconds old. The light reflected from the surface of the Moon takes only a second to reach Earth. The Sun is more than 8 light-minutes away. And so, if the light from the nearest star (Alpha Centauri) takes more than 4 years to reach us, we're seeing that star 4 years in the past.

There are galaxies millions of light-years away, which means the light we're seeing left the surface of those stars millions of years ago. For example, the galaxy M109 is located about 83.5 million light-years away.

If aliens lived in those galaxies, and had strong enough telescopes, they would see the Earth as it looked in the past. They might even see dinosaurs walking on the surface.

Source: Universe Today

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Sun and Earth

  • How Long Does It Take Sunlight To Reach Earth?

The sun is the largest, most massive object in the solar system , and it is also the primary source of light for all the planets . Everyday we feel the warmth of the sun’s light, and without it, life on Earth would not exist. Interestingly, light from the sun does not reach our planet instantaneously. Rather, since the speed of light is a finite number and the sun is located about 93-million miles (150-million kilometres) away, it takes about 8-minutes for light from the sun to reach Earth.

Speed Of Light

The sun

Light is the fastest known thing in the universe, and the speed of light represents a kind of cosmic speed limit that, as far as the laws of nature suggest, cannot be surpassed. The speed of light is approximately 186,000-miles per second (300,000-kilometres per second). That is extremely fast, yet it is still a finite number, and so the further away a source of light is, the longer it takes for that light to reach our eyes. 

Distance To The Sun

Earth and sun

The Earth orbits the sun at an average distance of 93-million miles (150-million kilometres). At this distance, there is a noticeable delay in when light emitted by the sun reaches our world. In fact, just by knowing the speed of light and the distance between the sun and Earth, you can calculate how long it takes for that light to reach us quite easily. You simply divide distance by speed and you get time. In this case, you divide 93-million miles by 186,000-miles per second to get 500-seconds, which can be converted to minutes by multiplying 500-seconds by 60, which gives you 8.3-minutes. Thus, it takes just a little over 8-minutes for light from the sun to reach the Earth. This also means that we never see the sun as it currently exists. Rather, we always see the sun as it was 8-minutes ago. For example, if the sun were to suddenly disappear, we would not notice for a full 8-minutes. 

What About The Other Planets?

Solar system

Since every planet in the solar system orbits the sun at a different distance, the amount of time it takes for sunlight to reach each of them is also different. The closer a planet is to the sun, the less time it takes. For example, Mercury is the closest planet to the sun and it takes about 3.2-minutes for sunlight to reach it. For Venus it’s about 6-minutes, and for Mars it takes 12.6-minutes. Thus, it takes less than 13-minutes for sunlight to reach all of the inner planets in our solar system. However, it takes much longer for sunlight to reach the gas giants. It takes 43.2-minutes for sunlight to reach Jupiter , and 79.3-minutes for it to reach Saturn . For Uranus , it takes 159.6-minutes, and for Neptune , it takes just over 4-hours. 

Time it Takes for Sunlight to Reach Each Planet

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  • Sound & Light (Physics): How are They Different?

How Does Light Travel From the Sun to Earth?

How Does Light Travel From the Sun to Earth

How is Light Transmitted?

Electromagnetic waves.

To understand how light travels from the sun to the Earth, you have to understand what light is. Light is an electromagnetic wave--a wave of electric and magnetic energy oscillating very quickly. There are many different electromagnetic waves, and the type is determined by the speed of oscillation. For example, radio waves oscillate more slowly than light, while X-rays oscillate much more quickly. These electromagnetic waves travel in small packets called photons. Because light travels in both waves and photon packets, it behaves both like a wave and a particle.

Traveling Through Space

Most waves require a medium to travel in. For example, if you drop a rock in a pond, it makes waves in the water. No water, no waves. Because light consists of photons, however, it can travel through space like a stream of tiny particles. The photons actually travel more quickly through space and lose less energy on the way, because there are no molecules in the way to slow them down.

The Atmosphere

When light travels through space from the sun, all of the frequencies of light travel in a straight line. When light hits the atmosphere, however, the photons begin to collide with gas molecules. Red, orange and yellow photons have long wavelengths and can travel right through the gas molecules. Green, blue and purple photons, however, have shorter wavelengths, which allows molecules to easily absorb them. The molecules hold onto the photon for only an instant, then shoot them out again in a random direction. This is why the sky looks blue. Many of these scattered photons fly toward the Earth, making the sky appear to glow. This is also why sunsets look red. At sunset, the photons have to travel through a larger layer of atmosphere before they reach your eyes. More of the higher frequency photons are absorbed, leaving layers of red, orange and yellow.

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  • NASA on Electromagnetic Waves
  • Why is the Sky Blue? from Science Made Simple

About the Author

Isaiah David is a freelance writer and musician living in Portland, Ore. He has over five years experience as a professional writer and has been published on various online outlets. He holds a degree in creative writing from the University of Michigan.

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How long does it take light from the sun to get to earth.

The speed of light is approximately 1,079,000,000 kilometers/hour (670,600,000 miles/hour). On average, the Sun is 150 million kilometers (93 million miles) away from Earth. This means that it takes a photon of light about 8 minutes and 20 seconds to travel from the Sun to Earth.

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It takes light a zippy 8 minutes to reach Earth from the surface of the Sun. But how long does it take that same light to travel from the Sun’s core to its surface? Oddly enough, the answer is many thousands of years. Sten Odenwald explains why by illustrating the random walk problem.

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The sun is an ordinary star, one of about 100 billion in our galaxy, the Milky Way. The sun has extremely important influences on our planet: It drives weather, ocean currents, seasons, and climate, and makes plant life possible through photosynthesis.

Biology, Earth Science, Astronomy, Physics

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The sun is an ordinary  star , one of about 100 billion in our galaxy , the Milky Way. The sun has extremely important influences on our planet: It drives weather, ocean currents, seasons, and  climate , and makes plant life possible through  photosynthesis . Without the sun’s heat and light, life on Earth would not exist. About 4.5 billion years ago, the sun began to take shape from a  molecular cloud  that was mainly composed of hydrogen and helium. A nearby  supernova  emitted a shockwave, which came in contact with the molecular cloud and energized it. The molecular cloud began to  compress , and some regions of gas collapsed under their own  gravitational pull . As one of these regions collapsed, it also began to  rotate  and heat up from increasing pressure. Much of the hydrogen and helium remained in the center of this hot, rotating mass. Eventually, the gases heated up enough to begin  nuclear fusion , and became the sun in our  solar system . Other parts of the molecular cloud cooled into a disc around the brand-new sun and became planets, asteroids, comets, and other bodies in our solar system. The sun is about 150 million kilometers (93 million miles) from Earth. This distance, called an  astronomical unit  (AU), is a standard measure of distance for  astronomers and astrophysicists. An AU can be measured at light speed, or the time it takes for a photon of light to travel from the sun to Earth. It takes light about eight minutes and 19 seconds to reach Earth from the sun. The  radius  of the sun, or the distance from the very center to the outer limits, is about 700,000 kilometers (432,000 miles). That distance is about 109 times the size of Earth’s radius. The sun not only has a much larger radius than Earth—it is also much more massive. The sun’s mass is more than 333,000 times that of Earth, and contains about 99.8 percent of all of the mass in the entire solar system! Composition The sun is made up of a blazing combination of gases. These gases are actually in the form of plasma . Plasma is a state of matter similar to gas, but with most of the particles  ionized . This means the particles have an increased or reduced number of electrons. About three quarters of the sun is hydrogen, which is constantly fusing together and creating helium by a process called nuclear fusion. Helium makes up almost the entire remaining quarter. A very small percentage (1.69 percent) of the sun’s mass is made up of other gases and metals: iron, nickel, oxygen, silicon, sulfur, magnesium, carbon, neon, calcium, and chromium This 1.69 percent may seem insignificant, but its mass is still 5,628 times the mass of Earth. The sun is not a solid mass. It does not have the easily identifiable boundaries of rocky planets like Earth. Instead, the sun is composed of layers made up almost entirely of hydrogen and helium. These gases carry out different functions in each layer, and the sun’s layers are measured by their percentage of the sun’s total radius. The sun is permeated and somewhat controlled by a  magnetic field . The magnetic field is defined by a combination of three complex mechanisms: a circular electric current that runs through the sun, layers of the sun that rotate at different speeds, and the sun’s ability to conduct  electricity . Near the sun’s  equator , magnetic field lines make small loops near the surface. Magnetic field lines that flow through the poles extend much farther, thousands of kilometers, before returning to the opposite pole. The sun rotates around its own axis, just like Earth. The sun rotates counterclockwise, and takes between 25 and 35 days to complete a single rotation. The sun  orbits clockwise around the center of the Milky Way. Its orbit is between 24,000 and 26,000 light-years away from the galactic center. The sun takes about 225 million to 250 million years to orbit one time around the galactic center. Electromagnetic Radiation The sun’s energy travels to Earth at the speed of light in the form of electromagnetic radiation (EMR). The  electromagnetic spectrum  exists as waves of different frequencies and  wavelengths . The  frequency  of a wave represents how many times the wave repeats itself in a certain unit of time. Waves with very short wavelengths repeat themselves several times in a given unit of time, so they are high-frequency. In contrast, low-frequency waves have much longer wavelengths. The vast majority of electromagnetic waves that come from the sun are invisible to us. The most high-frequency waves emitted by the sun are gamma rays, x-rays, and  ultraviolet radiation  (UV rays). The most harmful UV rays are almost completely absorbed by Earth’s atmosphere. Less potent UV rays travel through the atmosphere, and can cause sunburn. The sun also emits  infrared radiation —whose waves are a much lower-frequency. Most heat from the sun arrives as infrared energy. Sandwiched between infrared and UV is the visible spectrum, which contains all the colors we, as humans, can see. The color red has the longest wavelengths (closest to infrared), and violet (closest to UV) the shortest. The sun itself is white, which means it contains all the colors in the visible spectrum. The sun appears orangish-yellow because the blue light it emits has a shorter wavelength, and is scattered in the atmosphere—the same process that makes the sky appear blue. Astronomers, however, call the sun a “yellow dwarf” star because its colors fall within the yellow-green section of the electromagnetic spectrum. Evolution of the Sun The sun, although it has sustained all life on our planet, will not shine forever. The sun has already existed for about 4.5 billion years. The process of nuclear fusion, which creates the heat and light that make life on our planet possible, is also the process that slowly changes the sun’s composition. Through nuclear fusion, the sun is constantly using up the hydrogen in its core : Every second, the sun fuses around 620 million metric tons of hydrogen into helium. At this stage in the sun’s life, its core is about 74 percent hydrogen. Over the next five billion years, the sun will burn through most of its hydrogen, and helium will become its major source of fuel. Over those five billion years, the sun will go from “yellow dwarf” to “ red giant .” When almost all of the hydrogen in the sun’s core has been consumed, the core will contract and heat up, increasing the amount of nuclear fusion that takes place. The outer layers of the sun will expand from this extra energy. The sun will expand to about 200 times its current radius, swallowing Mercury and Venus. Astrophysicists debate whether Earth’s orbit would expand beyond the sun’s reach, or if our planet would be engulfed by the sun as well. As the sun expands, it will spread its energy over a larger surface area, which has an overall cooling effect on the star. This cooling will shift the sun’s visible light to a reddish color—a red giant. Eventually, the sun’s core reaches a temperature of about 100 million on the  Kelvin scale (almost 100 million degrees Celsius or 180 million degrees Farenheit), the common scientific scale for measuring temperature. When it reaches this temperature, helium will begin fusing to create carbon, a much heavier element. This will cause intense solar wind and other solar activity, which will eventually throw off the entire outer layers of the sun. The red giant phase will be over. Only the sun’s carbon core will be left, and as a “ white dwarf ,” it will not create or emit energy. Sun’s Structure The sun is made up of six layers: core, radiative zone , convective zone, photosphere , chromosphere , and corona . Core The sun’s  core , more than a thousand times the size of Earth and more than 10 times  denser than lead, is a huge furnace. Temperatures in the core exceed 15.7 million kelvin (also 15.7 million degrees Celsius, or 28 million degrees Fahrenheit). The core extends to about 25 percent of the sun’s radius. The core is the only place where nuclear fusion reactions can happen. The sun’s other layers are heated from the nuclear energy created there. Protons of hydrogen atoms violently collide and fuse, or join together, to create a helium atom. This process, known as a PP (proton-proton) chain reaction, emits an enormous amount of energy. The energy released during one second of solar fusion is far greater than that released in the explosion of hundreds of thousands of hydrogen bombs. During nuclear fusion in the core, two types of energy are released: photons and neutrinos . These particles carry and emit the light, heat, and energy of the sun. Photons are the smallest particle of light and other forms of electromagnetic radiation. Neutrinos are more difficult to detect, and only account for about two percent of the sun’s total energy. The sun emits both photons and neutrinos in all directions, all the time. Radiative Zone The radiative zone of the sun starts at about 25 percent of the radius, and extends to about 70 percent of the radius. In this broad zone, heat from the core cools dramatically, from between seven million K (12 million°F or 7 million°C) to two million K (2 million°C or 4 million°F). In the radiative zone, energy is transferred by a process called thermal radiation. During this process, photons that were released in the core travel a short distance, are absorbed by a nearby ion, released by that ion, and absorbed again by another. One photon can continue this process for almost 200,000 years! Transition Zone : Tachocline Between the radiative zone and the next layer, the convective zone, there is a transition zone called the tachocline. This region is created as a result of the sun’s differential rotation . Differential rotation happens when different parts of an object rotate at different velocities. The sun is made up of gases undergoing different processes at different layers and different latitudes. The sun’s equator rotates much faster than its poles, for instance. The rotation rate of the sun changes rapidly in the tachocline. Convective Zone At around 70 percent of the sun’s radius, the convective zone begins. In this zone, the sun’s temperature is not hot enough to transfer energy by thermal radiation. Instead, it transfers heat by thermal  convection  through thermal columns. Similar to water boiling in a pot, or hot wax in a lava lamp, gases deep in the sun’s convective zone are heated and “boil” outward, away from the sun’s core, through thermal columns. When the gases reach the outer limits of the convective zone, they cool down, and plunge back to the base of the convective zone, to be heated again. Photosphere The photosphere is the bright yellow, visible "surface" of the sun. The photosphere is about 400 kilometers (250 miles) thick, and temperatures there reach about 6,000K (5,700°C, 10,300°F). The thermal columns of the convection zone are visible in the photosphere, bubbling like boiling oatmeal. Through powerful telescopes, the tops of the columns appear as  granules crowded across the sun. Each granule has a bright center, which is the hot gas rising through a thermal column. The granules’ dark edges are the cool gas descending back down the column to the bottom of the convective zone. Although the tops of the thermal columns look like small granules, they are usually more than 1,000 kilometers (621 miles) across. Most thermal columns exist for about eight to 20 minutes before they dissolve and form new columns. There are also “supergranules” that can be up to 30,000 kilometers (18,641 miles) across, and last for up to 24 hours. Sunspots , solar flares , and solar prominences take form in the photosphere, although they are the result of processes and disruptions in other layers of the sun. Photosphere: Sunspots A sunspot is just what it sounds like—a dark spot on the sun. A sunspot forms when intense magnetic activity in the convective zone  ruptures a thermal column. At the top of the ruptured column (visible in the photosphere), temperature is temporarily decreased because hot gases are not reaching it. Photosphere: Solar Flares The process of creating sunspots opens a connection between the corona (the very outer layer of the sun) and the sun’s interior. Solar matter surges out of this opening in formations called solar flares. These explosions are massive: In the period of a few minutes, solar flares release the equivalent of about 160 billion megatons of TNT, or about a sixth of the total energy the sun releases in one second. Clouds of ions, atoms, and electrons erupt from solar flares, and reach Earth in about two days. Solar flares and solar prominences contribute to  space weather , which can cause disturbances to Earth’s atmosphere and magnetic field, as well as disrupt satellite and telecommunications systems. Photosphere: Coronal Mass Ejections Coronal mass ejections (CMEs) are another type of solar activity caused by the constant movement and disturbances within the sun’s magnetic field. CMEs typically form near the active regions of sunspots, the correlation between the two has not been proven. The cause of CMEs is still being studied, and it is hypothesized that disruptions in either the photosphere or corona lead to these violent solar explosions. Photosphere: Solar Prominence Solar prominences are bright loops of solar matter. They can burst far into the coronal layer of the sun, expanding hundreds of kilometers per second. These curved and twisted features can reach hundreds of thousands of kilometers in height and width, and last anywhere from a few days to a few months. Solar prominences are cooler than the corona, and they appear as darker strands against the sun. For this reason, they are also known as filaments. Photosphere: Solar Cycle The sun does not constantly emit sunspots and solar ejecta; it goes through a cycle of about 11 years. During this solar cycle, the frequency of solar flares changes. During solar maximums, there can be several flares per day. During solar minimums, there may be fewer than one a week. The solar cycle is defined by the sun’s magnetic fields, which loop around the sun and connect at the two poles. Every 11 years, the magnetic fields reverse, causing a disruption that leads to solar activity and sunspots. The solar cycle can have effects on Earth’s climate. For example, the sun’s ultraviolet light splits oxygen in the stratosphere and strengthens Earth’s protective  ozone layer . During the solar minimum, there are low amounts of UV rays, which means that Earth’s ozone layer is temporarily thinned. This allows more UV rays to enter and heat Earth’s atmosphere. Solar Atmosphere The solar atmosphere is the hottest region of the sun. It is made up of the chromosphere, the corona, and a transition zone called the solar transition region that connects the two. The solar atmosphere is obscured by the bright light emitted by the photosphere, and it can rarely be seen without special instruments. Only during  solar eclipses , when the moon moves between Earth and the sun and hides the photosphere, can these layers be seen with the unaided eye. Chromosphere The pinkish-red chromosphere is about 2,000 kilometers (1,250 miles) thick and riddled with jets of hot gas. At the bottom of the chromosphere, where it meets the photosphere, the sun is at its coolest, at about 4,400K (4,100°C, 7,500°F). This low temperature gives the chromosphere its pink color. The temperature in the chromosphere increases with altitude, and reaches 25,000K (25,000°C, 45,000°F) at the outer edge of the region. The chromosphere gives off jets of burning gases called  spicules , similar to solar flares. These fiery wisps of gas reach out from the chromosphere like long, flaming fingers; they are usually about 500 kilometers (310 miles) in diameter. Spicules only last for about 15 minutes, but can reach thousands of kilometers in height before collapsing and dissolving. Solar Transition Region The solar transition region (STR) separates the chromosphere from the corona. Below the STR, the layers of the sun are controlled and stay separate because of gravity, gas pressure, and the different processes of exchanging energy. Above the STR, the motion and shape of the layers are much more dynamic. They are dominated by magnetic forces. These magnetic forces can put into action solar events such as coronal loops and the solar wind. The state of helium in these two regions has differences as well. Below the STR, helium is partially ionized. This means it has lost an electron, but still has one left. Around the STR, helium absorbs a bit more heat and loses its last electron. Its temperature soars to almost one million K (one million°C, 1.8 million°F). Corona The corona is the wispy outermost layer of the solar atmosphere, and can extend millions of kilometers into space. Gases in the corona burn at about one million K (one million°C, 1.8 million°F), and move about 145 kilometers (90 miles) per second. Some of the particles reach an  escape velocity  of 400 kilometers per second (249 miles per second). They escape the sun’s gravitational pull and become the solar wind. The solar wind blasts from the sun to the edge of the solar system. Other particles form coronal loops. Coronal loops are bursts of particles that curve back around to a nearby sunspot. Near the sun’s poles are coronal holes. These areas are colder and darker than other regions of the sun, and allow some of the fastest-moving parts of the solar wind to pass through. Solar Wind The solar wind is a stream of extremely hot, charged particles that are thrown out from the upper atmosphere of the sun. This means that every 150 million years, the sun loses a mass equal to that of Earth. However, even at this rate of loss, the sun has only lost about 0.01 percent of its total mass from solar wind. The solar wind blows in all directions. It continues moving at that speed for about 10 billion kilometers (six billion miles). Some of the particles in the solar wind slip through Earth’s magnetic field and into its upper atmosphere near the poles. As they collide with our planet's atmosphere, these charged particles set the atmosphere aglow with color, creating  auroras , colorful light displays known as the Northern and Southern Lights. Solar winds can also cause solar storms . These storms can interfere with satellites and knock out  power grids on Earth. The solar wind fills the heliosphere, the massive bubble of charged particles that encompasses the solar system. The solar wind eventually slows down near the border of the heliosphere, at a theoretical boundary called the  heliopause . This boundary separates the matter and energy of our solar system from the matter in neighboring star systems and the  interstellar medium . The interstellar medium is the space between star systems. The solar wind, having traveled billions of kilometers, cannot extend beyond the interstellar medium. Studying the Sun The sun has not always been a subject of scientific discovery and inquiry. For thousands of years, the sun was known in cultures all over the world as a god, a goddess, and a symbol of life. To the ancient Aztecs, the sun was a powerful deity known as Tonatiuh, who required human sacrifice to travel across the sky. In Baltic mythology, the sun was a goddess named Saule, who brought fertility and health. Chinese mythology held that the sun is the only remaining of 10 sun gods. In 150 B.C.E., Greek scholar Claudius Ptolemy created a geocentric model of the solar system in which the moon, planets, and sun revolved around Earth. It was not until the 16th century that Polish astronomer Nicolaus Copernicus used mathematical and scientific reasoning to prove that planets rotated around the sun. This heliocentric model is the one we use today. In the 17th century, the telescope allowed people to examine the sun in detail. The sun is much too bright to allow us to study it with our eyes unprotected. With a telescope, it was possible for the first time to project a clear image of the sun onto a screen for examination. English scientist Sir  Isaac Newton  used a telescope and prism to scatter the light of the sun, and proved that sunlight was actually made of a spectrum of colors. In 1800, infrared and ultraviolet light were discovered to exist just outside of the visible spectrum. An optical instrument called a spectroscope made it possible to separate visible light and other electromagnetic radiation into its various wavelengths.  Spectroscopy  also helped scientists identify gases in the sun’s atmosphere—each element has its own wavelength pattern. However, the method by which the sun generated its energy remained a mystery. Many scientists hypothesized that the sun was contracting, and emitting heat from that process. In 1868, English astronomer Joseph Norman Lockyer was studying the sun’s electromagnetic spectrum. He observed bright lines in the photosphere that did not have a wavelength of any known element on Earth. He guessed that there was an element isolated on the sun, and named it helium after the Greek sun god, Helios. Over the next 30 years, astronomers concluded that the sun had a hot, pressurized core that was capable of producing massive amounts of energy through nuclear fusion. Technology continued to improve and allowed scientists to uncover new features of the sun. Infrared telescopes were invented in the 1960s, and scientists observed energy outside the visible spectrum. Twentieth-century astronomers used balloons and rockets to send specialized telescopes high above Earth, and examined the sun without any interference from Earth's atmosphere. Solrad 1  was the first spacecraft designed to study the sun, and was launched by the United States in 1960. That decade, NASA sent five  Pioneer  satellites to orbit the sun and collect information about the star. In 1980, NASA launched a mission during the solar maximum to gather information about the high-frequency gamma rays, UV rays, and x-rays that are emitted during solar flares. The Solar and Heliospheric Observatory ( SOHO ) was developed in Europe and put into orbit in 1996 to collect information. SOHO has been successfully collecting data and forecasting space weather for 12 years. Voyager 1  and  2  are spacecraft traveling to the edge of the heliosphere to discover what the atmosphere is made of where solar wind meets the interstellar medium. Voyager 1 crossed this boundary in 2012 and Voyager 2 did so in 2018. Another development in the study of the sun is  helioseismology , the study of solar waves. The turbulence of the convective zone is hypothesized to contribute to solar waves that continuously transmit solar material to the outer layers of the sun. By studying these waves, scientists understand more about the sun’s interior and the cause of solar activity. Energy from the Sun Photosynthesis

Sunlight provides necessary light and energy to plants and other producers in the  food web . These producers absorb the sun’s radiation and convert it into energy through a process called photosynthesis. Producers are mostly plants (on land) and algae (in aquatic regions). They are the foundation of the food web, and their energy and  nutrients are passed on to every other living organism. Fossil Fuels Photosynthesis is also responsible for all of the fossil fuels on Earth. Scientists estimate that about three billion years ago, the first producers evolved in aquatic settings. Sunlight allowed plant life to thrive and adapt. After the plants died, they decomposed and shifted deeper into the earth, sometimes thousands of meters. This process continued for millions of years. Under intense pressure and high temperatures, these remains became what we know as fossil fuels. These microorganisms became petroleum, natural gas, and coal. People have developed processes for extracting these fossil fuels and using them for energy. However, fossil fuels are a  nonrenewable resource . They take millions of years to form. Solar Energy Technology Solar energy technology harnesses the sun’s radiation and converts it into heat, light, or electricity. Solar energy is a  renewable resource , and many technologies can harvest it directly for use in homes, businesses, schools, and hospitals. Some solar energy technologies include solar voltaic cells and panels, solar thermal collectors, solar thermal electricity, and solar architecture . Photovoltaics use the sun’s energy to speed up electrons in solar cells and generate electricity. This form of technology has been used widely, and can provide electricity for rural areas, large power stations, buildings, and smaller devices such as parking meters and trash compactors. The sun’s energy can also be harnessed by a method called “concentrated solar power,” in which the sun’s rays are reflected and magnified by mirrors and lenses. The intensified ray of sunlight heats a fluid, which creates steam and powers an electric  generator . Solar power can also be collected and distributed without machinery or electronics. For example, roofs can be covered with vegetation or painted white to decrease the amount of heat absorbed into the building, thereby decreasing the amount of electricity needed for air conditioning. This is solar architecture. Sunlight is abundant: In one hour, Earth’s atmosphere receives enough sunlight to power the electricity needs of all people for a year. However, solar technology is expensive, and depends on sunny and cloudless local weather to be effective. Methods of harnessing the sun’s energy are still being developed and improved.

Like a Diamond in the Sky White dwarf stars are made of crystallized carbon diamond. A typical white dwarf is about 10 billion trillion trillion carats. In about five billion years, says Travis Metcalfe of the Harvard-Smithsonian Center for Astrophysics, our sun will become a diamond that truly is forever.

Solar Constant The solar constant is the average amount of solar energy reaching Earth's atmosphere. The solar constant is about 1.37 kilowatts of electricity per square meter.

Solarmax 2013 will bring the next solar maximum (solarmax), a period astronomers say will bring more solar flares, coronal mass ejections, solar storms, and auroras.

Sun is the Loneliest Number The sun is pretty isolated, way out on the inner rim of the Orion Arm of the Milky Way. Its nearest stellar neighbor, a red dwarf named Proxima Centauri, is about 4.24 light-years away.

Sunny Days at Space Agencies NASA and other space agencies have more than a dozen heliophysics missions, which study the sun, heliosphere, and planetary environments as a single connected system. A few of the ongoing missions are: ACE : observing particles of solar, interplanetary, interstellar, and galactic origins AIM : determining the causes of the highest-altitude clouds in Earths atmosphere Hinode : studying the sun with the worlds highest-resolution solar telescopes IBEX : mapping the entire boundary of the solar system RHESSI : researching gamma rays and X-rays, the most powerful energy emitted by the sun SOHO : understanding the structure and dynamics of the sun SDO : a crown jewel of NASA, aimed at developing the scientific understanding necessary to address those aspects of the sun and solar system that directly affect life and society STEREO : understanding coronal mass ejections Voyager : studying space at the edge of the solar system Wind : understanding the solar wind

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expanding universe light waves

How Light Travels: The Reason Why Telescopes Can See the Invisible Parts of Our Universe

Due to how light travels, we can only see the most eye-popping details of space—like nebulas, supernovas, and black holes—with specialized telescopes.

  • Our eyes can see only a tiny fraction of these wavelengths , but our instruments enable us to learn far more.
  • Here, we outline how various telescopes detect different wavelengths of light from space.

Light travels only one way: in a straight line. But the path it takes from Point A to Point B is always a waveform, with higher-energy light traveling in shorter wavelengths. Photons , which are tiny parcels of energy, have been traveling across the universe since they first exploded from the Big Bang . They always travel through the vacuum of space at 186,400 miles per second—the speed of light—which is faster than anything else.

Too bad we can glimpse only about 0.0035 percent of the light in the universe with our naked eyes. Humans can perceive just a tiny sliver of the electromagnetic spectrum: wavelengths from about 380–750 nanometers. This is what we call the visible part of the electromagnetic spectrum. The universe may be lovely to look at in this band, but our vision skips right over vast ranges of wavelengths that are either shorter or longer than this limited range. On either side of the visible band lies evidence of interstellar gas clouds, the hottest stars in the universe, gas clouds between galaxies , the gas that rushes into black holes, and much more.

electromagnetic spectrum the visible range shaded portion is shown enlarged on the right

Fortunately, telescopes allow us to see what would otherwise remain hidden. To perceive gas clouds between stars and galaxies, we use detectors that can capture infrared wavelengths. Super-hot stars require instruments that see short, ultraviolet wavelengths. To see the gas clouds between galaxies, we need X-ray detectors.

We’ve been using telescopes designed to reveal the invisible parts of the cosmos for more than 60 years. Because Earth’s atmosphere absorbs most wavelengths of light, many of our telescopes must observe the cosmos from orbit or outer space.

Here’s a snapshot of how we use specialized detectors to explore how light travels across the universe.

Infrared Waves

galaxy glass z13 through webb

We can’t see infrared waves, but we can feel them as heat . A sensitive detector like the James Webb Space Telescope can discern this thermal energy from far across the universe. But we use infrared in more down-to-Earth ways as well. For example, remote-control devices work by sending infrared signals at about 940 nanometers to your television or stereo. These heat waves also emanate from incubators to help hatch a chick or keep a pet reptile warm. As a warm being, you radiate infrared waves too; a person using night vision goggles can see you, because the goggles turn infrared energy into false-color optical energy that your eyes can perceive. Infrared telescopes let us see outer space in a similar way.

Astronomers began the first sky surveys with infrared telescopes in the 1960s and 1970s. Webb , launched in 2021, takes advantage of the infrared spectrum to probe the deepest regions of the universe. Orbiting the sun at a truly cold expanse—about one million miles from Earth—Webb has three infrared detectors with the ability to peer farther back in time than any other telescope has so far.

Its primary imaging device, the Near Infrared Camera (NIRCam), observes the universe through detectors tuned to incoming wavelengths ranging from 0.6 to 5 microns, ideal for seeing light from the universe’s earliest stars and galaxies. Webb’s Mid-Infrared Instrument (MIRI) covers the wavelength range from 5 to 28 microns, its sensitive detectors collecting the redshifted light of distant galaxies. Conveniently for us, infrared passes more cleanly through deep space gas and dust clouds, revealing the objects behind them; for this and many other reasons, the infrared spectrum has gained a crucial foothold in our cosmic investigations. Earth-orbiting satellites like NASA’s Wide Field Infrared Survey Telescope ( WFIRST ) observe deep space via longer infrared wavelengths, too.

Yet, when stars first form, they mostly issue ultraviolet light . So why don’t we use ultraviolet detectors to find distant galaxies? It’s because the universe has been stretching since its beginning, and the light that travels through it has been stretching, too; every planet, star, and galaxy continually moves away from everything else. By the time light from GLASS-z13—formed 300 million years after the Big Bang—reaches our telescopes, it has been traveling for more than 13 billion years , a vast distance all the way from a younger universe. The light may have started as ultraviolet waves, but over vast scales of time and space, it ended up as infrared. So, this fledgling galaxy appears as a red dot to NIRCam. We are gazing back in time at a galaxy that is rushing away from us.

Radio Waves

m87 supermassive black hole in polarised light

If we could see the night sky only through radio waves, we would notice swaths of supernovae , pulsars, quasars, and gassy star-forming regions instead of the usual pinprick fairy lights of stars and planets.

Tools like the Arecibo Observatory in Puerto Rico can do the job our eyes can’t: detect some of the longest electromagnetic waves in the universe. Radio waves are typically the length of a football field, but they can be even longer than our planet’s diameter. Though the 1,000-foot-wide dish at Arecibo collapsed in 2020 due to structural problems, other large telescopes carry on the work of looking at radio waves from space. Large radio telescopes are special because they actually employ many smaller dishes, integrating their data to produce a really sharp image.

Unlike optical astronomy, ground-based radio telescopes don’t need to contend with clouds and rain. They can make out the composition, structure, and motion of planets and stars no matter the weather. However, the dishes of radio telescopes need to be much larger than optical ones to generate a comparable image, since radio waves are so long. The Parkes Observatory’s dish is 64 meters wide, but its imaging is comparable to a small backyard optical telescope, according to NASA .

Eight different radio telescopes all over the world coordinated their observations for the Event Horizon Telescope in 2019 to put together the eye-opening image of a black hole in the heart of the M87 galaxy (above).

Ultraviolet Waves

sun in ultraviolet nasa image

You may be most familiar with ultraviolet, or UV rays, in warnings to use sunscreen . The sun is our greatest local emitter of these higher-frequency, shorter wavelengths just beyond the human visible spectrum, ranging from 100 to 400 nanometers. The Hubble Space Telescope has been our main instrument for observing UV light from space, including young stars forming in Spiral Galaxy NGC 3627, the auroras of Jupiter, and a giant cloud of hydrogen evaporating from an exoplanet that is reacting to its star’s extreme radiation.

Our sun and other stars emit a full range of UV light, telling astronomers how relatively hot or cool they are according to the subdivisions of ultraviolet radiation: near ultraviolet, middle ultraviolet, far ultraviolet, and extreme ultraviolet. Applying a false-color visible light composite lets us see with our own eyes the differences in a star’s gas temperatures.

Hubble’s Wide Field Camera 3 (WFC3) breaks down ultraviolet light into specific present colors with filters. “Science visuals developers assign primary colors and reconstruct the data into a picture our eyes can clearly identify,” according to the Hubble website . Using image-processing software, astronomers and even amateur enthusiasts can turn the UV data into images that are not only beautiful, but also informative.

X-Ray Light

chandra xray telescope image of two galaxies colliding and forming a gas bridge between them

Since 1999, the orbiting Chandra X-Ray Observatory is the most sensitive radio telescope ever built. During one observation that lasted a few hours, its X-ray vision saw only four photons from a galaxy 240 million light-years away, but it was enough to ascertain a novel type of exploding star . The observatory, located 86,500 miles above Earth, can produce detailed, full-color images of hot X-ray-emitting objects, like supernovas, clusters of galaxies and gases, and jets of energy surrounding black holes that are millions of degrees Celsius. It can also measure the intensity of an individual X-ray wavelength, which ranges from just 0.01 to 10 nanometers. Its four sensitive mirrors pick up energetic photons and then electronic detectors at the end of a 30-foot optical apparatus focus the beams of X-rays.

Closer to home, the Aurora Borealis at the poles emits X-rays too. And down on Earth, this high-frequency, low-wavelength light passes easily through the soft tissue of our bodies, but not our bones, yielding stellar X-ray images of our skeletons and teeth.

Visible Light

visible light image of mystic mountain, a pillar of gas dust and newborn stars in the carina nebula taken by the hubble telescope

Visible color gives astronomers essential clues to a whole world of information about a star, including temperature, distance, mass, and chemical composition. The Hubble Telescope, perched 340 miles above our planet, has been a major source of visible light images of the cosmos since 1990.

Hotter objects, like young stars, radiate energy at shorter wavelengths of light; that’s why younger stars at temperatures up to 12,000 degrees Celsius, like the star Rigel, look blue to us. Astronomers can also tell the mass of a star from its color. Because mass corresponds to temperature, observers know that hot blue stars are at least three times the mass of the sun. For instance, the extremely hot, luminous blue variable star Eta Carina’s bulk is 150 times the mass of our sun, and it radiates 1,000,000 times our sun’s energy.

Our comparatively older, dimmer sun is about 5,500 degrees Celsius, so it appears yellow. At the other end of the scale, the old star Betelgeuse has been blowing off its outer layer for the past few years, and it looks red because it’s only about 3,000 degrees Celsius.

A View of Earth

space telescopes and what lightwave ranges they detect

Scientists use different wavelengths of light to study phenomena closer to home, too.

Detectors in orbit can distinguish between geophysical and environmental features on Earth’s changing surface, such as volcanic action. For example, infrared light used alongside visible light detection reveals areas covered in snow, volcanic ash, and vegetation. The Moderate Resolution Imaging Spectroradiometer ( MODIS ) infrared instrument onboard the Aqua and Terra satellites monitors forest fire smoke and locates the source of a fire so humans don’t have to fly through smoke to evaluate the situation.

Next year, a satellite will be launched to gauge forest biomass using a special radar wavelength of about 70 centimeters that can penetrate the leafy canopy.

💡 Why is the sky blue? During the day, oxygen and nitrogen in Earth’s atmosphere scatters electromagnetic energy at the wavelengths of blue light (450–485 nanometers). At sunset, the sun’s light makes a longer journey through the atmosphere before greeting your eyes. Along the way, more of the sun’s light is scattered out of the blue spectrum and deeper into yellow and red.

Headshot of Manasee Wagh

Before joining Popular Mechanics , Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.

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The Race Against Time: Understanding How Long it Takes for Light to Reach Earth

Last Updated: March 7, 2023

The speed of light is incredibly fast and considered unreachable by technology or anything other than light. And yet, even if we could travel at the speed of light, it would take many lifetimes, if at all, to reach many destinations. Let’s explore the speed of light and how we use it to study the universe, including how we look into the past in the night sky.

The speed of light

When someone turns on a light, how long does it take for someone else to see it? When the first ray of sunlight breaches the horizon, how long has it been since it was emitted from the Sun?

Greek philosophers argued whether light had a rate of travel (since it could indeed travel) or if it was simply instantaneous, but they didn’t have any way to prove either hypothesis. Galileo attempted to measure the speed of light in the mid-1600s by having two people stand on hills less than a mile apart with shielded lanterns. Person A would uncover their lantern and when Person B saw the flash, he uncovered his too. Less than a mile was not nearly far enough to measure an accurate speed, but, he did conclude that light traveled at least ten times faster than the speed of sound (343 m/s or 1,087 ft/sec). 

In the 1670s, Danish astronomer Ole Rømer was studying the exact time of the eclipses of Jupiter’s moon Io to create an astronomical calendar for sailors, but noticed the eclipses didn’t follow his calculations. When Jupiter and Earth were moving away from each other, the eclipses lagged. When the planets were moving toward each other, the time eclipse arrived early. When they were at their closest or furthest distance, the eclipse arrived right on schedule. From this data, he was able to infer that light had a measurable rate at which it traveled. He was able to estimate the speed of light from his data at about 124,000 miles per second (200,000 km/s). Since scientists had not known the exact size of the solar system and the Earth’s orbit at that time, it was not correct, but it was the best estimate at the time and his findings changed astronomy. 

Other scientists, began analyzing the speed of light using precisely aligned mirrors that reflected light over great distances to determine how long it took for light to travel that far. English physicist James Bradley, French physicist Hippolyte Fizeau, and French physicist Leon Foucault each measured more and more precisely, getting to within 1% and then about 1,000 miles per second (1,609 km/s) of the speed of light. 

Albert A. Michelson dedicated himself to the speed of light and light in general, creating the new best estimate of 186,355 miles per second (299,910 km/s) in 1879 and then fine-tuning it not just once, but twice with his final experiment shortly before his death in 1931 resulting in just under today’s accepted value of the speed of light. He also proved that light did not operate as a wave (like sound) as was the prevailing theory because that would require a medium (generally referred to as a luminiferous aether or simply aether to travel through space) and he found no evidence of any such aether meaning light can and does travel through the vacuum of space, earning him a Nobel Prize. 

With more precise instruments in better-controlled environments, we have been able to confirm that the speed of light is 299,792,458 meters (983,571,056 feet) per second which is 299,792 kilometers per second or about 186,282 miles per second. That’s fast enough to travel around the equator about 7.5 times in one second.

The speed of light is a universal constant known in equations as “c,” or light speed. The speed of light is constant because light acts as both a particle and a wave, meaning it does not need a medium to travel through. In fact, light’s speed from either a stationary object or a moving object is always the same.

Light traveling through a vacuum (or imperfect vacuum such as space) travels at the constant speed of light, but light that travels through a medium bends when it comes in contact with particles, slowing it down. The measure of how much a material slows down light is known as its refractive index. Earth’s atmosphere, for instance, slows down light by about three ten-thousandths of the speed of light whereas light traveling through a diamond slows down to less than half its original speed.

Astronomy/ astrophysics is essentially the only field of study where you can’t physically be in the room with what you are studying. The only way we can study it is by observing the light that reaches us. Understanding how light behaves and travels is crucial to being able to analyze that light and data. The speed of light helps us calculate distance, size, location, and more. 

In addition, the speed of light is integral in understanding the conversion of mass into energy with Einstein’s famous equation E=mc2 which explains how tiny amounts of mass can turn into massive amounts of energy. However, for an object to reach the speed of light, its mass would have to become infinite, meaning the energy needed to move the object would also become infinite which is an impossibility. (However, we have observed that the universe is expanding faster than the speed of light, which is a paradox we still do not understand). Understanding these possibilities and limitations allows us to study the universe, and create more accurate algorithms and simulations to better understand how the universe works.

speed of light

The distance to other celestial objects

If you hold out your thumb at arm’s length (ideally covering something in front of you, but not necessary), close one eye, and then switch which eye is open, you will see your thumb move slightly. This is a very simple, Earth-based example of parallax, when an object’s location is displaced based on point of view.   By measuring this small displacement and the distance between your eyes, you can calculate the distance to your thumb using trigonometry to find the top of the cone that engulfs the space in between.

While it may look like a two-dimensional projection above our heads, we must remember that space is three-dimensional. Stars may seem like they are right next to each other, but in reality, not only are they far apart from each other, but one is likely closer than the other and it may be the one you don’t expect. In addition, we are on a planet orbiting the Sun, meaning our location in our orbit impacts how we see stars in the night sky. Every six months, the Earth moves about 186 million miles (300 million kilometers) to the other side of the Sun, displacing its view of the universe around it. This is a relatively small amount based on the size of the universe, but enough to cause stars to perform tiny circles throughout the year, so tiny that detecting and measuring them is difficult. 

The ancient Greek astronomer Hipparchus was the first to use parallax to determine the location of a celestial object: the Moon. He had a solar eclipse observed from two separate locations to calculate how far away the moon was. Mars’ distance from Earth was determined using parallax in 1672 by Giovanni Cassini and his colleague Jean Richer during simultaneous observations in Paris and French Guiana. To measure the exact location of small objects in the sky, we break up the sky into degrees. From one horizon to the opposite is 180 degrees, half of a full circle which would go around the Earth. From one horizon to the point straight above your head (the zenith) is 90 degrees. Special tools on telescopes help to measure these degrees similar to a large protractor from your school days. You can also estimate using a closed fist held out at arm’s length being about 10 degrees. A degree is about the width of your pinky held at arm’s length. A degree is then broken into 60 arcminutes and each arc minute is broken into 60 arcseconds. Arcminutes and arcseconds definitely need special tools to measure accurately. Even the closest stars have a shift of less than an arcsecond when analyzing parallax. 

Simultaneous observations from 2 locations on opposite sides of the world worked for objects within our solar system, but for objects outside of the solar system, like stars, we needed to factor in the orbit of Earth, taking measurements from opposite ends. Friedrich Bessel in 1838 then used the method to determine the distance from star 61 Cygni, in the Cygnus constellation, from Earth, springboarding the long process of creating a three-dimensional map of the night sky which took decades and improvements in the telescope to make the process easier. 

Today, we also use parallax by comparing earth observations with observations from probes and missions out in the solar system such as New Horizons, providing even more distance to calculate even more specific measurements. Even using parallax and our updated technology, we eventually find that stars are too far away to calculate the minute changes. Instead, we build on the parallax method using stars that are known as “standard candles” for comparison in intrinsic brightness to determine distance. Using parallax and similar methods we can determine how far away a star is. A light-year is the distance light can travel in 1 year or about 6 trillion miles (10 trillion kilometers) and is the main measurement we use for calculating the distance to other stars. If we took the circumference of the Earth, laid it out on a straight line, and multiplied it by 7.5, we would get one light second. We would then need 31.6 million of those light-second lines to create the distance of a light-year.

Light from the sun takes about 8 minutes to reach Earth meaning our Sun is 8 light-minutes away from us. The closest stars to us are the Alpha Centauri triple-star system which is about 4.37 light-years away, meaning if we could travel at the speed of light (which again is impossible) it would take us just under 4 years and 4.5 months to travel the distance. The closest of these stars is Proxima Centauri which is about 4.24 lightyears away from us, meaning just under 4 years and three months at the speed of light. Remember that a light-year is about 6 trillion miles meaning they are over 26 trillion miles or 43.7 trillion km away. Voyager 1 would take over 73,000 years to arrive at Proxima Centauri.

Another important factor in this section before we move on is redshift. The frequency of light or sound emitted by a moving object changes based on if it is moving closer to the observer or farther away. Objects moving away from the observer shift towards “redder”, longer wavelengths, which is where we get the term redshift. If they are moving toward us, the frequencies shift to shorter, bluer frequencies. This is also known as the Doppler Effect, especially with sound waves, which you can hear as the change in tone when a siren passes by you. Understanding redshift helps us to better calculate the distances and locations of celestial objects, which are always moving. This was first discovered when Ole Rømer was studying the eclipses of Io and fundamentally changed our understanding of astronomy.

stars and nebula

The time it takes for light to reach Earth

If we know how many lightyears away an object is, that is also how many years it would take for light from that star to reach us (and vice versa). When we are looking at stars, we are looking back in time, seeing them as they were when their light left, whether that is dozens, hundreds, millions, or hundreds of millions of years ago. Let’s explore a few famous stars to better understand their light.

The brightest star in the night sky is Sirius A in Canis Major, the Big Dog Constellation and it is 8.6 light-years away. The light we see from it is almost nine years old.  

Looking at the Big Dipper in Ursa Major, most of the stars, excluding the far end of the handle and the top right of the dipper, are about 80-83 light-years away. Alkhaid and Dubhe (the two exceptions mentioned previously) sit at 104 and 123 light-years away respectively. On average, the light from the stars in the Big Dipper took about a century to reach us and we look at it as it was a hundred years ago. 

But this is abnormal. Many stars within a constellation are not close by at all in reality. If we look at the Northern Cross of the Cygnus/ Swan constellation, Epsilon (ε) Cygni in the eastern end of the crossbeam is located 73 light-years from Earth, and is almost 36 times closer than Deneb, or Alpha (α) Cygni, at the head of the cross. 

Betelgeuse, the bright red star in the armpit of Orion is 642.5 light-years away meaning the light we see from Betelgeuse is almost 650 years old. If there was intelligent life near Betelgeuse looking at us, they would see us as we were in the 1300s! 

Some of the most distant stars in the Milky Way are known as RR Lyrae stars in the halo surrounding our galaxy over a million light-years away. The furthest stars are almost halfway to Andromeda, our sister galaxy, which is about 2.5 million light-years away. Finally, in 2022, Hubble spotted Earendel, the most distant star detected 28 billion light-years away.

When considering how bright a star is, we have to consider both how much light it is putting out and how far away it is (in addition to other factors such as what’s between us and them like clouds of gas and dust). 

Apparent magnitude is how bright we see the star from Earth while absolute magnitude is how bright the star is at a standard distance of 32.6 light-years (equal to 10 parsecs, the next rung up on the distance measurements of the universe). To understand its actual luminosity or brightness, astronomers look at its color spectrum, what it’s emitting on the electromagnetic spectrum in the visible light range which correlates to temperature and therefore brightness. 

Knowing the difference between the apparent magnitude and the absolute magnitude or actual brightness/ luminosity helps astronomers calculate the distance to the star. The magnitude scale was determined by Ptolemy in the 2nd century C.E. with the brightest stars receiving a magnitude of 1 and the dimmest receiving a magnitude of 6 with a 100x difference in brightness in between, but has expanded as our instruments can spot dimmer objects. 

Brighter stars produce more light, but the farther away a star is, the dimmer it will appear. A brighter star might be further away if it is producing more light than a closer star. For instance, the brightest star in the night sky is Sirius A at 8.6 light-years away and about -1.33 magnitude, but the closest star to us is Proxima Centauri at 3.24 light-years away but much dimmer at a magnitude of 11. For more about brightness and magnitude see our previous article on the brightest planets .

light reaching earth surface

How long it takes for light from each planet in our solar system to reach Earth

Here are the average distances of each planet in our solar system from Earth, and the corresponding amount of time it takes for light to travel from that planet to Earth:

  • Mercury : 77 million km (48 million miles) away from Earth on average so it takes about 3.2 minutes for light from Mercury to reach Earth.
  • Venus : 261 million km (162 million miles) away from Earth on average so it takes about 6 minutes for light from Venus to reach Earth.
  • Mars : 78 million km (48 million miles) away from Earth on average so it takes about 3-22 minutes (depending on the positions of Mars and Earth in their respective orbits) for light from Mars to reach Earth.
  • Jupiter: 628 million km (390 million miles) away from Earth on average so it takes about 35-52 minutes (depending on the positions of Jupiter and Earth in their respective orbits) for light from Jupiter to reach Earth.
  • Saturn: 1.28 billion km (795 million miles) away from Earth on average so it takes about 1-2 hours for light from Saturn to reach Earth.
  • Uranus: 2.72 billion km (1.7 billion miles) away from Earth on average so it takes about 2.5-3 hours for light from Uranus to reach Earth.
  • Neptune: 4.35 billion km (2.7 billion miles) away from Earth on average so it takes about 4-4.5 hours for light from Neptune to reach Earth.

So, the time it takes for light from each planet in our solar system to reach Earth varies depending on the distance between the two.

Astronomy is the study of light from the night sky because, in general, that’s the only way we can study these specimens. Understanding how light works is vital to understanding the data we observe.

Understanding the speed of light (299,792 kilometers per second or about 186,282 miles per second) and the fact that it is constant even if the source is moving helps us to understand the dynamics of our universe as well as the impact of this speed on mass and energy. The speed of light is effectively a speed limit for objects in the universe as the mass and energy would need to be infinite to surpass it which is impossible. 

We also use the speed of light to calculate the distance to celestial objects using light-years, the distance light travels within a year, and better understand the apparent brightness we see on Earth compared with the actual brightness emitted from a star.

Understanding light helps us understand the universe because it helps us quantify what we see and therefore understand the mechanics of the universe.

Sarah H.

Written by Sarah Hoffschwelle

Sarah Hoffschwelle is a freelance writer who covers a combination of topics including astronomy, general science and STEM, self-development, art, and societal commentary. In the past, Sarah worked in educational nonprofits providing free-choice learning experiences for audiences ages 2-99. As a lifelong space nerd, she loves sharing the universe with others through her words. She currently writes on Medium at  https://medium.com/@sarah-marie  and authors self-help and children’s books.

Wow! There's more to read 🚀

This page is part of our collection of articles about astronomy . If you enjoyed the read, then you’ll love the following articles.

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proto star

From Dust to Stars: A Guide to The Process of Star Formation

waxing gibbous moon

What makes the Moon shine?

Image that reads Space Place and links to spaceplace.nasa.gov.

What Is a Light-Year?

An image of hundreds of small galaxies on the black background of space.

An image of distant galaxies captured by the NASA/ESA Hubble Space Telescope. Credit: ESA/Hubble & NASA, RELICS; Acknowledgment: D. Coe et al.

For most space objects, we use light-years to describe their distance. A light-year is the distance light travels in one Earth year. One light-year is about 6 trillion miles (9 trillion km). That is a 6 with 12 zeros behind it!

Looking Back in Time

When we use powerful telescopes to look at distant objects in space, we are actually looking back in time. How can this be?

Light travels at a speed of 186,000 miles (or 300,000 km) per second. This seems really fast, but objects in space are so far away that it takes a lot of time for their light to reach us. The farther an object is, the farther in the past we see it.

Our Sun is the closest star to us. It is about 93 million miles away. So, the Sun's light takes about 8.3 minutes to reach us. This means that we always see the Sun as it was about 8.3 minutes ago.

The next closest star to us is about 4.3 light-years away. So, when we see this star today, we’re actually seeing it as it was 4.3 years ago. All of the other stars we can see with our eyes are farther, some even thousands of light-years away.

A chart explaining how far away certain objects are from Earth. The Sun is 8.3 light-minutes away. Polaris is 320 light-years away. Andromeda is 2.5 million light years away. Proxima Centauri is 4.3 light-years away. The center of the Milky Way is 26,000 light-years away. GN-z11 is 13.4 billion light-years away.

Stars are found in large groups called galaxies . A galaxy can have millions or billions of stars. The nearest large galaxy to us, Andromeda, is 2.5 million light-years away. So, we see Andromeda as it was 2.5 million years in the past. The universe is filled with billions of galaxies, all farther away than this. Some of these galaxies are much farther away.

An image of the Andromeda galaxy, which appears as a blue and white swirling mass among hundreds more galaxies in the background.

An image of the Andromeda galaxy, as seen by NASA's GALEX observatory. Credit: NASA/JPL-Caltech

In 2016, NASA's Hubble Space Telescope looked at the farthest galaxy ever seen, called GN-z11. It is 13.4 billion light-years away, so today we can see it as it was 13.4 billion years ago. That is only 400 million years after the big bang . It is one of the first galaxies ever formed in the universe.

Learning about the very first galaxies that formed after the big bang, like this one, helps us understand what the early universe was like.

Picture of hundreds of galaxies with one shown zoomed in to see greater detail. The zoomed in part looks like a red blob.

This picture shows hundreds of very old and distant galaxies. The oldest one found so far in GN-z11 (shown in the close up image). The image is a bit blurry because this galaxy is so far away. Credit: NASA, ESA, P. Oesch (Yale University), G. Brammer (STScI), P. van Dokkum (Yale University), and G. Illingworth (University of California, Santa Cruz)

More to explore

Image of a pink and blue spiral galaxy.

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Illustration of a game controller that links to the Space Place Games menu.

The Sun's Energy: An Essential Part of the Earth System

Solar radiation, or energy produced by the Sun, is the primary energy source for most processes in the Earth system and drives Earth’s energy budget .

schematic showing incoming radiation and how it is absorbed and scattered by Earth's atmosphere and surface

The Sun is the primary energy source for our planet’s energy budget and contributes to processes throughout Earth.

UCAR/The COMET Program

Energy from the Sun is studied as part of heliophysics, which relates to the Sun’s physics and the Sun’s connection with the solar system.

How Does Energy from the Sun Reach Earth?

It takes solar energy an average of 8 ⅓ minutes to reach Earth from the Sun. This energy travels about 149 million km (93 million miles) through space to reach the top of Earth’s atmosphere. Waves of solar energy radiate, or spread out, from the Sun and travel at the speed of light through the vacuum of space as electromagnetic radiation. The majority of the Sun’s radiation reaching Earth is in the form of visible light we can see and invisible infrared energy that we can’t see. A smaller portion of sunlight is made up of ultraviolet radiation, which is also invisible to our eyes.

schematic showing the time it takes solar radiation, including visible light and infrared radiation, to travel to Earth

Most of the Sun’s energy reaching Earth includes visible light and infrared radiation but some is in the form of plasma and solar wind particles. 

Other forms of radiation from the Sun can reach Earth as part of the solar wind , but in smaller quantities and with longer travel times. The solar wind contains plasma and particles and can also include gamma rays and x-rays resulting from solar storms or other bursts of energy from the Sun’s surface.

The Energy We Receive Depends on Distance From the Sun

The Sun’s energy we receive is electromagnetic radiation that travels through space or a medium in the form of waves or particles. The visible light we see from the Sun is similar to the visible light from a light bulb, but the Sun provides much more of it. The energy released from the Sun is mind-bogglingly large, equal to about 36 octillion (3.6 x 10 28 ) or 36 thousand trillion trillion lumens, where a lumen is a measure of the total amount of visible light. This brightness (or intensity) falls off with distance from the source by a factor of 1/d 2 , where d 2 is the square of the distance. From the surface of Mercury, depending on the planet's position in its orbit, the Sun would appear from seven to eleven times brighter as it does when viewed from Earth. By the time the Sun's energy reaches Earth's surface, it has an average brightness of about 127,000 lumens per square meter, or about 127 light bulbs if we consider that a typical light bulb produces about 650-1600 lumens of visible light.

image of the Sun taken from space, with Earth's horizon in the foreground

The Sun is 93 million miles from Earth, yet it still provides us with all of the energy needed to sustain life.

The Sun’s Energy is Important to Life on Earth

Energy from the Sun makes it possible for life to exist on Earth. It is responsible for photosynthesis in plants, vision in animals, and many other natural processes, such as the movements of air and water that create weather. Most plants need at least some sunlight to grow, so without light, there would be no plants, and without plants, there would not be oxygen for us to breathe. Infrared radiation from the Sun is responsible for heating the Earth’s atmosphere and surface. Without energy from the Sun, Earth would freeze. There would be no winds, ocean currents, or clouds to transport water.

photo of the sun rising over grain fields

Energy from the Sun enables photosynthesis in plants, which provides the oxygen we breathe and helps grow the food we eat.

Creative Commons Stephen Bowler

How do Humans Benefit from the Sun's Energy?

Throughout history, humans have used technology to harness the Sun’s energy as a source of light and heat and for growing crops. As early as 30 CE, people were constructing greenhouses to grow plants out of season. Did you know that one of the earliest greenhouses was built for the Roman emperor Tiberius using translucent sheets of mica, all so that he could eat cucumbers in any season?

Early civilizations knew that setting up shelters to capture or block sunlight could help with heating and cooling. In cold climates, they would position buildings to face southward, allowing interior spaces to gather heat and light. In warm climates, shelters might be constructed under cliffs or natural overhangs to protect the residents from the Sun’s radiation in the hottest part of the day. Using energy from the Sun effectively also helped with growing crops.

photo of cliff dwelling situated under a rock overhang at Mesa Verde National Park

The cliff dwellings at Mesa Verde National Park in Colorado, USA, were built in the shadows of rock overhangs to help keep the structures cool in the hot desert sunlight.

Creative Commons Steven Zucker

Today, we can intentionally position windows and skylights to help heat or cool our homes through passive solar design. Solar panels can also capture energy from the Sun by gathering sunlight and converting it to electricity. As of 2023, solar power is the third largest source of renewable energy worldwide, behind hydropower and wind.

How is Energy from the Sun Harmful?

photo of a parent applying sunscreen to protect a child's skin from ultraviolet radiation

UV radiation can damage skin and cause sunburn, but simple actions such as wearing a hat and sunscreen provide essential protection from the Sun’s harmful UV rays. 

Some of the Sun’s energy reaches Earth in the form ultraviolet (or UV) radiation. Fortunately, the ozone layer high in Earth’s atmosphere absorbs a lot of this UV radiation and blocks it from reaching Earth’s surface. But some UV still makes it through. UV radiation from the Sun causes sunburn and skin damage but can be blocked with clothes and sunscreen. Wearing hats and sunglasses in bright sunlight and trying to stay in the shade when possible can also help protect you from harmful UV rays. Note that it’s not realistic or advisable to avoid all exposure to sunlight, as sunlight also helps our bodies produce vitamin D, which we need to be healthy.

UV radiation can damage plants and limit photosynthesis. Scientists have found that overexposure to UV radiation reduces size, productivity, and quality in several crop species, including many varieties of rice, soybeans, winter wheat, cotton, and corn. Sometimes, UV can cause bleaching of plants, visible as damage or discoloration. Planting tender young plants in the shadow of larger plants can help protect them from UV radiation.

  • Electromagnetic Spectrum
  • Types of Light from the Sun
  • Visible Light
  • Ultraviolet Radiation
  • What is Space Weather and How Does it Affect the Earth?

Related Links

  • Sun's Spectrum
  • NASA - Play Helios: A Game About How the Sun Makes Energy

Teach Astronomy logo

Chapter 15: Galaxies

Chapter 1 how science works.

  • The Scientific Method
  • Measurements
  • Units and the Metric System
  • Measurement Errors
  • Mass, Length, and Time
  • Observations and Uncertainty
  • Precision and Significant Figures
  • Errors and Statistics
  • Scientific Notation
  • Ways of Representing Data
  • Mathematics
  • Testing a Hypothesis
  • Case Study of Life on Mars
  • Systems of Knowledge
  • The Culture of Science
  • Computer Simulations
  • Modern Scientific Research
  • The Scope of Astronomy
  • Astronomy as a Science
  • A Scale Model of Space
  • A Scale Model of Time

Chapter 2 Early Astronomy

  • The Night Sky
  • Motions in the Sky
  • Constellations and Seasons
  • Cause of the Seasons
  • The Magnitude System
  • Angular Size and Linear Size
  • Phases of the Moon
  • Dividing Time
  • Solar and Lunar Calendars
  • History of Astronomy
  • Ancient Observatories
  • Counting and Measurement
  • Greek Astronomy
  • Aristotle and Geocentric Cosmology
  • Aristarchus and Heliocentric Cosmology
  • The Dark Ages
  • Arab Astronomy
  • Indian Astronomy
  • Chinese Astronomy
  • Mayan Astronomy

Chapter 3 The Copernican Revolution

  • Ptolemy and the Geocentric Model
  • The Renaissance
  • Copernicus and the Heliocentric Model
  • Tycho Brahe
  • Johannes Kepler
  • Elliptical Orbits
  • Kepler's Laws
  • Galileo Galilei
  • The Trial of Galileo
  • Isaac Newton
  • Newton's Law of Gravity
  • The Plurality of Worlds
  • The Birth of Modern Science
  • Layout of the Solar System
  • Scale of the Solar System
  • The Idea of Space Exploration
  • History of Space Exploration
  • Moon Landings
  • International Space Station
  • Manned versus Robotic Missions
  • Commercial Space Flight
  • Future of Space Exploration
  • Living in Space
  • Moon, Mars, and Beyond
  • Societies in Space

Chapter 4 Matter and Energy in the Universe

  • Matter and Energy
  • Rutherford and Atomic Structure
  • Early Greek Physics
  • Dalton and Atoms
  • The Periodic Table
  • Structure of the Atom
  • Heat and Temperature
  • Potential and Kinetic Energy
  • Conservation of Energy
  • Velocity of Gas Particles
  • States of Matter
  • Thermodynamics
  • Laws of Thermodynamics
  • Heat Transfer
  • Thermal Radiation
  • Radiation from Planets and Stars
  • Internal Heat in Planets and Stars
  • Periodic Processes
  • Random Processes

Chapter 5 The Earth-Moon System

  • Earth and Moon
  • Early Estimates of Earth's Age
  • How the Earth Cooled
  • Ages Using Radioactivity
  • Radioactive Half-Life
  • Ages of the Earth and Moon
  • Geological Activity
  • Internal Structure of the Earth and Moon
  • Basic Rock Types
  • Layers of the Earth and Moon
  • Origin of Water on Earth
  • The Evolving Earth
  • Plate Tectonics
  • Geological Processes
  • Impact Craters
  • The Geological Timescale
  • Mass Extinctions
  • Evolution and the Cosmic Environment
  • Earth's Atmosphere and Oceans
  • Weather Circulation
  • Environmental Change on Earth
  • The Earth-Moon System
  • Geological History of the Moon
  • Tidal Forces
  • Effects of Tidal Forces
  • Historical Studies of the Moon
  • Lunar Surface
  • Ice on the Moon
  • Origin of the Moon
  • Humans on the Moon

Chapter 6 The Terrestrial Planets

  • Studying Other Planets
  • The Planets
  • The Terrestrial Planets
  • Mercury's Orbit
  • Mercury's Surface
  • Volcanism on Venus
  • Venus and the Greenhouse Effect
  • Tectonics on Venus
  • Exploring Venus
  • Mars in Myth and Legend
  • Early Studies of Mars
  • Mars Close-Up
  • Modern Views of Mars
  • Missions to Mars
  • Geology of Mars
  • Water on Mars
  • Polar Caps of Mars
  • Climate Change on Mars
  • Terraforming Mars
  • Life on Mars
  • The Moons of Mars
  • Martian Meteorites
  • Comparative Planetology
  • Incidence of Craters
  • Counting Craters
  • Counting Statistics
  • Internal Heat and Geological Activity
  • Magnetic Fields of the Terrestrial Planets
  • Mountains and Rifts
  • Radar Studies of Planetary Surfaces
  • Laser Ranging and Altimetry
  • Gravity and Atmospheres
  • Normal Atmospheric Composition
  • The Significance of Oxygen

Chapter 7 The Giant Planets and Their Moons

  • The Gas Giant Planets
  • Atmospheres of the Gas Giant Planets
  • Clouds and Weather on Gas Giant Planets
  • Internal Structure of the Gas Giant Planets
  • Thermal Radiation from Gas Giant Planets
  • Life on Gas Giant Planets?
  • Why Giant Planets are Giant
  • Ring Systems of the Giant Planets
  • Structure Within Ring Systems
  • The Origin of Ring Particles
  • The Roche Limit
  • Resonance and Harmonics
  • Tidal Forces in the Solar System
  • Moons of Gas Giant Planets
  • Geology of Large Moons
  • The Voyager Missions
  • Jupiter's Galilean Moons
  • Jupiter's Ganymede
  • Jupiter's Europa
  • Jupiter's Callisto
  • Jupiter's Io
  • Volcanoes on Io
  • Cassini Mission to Saturn
  • Saturn's Titan
  • Saturn's Enceladus
  • Discovery of Uranus and Neptune
  • Uranus' Miranda
  • Neptune's Triton
  • The Discovery of Pluto
  • Pluto as a Dwarf Planet
  • Dwarf Planets

Chapter 8 Interplanetary Bodies

  • Interplanetary Bodies
  • Early Observations of Comets
  • Structure of the Comet Nucleus
  • Comet Chemistry
  • Oort Cloud and Kuiper Belt
  • Kuiper Belt
  • Comet Orbits
  • Life Story of Comets
  • The Largest Kuiper Belt Objects
  • Meteors and Meteor Showers
  • Gravitational Perturbations
  • Surveys for Earth Crossing Asteroids
  • Asteroid Shapes
  • Composition of Asteroids
  • Introduction to Meteorites
  • Origin of Meteorites
  • Types of Meteorites
  • The Tunguska Event
  • The Threat from Space
  • Probability and Impacts
  • Impact on Jupiter
  • Interplanetary Opportunity

Chapter 9 Planet Formation and Exoplanets

  • Formation of the Solar System
  • Early History of the Solar System
  • Conservation of Angular Momentum
  • Angular Momentum in a Collapsing Cloud
  • Helmholtz Contraction
  • Safronov and Planet Formation
  • Collapse of the Solar Nebula
  • Why the Solar System Collapsed
  • From Planetesimals to Planets
  • Accretion and Solar System Bodies
  • Differentiation
  • Planetary Magnetic Fields
  • The Origin of Satellites
  • Solar System Debris and Formation
  • Gradual Evolution and a Few Catastrophies
  • Chaos and Determinism
  • Extrasolar Planets
  • Discoveries of Exoplanets
  • Doppler Detection of Exoplanets
  • Transit Detection of Exoplanets
  • The Kepler Mission
  • Direct Detection of Exoplanets
  • Properties of Exoplanets
  • Implications of Exoplanet Surveys
  • Future Detection of Exoplanets

Chapter 10 Detecting Radiation from Space

  • Observing the Universe
  • Radiation and the Universe
  • The Nature of Light
  • The Electromagnetic Spectrum
  • Properties of Waves
  • Waves and Particles
  • How Radiation Travels
  • Properties of Electromagnetic Radiation
  • The Doppler Effect
  • Invisible Radiation
  • Thermal Spectra
  • The Quantum Theory
  • The Uncertainty Principle
  • Spectral Lines
  • Emission Lines and Bands
  • Absorption and Emission Spectra
  • Kirchoff's Laws
  • Astronomical Detection of Radiation
  • The Telescope
  • Optical Telescopes
  • Optical Detectors
  • Adaptive Optics
  • Image Processing
  • Digital Information
  • Radio Telescopes
  • Telescopes in Space
  • Hubble Space Telescope
  • Interferometry
  • Collecting Area and Resolution
  • Frontier Observatories

Chapter 11 Our Sun: The Nearest Star

  • The Nearest Star
  • Properties of the Sun
  • Kelvin and the Sun's Age
  • The Sun's Composition
  • Energy From Atomic Nuclei
  • Mass-Energy Conversion
  • Examples of Mass-Energy Conversion
  • Energy From Nuclear Fission
  • Energy From Nuclear Fusion
  • Nuclear Reactions in the Sun
  • The Sun's Interior
  • Energy Flow in the Sun
  • Collisions and Opacity
  • Solar Neutrinos
  • Solar Oscillations
  • The Sun's Atmosphere
  • Solar Chromosphere and Corona
  • The Solar Cycle
  • The Solar Wind
  • Effects of the Sun on the Earth
  • Cosmic Energy Sources

Chapter 12 Properties of Stars

  • Star Properties
  • The Distance to Stars
  • Apparent Brightness
  • Absolute Brightness
  • Measuring Star Distances
  • Stellar Parallax
  • Spectra of Stars
  • Spectral Classification
  • Temperature and Spectral Class
  • Stellar Composition
  • Stellar Motion
  • Stellar Luminosity
  • The Size of Stars
  • Stefan-Boltzmann Law
  • Stellar Mass
  • Hydrostatic Equilibrium
  • Stellar Classification
  • The Hertzsprung-Russell Diagram
  • Volume and Brightness Selected Samples
  • Stars of Different Sizes
  • Understanding the Main Sequence
  • Stellar Structure
  • Stellar Evolution

Chapter 13 Star Birth and Death

  • Star Birth and Death
  • Understanding Star Birth and Death
  • Cosmic Abundance of Elements
  • Star Formation
  • Molecular Clouds
  • Young Stars
  • T Tauri Stars
  • Mass Limits for Stars
  • Brown Dwarfs
  • Young Star Clusters
  • Cauldron of the Elements
  • Main Sequence Stars
  • Nuclear Reactions in Main Sequence Stars
  • Main Sequence Lifetimes
  • Evolved Stars
  • Cycles of Star Life and Death
  • The Creation of Heavy Elements
  • Horizontal Branch and Asymptotic Giant Branch Stars
  • Variable Stars
  • Magnetic Stars
  • Stellar Mass Loss
  • White Dwarfs
  • Seeing the Death of a Star
  • Supernova 1987A
  • Neutron Stars and Pulsars
  • Special Theory of Relativity
  • General Theory of Relativity
  • Black Holes
  • Properties of Black Holes

Chapter 14 The Milky Way

  • The Distribution of Stars in Space
  • Stellar Companions
  • Binary Star Systems
  • Binary and Multiple Stars
  • Mass Transfer in Binaries
  • Binaries and Stellar Mass
  • Nova and Supernova
  • Exotic Binary Systems
  • Gamma Ray Bursts
  • How Multiple Stars Form
  • Environments of Stars
  • The Interstellar Medium
  • Effects of Interstellar Material on Starlight
  • Structure of the Interstellar Medium
  • Dust Extinction and Reddening
  • Groups of Stars
  • Open Star Clusters
  • Globular Star Clusters
  • Distances to Groups of Stars
  • Ages of Groups of Stars
  • Layout of the Milky Way
  • William Herschel
  • Isotropy and Anisotropy
  • Mapping the Milky Way

Chapter 15 Galaxies

  • The Milky Way Galaxy
  • Mapping the Galaxy Disk
  • Spiral Structure in Galaxies
  • Mass of the Milky Way
  • Dark Matter in the Milky Way
  • Galaxy Mass
  • The Galactic Center
  • Black Hole in the Galactic Center
  • Stellar Populations
  • Formation of the Milky Way
  • The Shapley-Curtis Debate
  • Edwin Hubble
  • Distances to Galaxies
  • Classifying Galaxies
  • Spiral Galaxies
  • Elliptical Galaxies
  • Lenticular Galaxies
  • Dwarf and Irregular Galaxies
  • Overview of Galaxy Structures
  • The Local Group

Light Travel Time

  • Galaxy Size and Luminosity
  • Mass to Light Ratios
  • Dark Matter in Galaxies
  • Gravity of Many Bodies
  • Galaxy Evolution
  • Galaxy Interactions
  • Galaxy Formation

Chapter 16 The Expanding Universe

  • Galaxy Redshifts
  • The Expanding Universe
  • Cosmological Redshifts
  • The Hubble Relation
  • Relating Redshift and Distance
  • Galaxy Distance Indicators
  • Size and Age of the Universe
  • The Hubble Constant
  • Large Scale Structure
  • Galaxy Clustering
  • Clusters of Galaxies
  • Overview of Large Scale Structure
  • Dark Matter on the Largest Scales
  • The Most Distant Galaxies
  • Black Holes in Nearby Galaxies
  • Active Galaxies
  • Radio Galaxies
  • The Discovery of Quasars
  • Types of Gravitational Lensing
  • Properties of Quasars
  • The Quasar Power Source
  • Quasars as Probes of the Universe
  • Star Formation History of the Universe
  • Expansion History of the Universe

Chapter 17 Cosmology

  • Early Cosmologies
  • Relativity and Cosmology
  • The Big Bang Model
  • The Cosmological Principle
  • Universal Expansion
  • Cosmic Nucleosynthesis
  • Cosmic Microwave Background Radiation
  • Discovery of the Microwave Background Radiation
  • Measuring Space Curvature
  • Cosmic Evolution
  • Evolution of Structure
  • Mean Cosmic Density
  • Critical Density
  • Dark Matter and Dark Energy
  • Age of the Universe
  • Precision Cosmology
  • The Future of the Contents of the Universe
  • Fate of the Universe
  • Alternatives to the Big Bang Model
  • Particles and Radiation
  • The Very Early Universe
  • Mass and Energy in the Early Universe
  • Matter and Antimatter
  • The Forces of Nature
  • Fine-Tuning in Cosmology
  • The Anthropic Principle in Cosmology
  • String Theory and Cosmology
  • The Multiverse
  • The Limits of Knowledge

Chapter 18 Life On Earth

  • Nature of Life
  • Chemistry of Life
  • Molecules of Life
  • The Origin of Life on Earth
  • Origin of Complex Molecules
  • Miller-Urey Experiment
  • Pre-RNA World
  • From Molecules to Cells
  • Extremophiles
  • Thermophiles
  • Psychrophiles
  • Acidophiles
  • Alkaliphiles
  • Radiation Resistant Biology
  • Importance of Water for Life
  • Hydrothermal Systems
  • Silicon Versus Carbon
  • DNA and Heredity
  • Life as Digital Information
  • Synthetic Biology
  • Life in a Computer
  • Natural Selection
  • Tree Of Life
  • Evolution and Intelligence
  • Culture and Technology
  • The Gaia Hypothesis
  • Life and the Cosmic Environment

Chapter 19 Life in the Universe

  • Life in the Universe
  • Astrobiology
  • Life Beyond Earth
  • Sites for Life
  • Complex Molecules in Space
  • Life in the Solar System
  • Lowell and Canals on Mars
  • Implications of Life on Mars
  • Extreme Environments in the Solar System
  • Rare Earth Hypothesis
  • Are We Alone?
  • Unidentified Flying Objects or UFOs
  • The Search for Extraterrestrial Intelligence
  • The Drake Equation
  • The History of SETI
  • Recent SETI Projects
  • Recognizing a Message
  • The Best Way to Communicate
  • The Fermi Question
  • The Anthropic Principle
  • Where Are They?

In the everyday world, as perceived by the human senses, light seems to travel instantaneously from one place to another. In fact, the speed of light is not infinite, and light doesn't instantly jump from your ceiling light to your desk and then to your eye. We perceive light as moving instantly because its actual velocity is almost unimaginably high; light travels at 300,000 km/s, denoted c. Using the equation Rate × Time = Distance, you can divide any distance by this number to figure out the time it would take light to cross that distance. In this way, we can see that light takes 1.5 × 10 8 / 3 × 10 5 = 500 seconds to reach Earth from the Sun, or just over 8 minutes. It takes light about 40 times longer ( Pluto at a distance of 39.4 A.U.) to leave the Solar System or about 5 hours.

The speed of light is a built-in quality of our universe . All evidence to date indicates that light has always traveled at this speed, that the speed is exact, and that the same speed is observed for all observers. The vast size of the universe, coupled with the finite (albeit large) speed of light, means that as we look out in space, we look back in time. Distant light is old light.

light travel to earth from sun

The 5 hours it takes light to travel across our Solar System may seem like a short period to cross such a large distance, but we have to think about scale. While distances within the Solar System are large to us, they are dwarfed by the distances between the stars. Considering larger regions of the Milky Way, a natural distance unit is the distance light travels in one year. This is called a light year. We can easily calculate the size of this unit by remembering that distance has the units of velocity times time. So:

D ly = vt = c x 1 year = 3 × 10 5 x (3600 × 24 × 365) = 9.5 × 10 12 km

light travel to earth from sun

A light year is the typical distance between stars in the neighborhood of the Sun. It is nearly 10 trillion kilometers or 6 trillion miles! The fundamental unit of distance defined by geometry is the 13 km; defined as the distance corresponding to a parallax of 1 second of arc.">parsec , equal to 3.1 × 10 13 km. This is described in more detail in the article on parallax . Geometrically, one parsec is the height of a right triangle with an angle of 1 arcsec describing its apex , and a distance of 1 AU describing its base. The units are related by a small numerical constant D ly = 3.26 D pc . So to roughly convert from parsecs to light years, multiply by 3.3.

The following list gives the distance to various points within the Milky Way and beyond, both in terms of parsecs and the light travel time in years (which is also the distance in light years or 3.3 times the distance in parsecs). To fully appreciate how isolated we are in space, remember that light is the fastest thing we know of. The fastest spacecraft can not reach 1% of the speed of light. So you would have to multiply the numbers on the right-hand side of the table by at least 100 to estimate how long it would take to send a probe through the Milky Way and into the Local Group with current technology.

light travel to earth from sun

• Nearest star (α Centauri) - 1.3 pc, 4.2 years • Sirius - 2.7 pc, 8.8 years • Vega - 8.1 pc, 26 years • Hyades cluster - 42 pc, 134 years • Pleiades cluster - 125 pc, 411 years • Orion nebula - 460 pc, 1500 years • Nearest spiral arm - 1200 pc, 3900 years • Center of the 8 to 10 13 solar masses.">galaxy - 8500 pc, 29,000 years • Far edge of the galaxy - 24,000 pc, 78,000 years • Large Magellanic Cloud - 50,000 pc, 163,000 years • Andromeda galaxy (M31) - 670,000 pc, 2.2 million years

light travel to earth from sun

What does Andromeda look like now? Nobody knows. Since nothing travels faster than light (and this applies to all the colors of light across the electromagnetic spectrum ), there is no quicker way to send information from one place to another. We are stuck with collecting and measuring "old" light. While this seems like a limitation, scientists actually find that it turns out that light travel time is a wonderful tool. By looking further out in space we look further back in time. In this way, astronomers get to explore the earlier stages of the universe seeing firsthand (with a delay) what the early universe looked like.

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How long does it take light to travel from the sun to Earth?

light travel to earth from sun

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The Solar System

About the image.

solar system

The image of the Solar System was made using real images of the planets. It is not to scale; the Solar System is so large with respect to the size of the planets, that to fit it on the screen, the planets would have to be small dots. Thus, some artistic license is involved. However, the planets are in the correct order. Also shown is comet Hale-Bopp, photographed by the author.

One way to help visualize the relative distances in the solar system is to imagine a model in which the solar system is reduced in size by a factor of a billion (10 9 ). The Earth is then about 1.3 cm in diameter (the size of a grape). The Moon orbits about a foot away. The Sun is 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter is 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) is 10 blocks away; Uranus and Neptune (lemons) are 20 and 30 blocks away. A human on this scale is the size of an atom; the nearest star would be over 40,000 km away!

Distance Information

Distances in the solar system are commonly measured in Astronomical Units (AU). An AU is simply the average distance between the Earth and the Sun. Because the Earth's orbit around the Sun is an ellipse, the Earth is not always the same distance from the Sun. An AU is equal to ~149,600,000 km. It takes 8 minutes for light to travel from the Sun to the Earth,traveling at the speed of light, of course.

The Moon, the closest solar system body to us, is about 400,000 km away from the Earth, which means it takes about 2 seconds for radio signal from Earth to reach the Moon and travel back. You could hear this delay in communications between the Apollo astronauts and the ground control.

The most distant planet from the Earth isn't Pluto anymore. Pluto was reclassified as a "dwarf planet"; a dwarf planet is not just a small planet - it belongs to a separate class of objects. Neptune is now the outer-most planet in our solar system. Its orbit places it at ~ 4,500,000,000 km or 30 AU from the Sun.

Pluto is still an interesting member of the solar system, however - its orbit is actually very eccentric and takes Pluto 4,400,000,000 - 7,400,000,000 km (30 - 49 AU) from the Sun. Pluto's orbit is also inclined with respect to the planets and doesn't fall within the same plane. As a result of its eccentricity, Pluto occasionally comes closer to the Sun than the planet Neptune does!

The Outer Reaches of the Solar System

There are objects belonging to our Solar System that are even farther than the orbit of our planets. The Kuiper Belt is a disk-shaped region past the orbit of Neptune, roughly 4,400,000,000 to 14,900,000,000 km (30 to 100 AU) from the Sun, that consists mainly of small bodies which are the remnants from the Solar System's formation. It also contains at least one dwarf planet - Pluto. Pluto is indeed now considered to be a member of the Kuiper Belt - the largest object belonging to it, in fact! Like other members of the Belt, it is composed primarily of rock and ice and is relatively small. There is an excellent discussion on why Pluto was reclassified from "planet" to "dwarf planet" and Kuiper Belt Object (KBO) here . The Kuiper Belt is also believed to be the source for short-period comets (ie, those that take less than 200 years or orbit).

The Kuiper Belt and Solar System

Pluto is not the only dwarf planet in our solar system - Eris, 27% more massive than Pluto, was discovered in 2003. Eris and its moon Dysnomia have a current distance from the Sun of 97 AU, which is nearly 3 times as far from the Sun as Pluto is. Eris is part of a region of space beyond the Kuiper Belt known as the scattered disc. The scattered disc is sparesely populated with icy minor planets. These so-called Scattered Disc Objects or SDO's are among the most distant and thus the most cold objects in the solar system. The innermost portion of the scattered disc overlaps with the Kuiper Belt, but its outer limits extend much farther away from the Sun and farther above and below the ecliptic than Belt. Although their origin is not completely understood, it is thought that Scattered Disc Objects were previously members of the Kuiper Belt, which got ejected into eccentric, scattered orbits through close encounters with Neptune.

From the surface of a Scattered Disc Object, the Sun would look like little more than an exceptionally bright star.


In 1977, the Voyager satellites were launched - and after tours that took them near the outer planets in our Solar System, they continued going, with a new mission to explore interstellar space. As of 2007, Voyager 1 was nearing the heliopause - the region where the Sun's dominance of the environment ends and interstellar space begins. This made Voyager the farthest human-made object, at more than three times the distance of Pluto. Voyager 1 continues to move away from us at 17.3 kilometers per second.

Moving still further away from the Sun, we reach the Oort Cloud. In 1950, astronomer Jan Oort proposed that long-period comets reside in a vast spherical cloud residing 5,000 to 50,000+ AU from the Sun, at the outer reaches of the Solar System. This major reservoir of comets has come to be known as the Oort Cloud. The Kuiper belt can be described as disc or doughnut-shaped, but the Oort cloud is more like a very thick "bubble" that surrounds the entire solar system, reaching about half-way from the Sun to the next nearest star. Statistics imply that it may contain as many as a trillion (10 12 ) comets. Unfortunately, since the individual comets are so small and at such large distances, we have no direct evidence for the Oort Cloud. The Oort Cloud is, however the best theory to explain how long-period comets exist.

The Oort Cloud

50,000 AU seems like a very large distance from the Sun - but the nearest star to us is over 271,000 AU away!

How Do We Calculate Distances of This Magnitude?

Johannes Kepler, born in 1571, was the first to explain the motions of the planets in the sky, by realizing that the planets revolved around the Sun - and that their orbits were actually ellipses, not perfect circles. He also knew that the movement of the planets around the Sun could be described by physics - and in mathematical terms. The closer the planet was to the Sun, the faster it moved. Conversely, farther planets orbited the Sun more slowly. Knowing this, he was able to connect the average distance of a planet from the Sun with the time it takes that planet to orbit the Sun once.

Though he wasn't able to come up with distance measurements in kilometers, Kepler was able to order the planets by distance and to figure out their proportional distances. For example, he knew that Mars was about 1.5 times farther from the Sun than the Earth.

If you hold your finger in front of your face, close one eye and look with the other, then switch eyes, you'll see your finger seem to "shift " with respect to more distant objects behind it. This is because your eyes are separated from each other by a distance of a few inches - so each eye sees the finger in front of you from a slightly different angle. The amount your finger seems to shift is called its "parallax".

Diagram showing parallax

In the late 17th century, Giovanni Cassini used the parallax technique to measure the distance to Mars. Cassini knew that a larger parallax would be easier to measure - but this required a larger baseline (ie, the baseline would be like the distance between your eyes). He took measurements of the position of Mars from Paris and sent a fellow astronomer to French Guiana in South America to do the same. This gave him a baseline of several thousand kilometers - using geometry, he was able to calculate a distance for Mars that is only 7% off today's more precise measurements! (Try this page for the mathematics for the calculation.)

Even with modern technology, measuring distances by parallax isn't trivial - and the errors can be big - as we can see from Cassini's measurement of the Earth-Mars distance.

Oneof the most accurate ways to measure the distances to the planets is by bouncing radar off them, or sending a spacecraft there, which can send a radio signal back to the Earth that can be timed. Radar is essentially microwave electromagnetic radiation (microwaves fall under the radio spectrum). Since electromagnetic radiation, in all of its forms, is light, we know that radar travels at the speed of light - 2.99 x 10 5 km/s. Simply, distance traveled is equal to the time multiplied by the velocity. If we bounce radar off a planet, and measure the time it takes the signal to go there and back, we can use this information to calculate the distance of the planet.

Distant Solar System Objects

There are other modern methods to calculate the distances to objects on the fringes of our Solar System, like Kuiper Belt or Scattered Disc Objects. However, these techniques are often based on those Kepler employed! Several observations of the object's position in the sky are recorded, which are then used to determine the orbit of the object - then the position of the object along each point can be calculated. Nowadays, even home PCs are powerful enough that there are some advanced amateur astronomers who not only discover comets and asteroids but determine their orbits.

Why Are These Distances Important To Astronomers?

Knowing the distances to objects in our solar system, tells us how big it is - and how far away our neighboring planets are. How far the planets are from the Sun is particularly meaningful - here's why.

If youplace a candle at arm's length in an otherwise dark room, you'll see a bright flame. If you stand twice as far from the candle, you will see that it is now a quarter as bright as before. When you increase the distance by a factor of 2 (or 3 or 4, ...), the same amount of light has spread to an area 4 times (or 9 or 16, ...) bigger. This means the amount of light per unit area is 1/4 (or 1/9, 1/16, ...); since our eyes have the same area no matter how far the candle is, the brightness we perceive is also decreased by the same factor.

Similarly, the distance from the Sun determines how much sunlight a planet receives. On Mars, which is 1.6 AU from the Sun (AU being the average Sun-Earth distance), the sunlight is about 2.5 times weaker than on Earth. That is the major reason why it is so cold on Mars, so cold that water does not exist as liquid on the Martian surface today. On Venus, which is 0.7 AU from the Sun, the sunlight is twice as intense as on Earth. This (combined with the greenhouse effect of its thick atmosphere) makes Venus a boiling hot place, unsuitable for human habitation. This leads to the idea of a 'habitable zone' --- for a star of a given brightness, you can determine the approximate range of distances a planet has to be for liquid water to exist. Life as we know it will not be able to evolve on a planet outside such a habitable zone.

Travel Time

Cassini, launched in 1997, is a spacecraft that was bound for Saturn. It traveled towards Saturn at 18,720 miles per hour, or 5.2 kilometers per second. Using gravitational assists to aid it, Cassini still took 6.7 years to reach Saturn. If Cassini left Saturn and continued on to Pluto at a rate of 5.2 km/s, it would arrive there about 27 years later.

For detailed, up-to-date, information about our Solar System, see the wonderful "Nine Planets" page, written by Bill Arnett. See also NASA JPL's page on the planets and A Comprehensive Guide to Our Solar System . For more information about the Oort Cloud and the Kuiper Belt, see Phil Plait's Bad Astronomy page .

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Distances in Light Time between Planets and the Sun

Light time introduces the concept of length associated with the distance traveled by the light itself over a period of time.

In particular, the light year is a unit of length measurement, defined as the distance traveled by electromagnetic radiation (with the term light referring to the spectral portion visible from the human eye) in the vacuum, in a sidereal year (365 Days, 6 hours, 9 minutes, and 10 seconds).

The speed of light (c) in the void is 299 792,458 (km/s) . To have that distance in miles, just multiply that value over the time range considered. Approximately 9461 billion kilometers (or 63 241 times the distance between Earth and the Sun, also called astronomical units, is 149 597 870,700 km ).

The light year, like parsec (about 3.26 light years), is mainly used for galactic distances. Below are the distances between the Sun and the planets belonging to the Solar System in light time .

  • Mercury : 3,3 light minutes
  • Venus : 6 light minutes
  • Earth : 8,3 light minutes
  • Mars : 12,7 light minutes
  • Jupiter : 43 light minutes
  • Saturn : 1,3 light hours
  • Uranus : 2,7 light hours
  • Neptune : 4,2 light hours

Pluto, the dwarf planet, is at 5.3 light hours from us. You must travel a distance of one light year to get out of the Solar System; While you should travel at the speed of light for just over 4 years before encountering another star, Proxima Centauri.

  • distances in light time
  • solar system
  • speed of light

light travel to earth from sun

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Four Forces

light travel to earth from sun

The Four Fundamental Forces

Why does Earth stay in orbit around the Sun? How does light travel? What holds atoms and nuclei together?

For centuries, scientists have sought to describe the forces that dictate interactions on the largest and smallest scales, from planets to particles. They understand that there are four fundamental forces — gravity, electromagnetism, and the strong and weak nuclear forces — that are responsible for shaping the universe we inhabit.

Gravitational Force

graphic demonstrating the Gravitational Force

The most familiar force is gravity. It is responsible for keeping our feet on the ground and holding Earth in its orbit around the Sun.

According to the general theory of relativity, gravity can be understood as bends and curves in the fabric of space-time that affect the motions of galaxies, stars, planets, and even light. Anything with mass makes a dent in space-time, causing objects to be attracted to each other.

Gravity is an attractive force that draws two objects together. Its strength approximately increases with the masses of the two objects but decreases with the square of the distance between them. That means that if the Moon were twice its current distance from Earth, the gravitational tug between the two would be just one fourth of what it is now.

Despite being the weakest force, gravity works across infinite distances, making it responsible for the formation of the universe's structure.

Electromagnetic Force

graphic demonstrating the Electromagnetic Force

Our television sets are powered by electromagnetism. Light carries this force, which illuminates our houses at night, keeps electrons in orbit around atomic nuclei, and allows chemical compounds to form.

As the name implies, electromagnetism is the force that includes both electricity and magnetism. They are intertwined — a moving electric field produces a magnetic field, and vice versa.

Like gravity, the strength of electromagnetism drops off with the square of the distance between objects and works at infinite range. However, electromagnetism only comes into play for charged objects, and whether it attracts or repels depends on the charges of each. Two negative or positive charges repel each other; one of each attracts.

While electromagnetism is stronger than gravity, it is often balanced out in large objects by the equal numbers of positive and negative charges that form neutral atoms. For example, Earth has a magnetic field due to electric currents in its liquid core; however, Earth itself is electrically neutral.

Strong Nuclear Force

graphic demonstrating the Strong Force

Nuclear forces affect our daily lives, but they work on distances smaller than atoms.

The strong nuclear force, or strong force for short, holds together the building blocks of atoms. It always attracts and works at two different size scales in atoms. At the level of an atomic nucleus, the strong force holds together the protons and neutrons that form the essence of the elements. On an even smaller scale, the strong force holds together the oppositely charged quarks that make up the neutrons and protons themselves.

As suggested by its name, the strong force is the strongest of the fundamental forces. It is about 100 times stronger than electromagnetism and 100 trillion trillion trillion times stronger than gravity.

However, the strong force only has influence over very, very small distances. For anything larger than the nucleus of a medium-sized atom (about 100 million times smaller than the width of a human hair), its influence quickly drops, and other forces will be stronger.

Weak Nuclear Force

graphic demonstrating the Weak Force

The weak force is responsible for interactions between subatomic particles – the tiny particles that are the building blocks for matter, like protons, neutrons, and electrons.

In particular, the weak force can change one quark type into another. Protons and neutrons are made of two quark varieties, up and down. The weak force can turn a down quark in a neutron into an up quark, which would change the neutron into a proton and switch its electric charge from neutral to positive. If that neutron were in the nucleus of an atom, the change to a proton would turn that atom into a different type of element. Such reactions are happening all the time in our Sun, giving it the energy to shine. This type of action also occurs in radioactive decay, which happens when atoms spontaneously shed energy and subatomic particles.

The weak force works on the smallest distance scales, another 1,000 times smaller than the strong force. It is about a million times weaker than the strong force, too, though it is still considerably stronger than gravity.

Discover More Topics From NASA

Dark Matter & Dark Energy

light travel to earth from sun

The Big Bang

light travel to earth from sun

The Nine Planets

The Nine Planets

How Far is the Sun From Earth?

Everything in the Universe is moving, from a couple of kilometers/miles per second to more than 200 km / 124 mi per second, with space itself theorized to expand faster than even the speed of light, which is 299,792 km / 186,282 mi per second.

Most of the objects in our universe appear to be drifting away from us, while others, such as the Andromeda Galaxy , are closing in on us, but let us take a closer look at the celestial objects in our vicinity, like the Sun.

light travel to earth from sun

How far is the Sun from Earth? Well, when it comes to space, we apply different measurements, and in terms of distance, we speak trough astronomical units.

An astronomical unit (AU) is the equivalent of 150 million km / 93 million mi, and the Sun is 1 AU away from Earth. In light-years, the Sun is 0.00001581 light-years away, while in light minutes, the Sun is 8.20 light minutes away, or 500 light-seconds away from Earth.

If we were to speak in meters, then the Sun would be 150.4 billion meters away from Earth. The Earth orbits the Sun once every 365.3 days, while farther planets such as Mars, completes an orbit around the Sun in 687 days. For comparison, Mars is 1.5 AU away from the Sun, which would translate to 227.94 million km / 141.70 million mi.

light travel to earth from sun

Since the Earth moves around the Sun, the distance differs, with Earth’s closest point from the Sun – perihelion – reaching 147.5 million km / 91.3 million mi.

When it comes to Earth’s farthest point from the Sun – aphelion –  it is around 152 million km / 94.5 million mi, a little over 1 AU away from the Sun.

How Long Would It Take to Get to the Sun From Earth?

It’s tough to predict a spacecraft’s journey towards the Sun. If we were to launch an imaginary spacecraft from Earth that would travel around 153,454 mi / 246,960 km per hour constantly, it would reach the Sun in 606 hours, or 25 days.

However, what clouds our estimation is the fact that it is impossible to launch a spacecraft that would constantly maintain its top speed from the start. When the spacecraft is launched, it will take several minutes or hours to reach its top speed.

light travel to earth from sun

One of the fastest planned spacecraft on Earth is the Parker Solar Probe. This probe might reach a maximum speed of 430,000 mi / 692,017 km per hour. This means that the spaceship may get to the Sun in around 216 hours or nine days.

However, there is an additional problem. Nothing in space stays still, so we can’t launch anything directly at the Sun, because the moment the object would arrive at the Sun’s position, it would no longer be there.

Therefore, we first have to predict where the Sun would be, based upon its moving speed, and then calculate our object’s moving rate, and most of all, we even have to consider our Earth’s movement.

How Long Would It Take to Get to the Sun in a Car?

Let’s say we could drive our cars towards the Sun. Since when it comes to space, distances take on a whole new value, maybe with this hypothetical scenario, we might more easily familiarize ourselves with the actual length of the Sun, how far away it is.

So how long would it take to get to the Sun in a car? If our car would travel at a constant speed of around 100 mph, and if we could drive for 24 hours without rest, then we would reach the Sun with our car in more than 106 years.

light travel to earth from sun

However, this also implies that our oxygen, food, and fuel reserves are infinite, and we would travel towards a correct estimation of where the Sun would be at, in around 106 years.

In a Jumbo Jet, it may take up to 19 years to get to the Sun from Earth, so regardless of our current daily traveling methods, it would take more than a lifetime to reach the Sun.

Is Earth Getting Closer to the Sun?

Since the Sun is so far away, and as discussed above, it would take more than a lifetime to reach it, would our chances of getting to the Sun increase in the distant future? Is our Earth getting closer to the Sun?

Now, getting closer to the Sun wouldn’t really help us in any way, shape, or form, probably not even scientifically. There may be other methods available to us in the future to collect data from our Sun, without the need to be near it.

light travel to earth from sun

In fact, we aren’t even getting closer to our Sun. Our Earth is actually slowly moving away from the Sun. This is because our Sun, like all stars, burns its fuel.

As the Sun burns its fuel,  it loses power, mass, and gravity. Since the Sun’s gravity / gravitational pull is weakening, since it loses mass, our Earth can slowly move away from it.

Our Earth is moving away from the Sun at around 15 cm every year. However, Earth will never escape from the Solar System , since the Sun will evolve in 5 billion years into a red giant star.

When this happens, Earth will be with 750,000 km / 466,028 mi farther away from the Sun; however, the Sun will also expand its radius for more than 256 times its current size (696,340 km / 432,685 mi), reaching over 178 million km / 110 million mi in radius, inevitably engulfing Earth in the end.

How Far is it From Earth to Space?

If you want to reach space from Earth, then you would have to fly straight up in the sky for around 100 km / 62 miles. This is where most scientists agree, is where our planet’s boundary ends, and suborbital space begins.

light travel to earth from sun

Did you know?

  • Our Solar System travels through space at a speed of around 515,000 mph / 828,000 km/h.
  • It takes our Solar System around 230 million years to travel around our galaxy, the Milky Way , once.
  • Neptune is the farthest planet from the Sun. It is situated at around 30 AU away – that’s 30 times farther away from the Sun than our Earth.
  • Though Neptune is regarded as the farthest planet away from the Sun when Pluto was categorized as a planet, it held this title. Now Pluto is considered a dwarf planet , and it is located at around 39 AU away from our Sun.

Image Sources:

  • https://earthsky.org/upl/2014/01/earth-sun-lg-nasa-artist-e1476952761550.jpg
  • https://scijinks.gov/review/earths-seasons/aphelion-perihelion-lrg.png
  • https://www.eteknix.com/wp-content/uploads/2015/07/solar-system-800×450.jpg
  • https://www.nasa.gov/sites/default/files/thumbnails/image/sweap_thumb.png
  • https://regmedia.co.uk/2018/01/18/sun_earth_messenger.jpg
  • https://images.immediate.co.uk/production/volatile/sites/4/2019/09/GettyImages-471296532-c-db7bc08.jpg?quality=90&resize=960%2C408

Does the sun move in the solar system?

Yes and no.

an illustration of the solar system showing the orbits of the planets around the sun

From an early age, we are taught to understand that the planets of our solar system change in position while orbiting a central star, the sun. But does the sun itself move within the solar system?

Well, in general the sun is far from static in the universe. We know, for instance, that our star orbits the heart of the Milky Way at staggering speeds reaching 450,000 miles per hour (720,000 kilometers per hour) and dragging the whole solar system along with it.

Over the course of the day, the sun certainly appears to move from our vantage point, too. It crosses across the sky over Earth , giving us lovely sunrises and sunsets. This movement, however, is a result of the Earth rotating; it's not the result of the actual motion of the sun .

Related: Space mysteries: Why do Earth's magnetic poles flip?

Further, during the course of an Earth year, a familiar 365.3 days, the sun's position changes in the sky from our perspective as well. Still, according to Royal Museums Greenwich , that is not the result of the sun actually moving, but rather the result of Earth's tilt , or the fact that our planet has a flattened or "elliptical orbit" and is therefore sometimes closer to the sun during a year than at other times.

The time it takes a planet to complete a full orbit of the sun determines the length of its year, with the shortest year being that of the closest planet to the sun, Mercury . Mercury's year is equivalent to 88 Earth days. The longest planetary orbit in our cosmic neighborhood belongs to Neptune , which has a year that lasts 60,182 Earth days (164.8 Earth years).

But returning to our main question, the short answer is that the sun does indeed shift position within the solar system, albeit by a tiny amount. That limited oscillating motion or "wobble" results from the gravitational influences of the planets that orbit the sun.

Patrick Antolin is a solar scientist at Northumbria University who specializes in phenomena that we observe in the solar atmosphere, and in particular, the solar corona, which is the most extended layer of the sun's atmosphere .

"Movement is always relative to the frame of reference. The solar system orbits around the center of the Milky Way  —  our galaxy  —  but even within the frame of the solar system, the sun is not exactly static because of the gravitational interaction with the other bodies in the system," Antolin told Space.com.

The solar scientist said that the gravitational interaction between two bodies is a two-way street. As body one pulls on body two, body one is also being pulled on itself, even if the size difference between the two bodies is immense, as is the case with the sun and the planets of the solar system. 

"Because of the large difference in mass between the sun and any other body in the solar system, the sun is the main gravitational attractor and is not very much affected by any of the other planets' gravity," he continued.

The net result of all this is that the planets of the solar system don't technically orbit their star. Instead, the sun and each planet orbit a point of mutual gravity called a " barycenter ," the location of which is determined by the masses of the bodies in question. 

Because the sun is so much more massive than the planets, these barycenters are located deep within the sun; if a planet's mass is small, the barycenter it orbits falls closer to the sun's heart. And the closer these barycenters are to the sun's center, the less the sun will wobble due to orbiting it.

"To a good approximation, one can neglect the small gravitational pull from any other planet," Antolin said. "However, our instruments and theory are precise and advanced enough that we can detect the small deviations from these additional gravitational pulls that the other bodies exert on the sun, and in particular that produced by Jupiter, which is more massive than all the other solar system planets combined."

The sun is around 1,000 times more massive than Jupiter, which is the fifth planet in the solar system, so the effect on the sun as a result of the gas giant is no more than a 40-mile-per-hour "wobble" over the planet’s 12-Earth year-long orbit around its star, according to Lick Observatory.  

Stellar wobbles caused by orbiting planets are detectable by the shift in the wavelength of light they produce, similar to the Doppler shift. This means the Doppler effect can be used to detect planets orbiting stars outside the solar system, called extrasolar planets or "exoplanets." As a star wobbles, the wavelength of its light is stretched and becomes redder as its motion is directed away from Earth, known as "redshift." As a star moves toward Earth, the wavelength of its light is compressed, making it relatively bluer in color, predictably referred to as "blueshift."

 —   What is the biggest star in the universe?

 —  What is the biggest planet ever found?

 —  Space mysteries: Why are there no gas moons?

Not only can this effect be used to spot a stellar wobble and, therefore, detect the presence of an exoplanet, but it can also be used to measure some of the properties of the bodies in distant planetary systems. 

"If one can detect the wobble as well as the speeds of the bodies, then one can infer the masses and distances between each other," Antolin said. "This can be applied to any star system in which we can detect the wobble and measure the speeds of the rotating bodies. 

"Of course, there is additional complexity when more than two massive bodies are involved, but numerical models can often help find out the most likely number of planets involved in the wobble."

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

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Robert Lea

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

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  • Helio When astronomers use the word "tiny", like "inconceivable" "it may not mean what you think it means." ;) As Wiki notes, the Sun can move, at most, just over 810,000 km, a little more than its radius and about 65x that of Earth. " To calculate the actual motion of the Sun, only the motions of the four giant planets (Jupiter, Saturn, Uranus, Neptune) need to be considered. The contributions of all other planets, dwarf planets, etc. are negligible. If the four giant planets were on a straight line on the same side of the Sun, the combined center of mass would lie at about 1.17 solar radii, or just over 810,000 km, above the Sun's surface ." Reply
Admin said: We know the sun is currently zooming through space, but does it move within our solar system? Space mysteries: Does the sun move in the solar system? : Read more
Astronomers Have Located The Centre of The Solar System to Within 100 Metres
This is where the team's software enters the picture. It's called BayesEphem, and it's designed to model and correct for those uncertainties in Solar System orbits most relevant to gravitational wave searches using pulsars - Jupiter in particular. When the team applied BayesEphem to the NANOGrav data, they were able to place a new upper limit on the gravitational wave background and detection statistics. And they were able to calculate a new, more precise location for the Solar System barycentre that, going forward, could enable much more accurate low-frequency gravitational wave detections.
Torbjorn Larsson said: Notably "the astronomical GPS system" of pulsar arrays had to improve on the knowledge of the system barycenter in order to later make the discovery of the gravitational background. 30 June 2020, MICHELLE STARR, ScienceAlert
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Despite appearences from Earth's surface, the sun is larger than the moon | Fact check

light travel to earth from sun

The claim: The sun and the moon are the same size and the same distance from Earth

A Dec. 28, 2023, Facebook post ( direct link , archive link ) claims humanity's basic understanding of the solar system is flawed.

"They say the sun is 400 times larger than the moon and that it only appears to be the same size as the moon because, miraculously, it is 400 times further away," reads the post. "Occam’s razor states the simplest explanation is usually the best one; they are of equal size and distance from us, and both reside within our local system."

The post was shared more than 100 times within three weeks.

More from the Fact-Check Team: How we pick and research claims | Email newsletter | Facebook page

Our rating: False

The sun and moon are different sizes and located at different distances from the Earth, as confirmed by an array of measurement approaches including radar, lasers and calculations based on planetary movement.

Sun is larger and farther from the Earth than the moon

The sun is around 400 times larger than the moon and also around 400 times farther away from Earth than the moon, according to NASA . Because of this, the moon and the sun may appear to be almost the same size when observed from Earth, such as during a total solar eclipse .

However, at other times, such as during an annular solar eclipse , the sun and moon appear to be different sizes. This is because the moon's orbit around Earth (and Earth's orbit around the sun) are elliptical, so the precise distances between the three bodies change, according to NASA .

An annular solar eclipse occurs when the moon is farther away from Earth in its orbit, making it appear smaller.

Fact check : Flat Earth claim based on gas pressure fails to account for gravity's impact

Researchers have confirmed the sun and moon's actual sizes and positions in space through various techniques.

For instance, the distance between Earth and the moon − 238,855 miles on average − has been measured by bouncing radar and lasers off the moon and counting the time it takes for the signal to return to Earth, according to the Institute of Physics . The diameter of the moon − around 2,159 miles − can be calculated mathematically based on the distance from the Earth to the moon and other measurements, Phil Sutton , an astrophysics lecturer at the University of Lincoln, explained on his YouTube channel.

The distance between Earth and the sun ( around 93 million miles on average) can be extrapolated from the distance between the Earth and Venus, Jo Dunkley , a Princeton astrophysicist, wrote in "Our Universe: An Astronomer's Guide ." This distance can be measured by observing a transit of Venus − the movement of the planet in front of the sun with respect to the Earth − from different positions on Earth.

The distance to Venus can also be measured with radar . The diameter of the sun − about 865,000 miles − can be determined by measuring the amount of time it takes for Mercury to transit in front of the sun.

USA TODAY reached out to the Facebook user who shared the post for comment but did not immediately receive a response.

Our fact-check sources:

  • Space, Aug. 22, 2023, What's the difference between a total solar eclipse and an annular solar eclipse?
  • Space, Nov. 1, 2023, Astronomical Unit: How far away is the sun?
  • NASA, accessed Jan. 15, Calculating the Astronomical Unit during a Transit of Venus using Satellite Data
  • NASA, accessed Jan. 15, Why Do Eclipses Happen?
  • NASA, accessed Jan. 16, Our Sun: Facts
  • NASA, accessed Jan. 16, Fun facts about the moon
  • NASA, accessed Jan. 16, How big is our solar system?
  • AstroPhil (YouTube), Jan. 1, 2022, How Do We Calculate The Diameter Of The Moon?
  • Our Universe: An Astronomer's Guide , 2019, p. 32-38
  • University of Hawaii, March 21, 2012, Scientists measure diameter of the Sun with unprecedented accuracy
  • Forbes, Nov. 6, 2019, A Transit Of Mercury Told Us The Scale Of The Universe
  • The Astronomical Journal, May 1962, A radar investigation of Venus
  • The Royal Museums Greenwich, accessed Jan. 16, How far away is the moon?
  • The New York Times, April 12, 1985, Lasers measure distance to moon to within an inch, scientists say
  • The Institute of Physics, accessed Jan. 16, The Moon’s distance from Earth

Thank you for supporting our journalism. You can subscribe to our print edition, ad-free app or e-newspaper here .

USA TODAY is a verified signatory of the International Fact-Checking Network, which requires a demonstrated commitment to nonpartisanship, fairness and transparency. Our fact-check work is supported in part by a grant from Meta .

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NASA announces new 'super-Earth': Exoplanet orbits in 'habitable zone,' is only 137 light-years away

The exoplanet, TOI-715 b, is about one and a half times the width of Earth.

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Could a recently discovered "super-Earth" have the potential temperature and conditions to sustain life?

The new exoplanet is situated "fairly close to us" -- only 137 light-years away -- and orbits within a "habitable zone," according to NASA .

Astronomers say the planet, dubbed TOI-715 b, is about one and a half times the width of Earth and orbits a small, reddish star. The same system also might harbor a second, Earth-sized planet, which, if confirmed, "would become the smallest habitable-zone planet discovered by TESS [ the Transiting Exoplanet Survey Satellite ] so far," NASA said in a Jan. 31 press release.

This illustration shows one way that planet TOI-715 b, a super-Earth in the habitable zone around its star, might appear to a nearby observer.

Due to the super-Earth's distance from its parent star, it could be in a conservative "habitable zone" and harbor the right temperature for liquid water to form on its surface, which is essential to sustain life, according to the agency, which also added that "several other factors would have to line up, of course."

NASA said the measurements of the habitable zone -- "a narrower and potentially more robust definition than the broader 'optimistic' habitable zone" -- put the newly discovered planet, and possibly the smaller Earth-sized planet, in "prime position" from its parent star.

The agency said that because of the short distance the super-Earth orbits from its parent star, a red dwarf that's smaller and cooler than our Earth's sun, a "year" for the planet is equal to 19 Earth days.

The tighter orbits mean the "planets can be more easily detected and more frequently observed," NASA said.

Since its launch in 2018, TESS has been adding to astronomers' stockpile of habitable-zone exoplanets, such as TOI-715 b, that could be more closely scrutinized by NASA's James Webb Space Telescope, the agency said.

The Webb telescope is designed to not only detect exoplanets but "explore the composition of their atmospheres, which could offer clues to the possible presence of life," NASA said.

The super-Earth research and discovery was led by Georgina Dransfield at the University of Birmingham in the U.K. and published in the "Monthly Notices of the Royal Astronomical Society" journal in January.

The findings mark another step forward in astronomers' mission to understand what atmospheric conditions are needed to sustain life and further explore the characteristics of exoplanets beyond our solar system, NASA said.

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