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# how long does it take for light from the sun to reach earth?

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### James

Guys, does anyone know the answer?

get how long does it take for light from the sun to reach earth? from EN Bilgi.

## Seeing in the Dark . Astronomy Topics . Light as a Cosmic Time Machine

The universe tells us its story mainly through light and other wavelengths of electromagnetic radiation. We learn about the planets, stars, and galaxies by their light—visible light, and also shorter-wavelength ultraviolet and longer-wavelength infrared light, invisible to the eye but detectable by certain telescopes on Earth and in space—and by the still longer waves of radio energy that they send us. These waves do not arrive instantaneously. Although they travel at the fastest possible speed (the speed of light,) they take a while to get here. The universe is big, so the news is delayed by the vast gulfs of space it has to cross to reach us. Light covers 186,000 miles EVERY SECOND (kids, please don't try traveling this fast without adult supervision!!!) In metric units, that's about 300,000 kilometers per second.

How long does light take to reach us from familiar objects? Let's take a quick tour of the solar system, asking at each place how long its light takes to reach us here on Earth.

## The Moon and the Sun

The closest object to us is the Moon. Its average distance is about 240,000 miles, so light from the Moon takes (240,000 miles divided by 186.000) 1 and 1/3 seconds to get from the Moon to Earth. When astronauts orbited the Moon and later walked on its surface in the 1960's, television viewers noticed that they were slow to answer questions transmitted from Earth. That was because it took 1.3 seconds for the question to travel to the Moon, and another 1.3 seconds for the answer to get back to Earth. Those 2.6 seconds were exactly the round-trip travel time for radio waves between the Earth and the Moon.

The Sun is 93 million miles away, so sunlight takes 8 and 1/3 minutes to get to us. Not much changes about the Sun in so short a time, but it still means that when you look at the Sun, you see it as it was 8 minutes ago. PhotoPhoto of the Sun in hydrogen-alpha light.

## The Planets

The giant planet Jupiter, whose large moons Galileo discovered with his "trouble-making" telescope, is more than 5 times farther from the Sun than the Earth is. We see a planet like Jupiter because its light—which like the other planets and the Moon originates on the Sun—takes about 43 minutes to reach Jupiter. The return trip to the Earth can take from 35 to 52 minutes, depending on whether we are on the same side of the Sun as Jupiter or on the other side.

Little Pluto is so small and remote it was not discovered until 1930, orbits 40 times farther from the Sun than we do. Light from the Sun takes about 5 and 1/2 hours to reach it and roughly the same time to return to Earth. By the time the light reaches us, it has spread out so much that the planet looks very dim, and requires a good telescope to spot. PhotoPhoto of Pluto and its moon Charon, as seen with the Hubble Space Telescope in 1994.

## Beyond the Solar System

Moving beyond the Solar System, our scale of distances and travel times needs to change. Now light will require years, not hours, to make its way to us. The star that is nearest to the Sun happens to be part of a system of three stars. (Unlike the Sun, which is a loner, many stars are found in groups of two, three, four or more.) The brightest star in our neighbor system is called Alpha Centauri (pronounced Al' fa Sen' to ree), and it is a virtual twin of the Sun. Light from Alpha Centauri takes more than 4 years to reach the Sun. (Astronomers use a special term for this way of measuring distance—they say the star is 4 light years away.)

The brightest star in our skies is the "dog star", Sirius (pronounced Sea' ree us). It's the primary star in the constellation of the big dog, Canis Major. Sirius is roughly 9 light years away. Think of what you were doing 9 years ago. That's when the light we see from Sirius tonight first began its journey to us. Not far from Sirius in the sky is the bright star Betelgeuse (pronounced Beetle' juice). It is so far that its light takes 430 years to reach us. Light that we see tonight from Betelgeuse left it in the late 1500's.

In the same part constellation, Orion, as Betelgeuse but even farther away is the Orion Nebula, a place where we see new stars forming. Its distance is 1500 light years, meaning that the light we see from it left more than a thousand years before the invention of the telescope.

The farther away an object in space lies, the longer it takes its light to get to us and the older that light is when it reaches Earth. As we look deeper and deeper into the Milky Way Galaxy (the island of stars in which we live), we are looking deeper into the past. Light can take tens of thousands of years or more to reach us from distant parts of our galaxy, which is roughly 100,000 light years wide.

## Other Galaxies

Once we move outside the galaxy, we encounter even larger spaces and longer light travel times. One of the great scientific ideas of the 20th century astronomy was the discovery that there are other galaxies out there—stretching as far as our great telescopes can see. Billions of other islands of stars are scattered through the great dark ocean of space.

The nearest large galaxy to the Milky Way is the Andromeda Galaxy. Sometimes astronomers call it M31, by its number in the famous Messier catalog of fuzzy celestial objects. The Andromeda (pronounced An drah' mid a) Galaxy lies about 2 1/2 million light years from Earth. The light we see from it tonight left it more than 2 million years ago, when our species was just beginning to establish its fragile foothold on planet Earth.

In this sense, astronomy is mostly ancient history: The farther away objects are, the older the story they have to tell us. Young people, raised on CNN, the Web, and "instant messaging" may at first bridle at the thought that the most recent information we can get from a neighbor galaxy might be 2 million years old. But for astronomers, this delay in the arrival of light is one of the universe's greatest gifts.

After all, one of the fundamental tasks of astronomy is to fill in the history of the universe—from the Big Bang to the moment you are reading this paragraph. Astronomers might not be able to undertake such a task if the information from the universe were limited to current events. But the universe is a time machine. Looking at more distant objects, we learn about more ancient times and phenomena. Large telescopes allow us to look billions of years into the past and to reconstruct the story of the cosmos eon by eon.

Source : www.pbs.org

## 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.

April 15, 2013

# How long does it take sunlight to reach the Earth?

by Fraser Cain, Universe Today

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.

Explore further

Source : phys.org

## How long does it take for sunlight to reach the earth?

Find out just how far back the warm sunshine hitting our face was emitted from the sun itself.

A bloke called George Harrison once sang ‘here comes the sun, dooby doo doo’.

Annoyingly, he failed to mention how fast it was travelling, or its estimated time of arrival.

With weather like this showing no signs of letting up, you might just be wondering just how far the golden rays on your face have traveled to greet you.

Have they been blasting through space all day or just for a wee while? When was this light born?

And how long does it take in earth terms for this light to reach us from the sun?

The speed of light is 299,792,458 metres per second (299,792.458 kilometres per second).

The distance between the earth and the sun is 149.6 million kilometres (that’s 92.95 million miles).

Scientists calculate that the average time it takes for light to reach the earth from the sun is 8 minutes and 20 seconds.

However, this time is variable because the earth is constantly orbiting the sun on a course which is elliptical, ie uneven.

It can be between 147 million to 152 million km away from the sun at any time.

When closest to the star, sunlight only takes 490 seconds (8 minutes 10 seconds) to reach Earth. And then at the most distant point, it takes 507 seconds (8 minutes 27 seconds) for sunlight to make the journey.

But interestingly, those travelling photons (light radiations) are much, much older than eight minutes. They were made thousands of years ago inside the sun itself.

Once they reach the surface and escape they travel fast. But they are made via fusion reactions right at the sun’s core and VERY slowly work their way out.

They start as gamma radiation and then become photons waiting to be released inside the giant star, with a diameter 109 times larger than earth.

A whopping 1,300,000 earths can fit inside the sun. Not that any right-thinking earth would want to do that.

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