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How do we see whole galaxies or even the whole sun? The photons of light travelling from the sun cover a vast distance, so shouldn't we only see the bit that hits the earth and the rest would be missing?

The earth is many times smaller than the sun and yet we see the whole sun. How does light carry the image of the whole sun to earth? From the same viewing position with a telescope you get greater detail of the sun. How do the photons carry all these different images, including such detail?

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    $\begingroup$ It's a matter of perspective. ;) We don't see orthographically. $\endgroup$
    – PM 2Ring
    Jul 3 at 15:55
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    $\begingroup$ This is fundamental optics. It doesn't even belong on Astronomy.SE $\endgroup$ Jul 5 at 12:13

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Actually, we don't see the whole Sun. We only see a bit less than half of it.

We see the half that faces us because every point over it radiates photons in every possible direction, including exactly towards our eyes or instruments.

Yes, the great majority of photons get lost (for some meaning of lost) in space. This is also how we get to see other stars.


What you possibly got wrong:

You may imagine that the Sun radiates every photon at right angle to its surface. It doesn't, every point radiates in every direction.

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    $\begingroup$ The "What you possibly got wrong:" is the really important bit. If stars would work like that we would indeed not see all of them $\endgroup$
    – Hakaishin
    Jul 4 at 13:26
  • $\begingroup$ Of course, this also happens to be true for nearly everything we can see. Aside from lasers, rather few things emit photons in a close approximation of only one direction. I can see the entire keyboard sitting in front of me from any angle I want because each part of it is reflecting light in nearly every direction, too. $\endgroup$
    – reirab
    Jul 6 at 22:34
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You only see the light that enters your eye!

But it doesn't matter that your eye is smaller or larger than the thing you are looking at.

Hold your hand up in front of your face at arms length. There is some light that reflects of the top of your hand into your eye, and some light from the middle and some from the bottom. It all comes into your eye from different directions and is then focused into an image on the back of your eye.

The "magic" is how a lens (or a curved mirror) can focus light that is coming from different directions into an image. But, of course, it is not really magic. Instead, it is just how light works.

It's the same with a galaxy. If you look at a galaxy there would be some light that comes from the top part, some from the middle and some from the lower part. It gets focused to an image. Galaxies are very faint, so a telescope will gather more light and allow you to magnify the image and make it appear larger.

But there is no greater mystery in why we can see the whole of a galaxy than why we can see the whole of a hand when it is in front of our eyes. We can see the whole as an image because that is what lenses do.

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  • $\begingroup$ It is also worth noting that to image very distant objects, VERY long exposure times are often used. There might not be very many photons striking the lens per second from a distant galaxy, but with a long enough exposure time (and some math to account for your velocity) you can gather a more detailed image. $\endgroup$
    – abestrange
    Jul 5 at 23:37
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Each individual atom of the Sun emits light in every direction (including back into the Sun). So some light from every point on the Sun's surface will hit the lens. Since they were emitted from different positions, they will arrive at the lens at slightly different angles. The job of a lens is to exploit this fact and bend the light hitting its surface at different locations at specific angles into a coherent image.

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There's a LOT of photons. Like, A LOT. Even for a far far faraway galaxy, if you wait long enough, you'll get uncountable billions hitting your eye/camera. But you have to wait for a while, hence the "long-exposure" setting on cameras. But all those photons, taken together, will give you a a good picture. Sure, each one of them will only describe the point from which it emanated, but since there's A LOT of them, and since they are distributed randomly, you get the full picture. In time.

For something as big and as bright as the sun though you get a gazillion photons hitting just your eyeball every second. So you don't need to wait very long to get a good picture. Did I say there was a LOT of photons?

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