# The Naked Scientists Forum

### Author Topic: How long does it take for sunlight to reach Earth?  (Read 22634 times)

#### Tom Ryan

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##### How long does it take for sunlight to reach Earth?
« on: 14/11/2010 20:30:03 »
Tom Ryan  asked the Naked Scientists:

In the October Playboy magazine, it said in a short info fact that "sunlight takes from 10 to 170 thousand years to get from the sun to earth." We know it's eight minutes, what are they talking about? Is heat also moving that fast?

Thanks,

Tom R

What do you think?
« Last Edit: 14/11/2010 20:30:03 by _system »

#### chris

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• The Naked Scientist
##### How long does it take for sunlight to reach Earth?
« Reply #1 on: 14/11/2010 21:08:36 »
The Sun's about 90 million miles away, which is about 150 million kilometres.

The speed of light is 300,000 km/s, so the time is 150*10^6 / 300*10^5 = 500 seconds or about 8 minutes, as you say.

But this relates to the time it takes a photon leaving the Sun's surface to cover the distance to Earth. What the statistic you read might have been referring to is the average time it takes the photon to also escape from inside the Sun, reach the surface and then begin to travel towards the Earth.

York University Professor of Astrophysics Brian Fulton once mentioned to me in an interview on the Naked Scientists that photons can take a million years to make it out of the Sun owing to the density of the material and hence being scattered internally so many times.

Therefore, although it sounds unlikely, Playboy might be on the right lines when considering the actual age of the photon that reaches the Earth, which could be up to millions of years old before it gets here!

Chris

#### maffsolo

• Sr. Member
• Posts: 280
##### How long does it take for sunlight to reach Earth?
« Reply #2 on: 14/11/2010 22:48:18 »
Tom Ryan  asked the Naked Scientists:

In the October Playboy magazine, it said in a short info fact that "sunlight takes from 10 to 170 thousand years to get from the sun to earth." We know it's eight minutes, what are they talking about? Is heat also moving that fast?

Thanks,

Tom R

What do you think?

I think I read that a photon takes that long to get out from the center of the sun...
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Light from the Center of the Sun
On a PBS science show I heard a statement made about the length of time it took for light to go from the center of the Sun to its outer edge. The time was in thousands of years, which does not sound plausible, but someone else on the show confirmed the statement. Can you provide any information about this?
Yes, it does take light thousands of years to get out of the Sun. The important thing to realize is that the Sun (especially at the center) is quite opaque, that is, light travels through it only slightly better than light travels through a rock. What happens is that light only travels a short distance before it is absorbed. It is then re-emitted, but in a random direction. It eventually random "walks" it's way out of the Sun, but that takes a long time.

Dr. Eric Christian
http://helios.gsfc.nasa.gov/qa_sun.html#center
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"The distance between the earth and the sun is 93,000,000 miles.
The time it takes light to travel this distance is 93,000,000
miles/186,000 miles/sec or 500 seconds. Hence the time is 500sec/60 or
8.333 minutes. What fraction of a year is 8.333 minutes, well that is
8.333/(60x24x365) = or 0.00001585 light years.
You can express the distance either way, but in this case 93 million
miles makes more sense."