On a boat in the ocean at the equator (earth rotating 28km) watching the sun dip below the horizon (4.7k away) do we see any photons that were emitted directly from the sun (8 minutes ago) or are they all reflected off of something? My Gedankenerfahrung
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$\begingroup$ Can you explain why you think we would or wouldn't see photons directly from the sun? $\endgroup$– user234190Commented Jul 8, 2019 at 21:17
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$\begingroup$ I'm thinking about the photons that left the sun when it was above the horizon but now we have rotated and it is below the horizon. Since it takes 8 min to get here are there any photons in that "eight minute stream" that reach our eyes before the sun disappears below the horizon (i.e. from the "stream" while sun is not currently visible) $\endgroup$– Parish ElliottCommented Jul 8, 2019 at 21:25
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$\begingroup$ Maybe you can reword the question so it fits the answer you chose. If the sun is not currently visible, then you aren't seeing any photons from atmospheric refraction. $\endgroup$– user234190Commented Jul 9, 2019 at 21:49
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$\begingroup$ Remember, it is the earth that rotates and not the sun that orbits the earth. So when we see the sun above the horizon then that is where the sun is located at that moment even though the light that we see left the sun 8 mins ago. $\endgroup$– flippiefanusCommented Jul 10, 2019 at 4:12
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$\begingroup$ "when we see the sun above the horizon then that is where the sun is located at that moment" I believe that's where it WAS minutes ago. $\endgroup$– Parish ElliottCommented Jul 10, 2019 at 18:00
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2 Answers
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We see the Sun with a time delay of a little over 8 minutes (about 490 to 507 seconds). So the location that the Sun appears to be now is actually where it was 8 minutes ago. In a comment you said:
I'm thinking about the photons that left the sun when it was above the horizon but now we have rotated and it is below the horizon. Since it takes 8 min to get here are there any photons in that "eight minute stream" that reach our eyes before the sun disappears below the horizon (i.e. from the "stream" while sun is not currently visible)
You see all the photons in that "eight minute stream" (assuming that stream consists of photons heading in the direction of your eye). Those photons continue to travel in a straight line towards your eye, the rotation of the Earth won't affect that.
However, all of the above assumes that light always travels in a straight line. But that's not actually true: the path of light can refract (bend) when it changes speed. The speed of light in air is lower than the speed of light in a vacuum. The exact speed of light in air depends on the air's temperature, pressure, and the amount of water vapour in the air.
The light from a celestial body near the horizon passes through a lot more air than a celestial body that's high in the sky, so refraction effects are greatest near the horizon. Atmospheric refraction causes the celestial body to appear higher in the sky than it would appear without air. Coincidentally, the amount of refraction at the horizon is roughly equal to the Sun's apparent diameter. So if you can see the lower limb of the Sun just touching the horizon, if all the air disappeared at that instant the Sun's upper limb would be on the horizon, i.e., the Sun would be on the point of setting.
For more details on astronomical refraction, please see the Wikipedia article on atmospheric refraction.
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$\begingroup$ This is interesting but it still doesn't answer the question, which is based on what we see, basically saying, 'do we see any photons directly from the sun when we can no longer see them?'. The question could be reworded so this stands as an interesting question and answer, but as of now, it's not so clear $\endgroup$– user234190Commented Jul 9, 2019 at 21:46
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$\begingroup$ @user47014 Sorry, I don't get what you're trying to say. If we neglect refraction, you see the Sun where it was 8 minutes ago, so the photons that you see for the last 8 minutes before sunset were emitted after the instantaneous line from your eye to the Sun intersected the Earth's surface. $\endgroup$– PM 2RingCommented Jul 10, 2019 at 5:11
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$\begingroup$ Yes I see what you are saying, But what is said is, '(i.e. from the "stream" while sun is not currently visible) ' so if the sun isn't visible, then you aren't seeing anything from refraction, because what you described is still being able to see the sun even though you wouldn't without refraction. So maybe the question could be worded as, 'Are any of the photons we see during a sunset actually coming from behind the earth?' or something like that. If you aren't seeing any sunlight, then refraction isn't strong enough at that point. $\endgroup$– user234190Commented Jul 10, 2019 at 5:19
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$\begingroup$ @user47014 No, the Sun is still currently visible. The question says: "watching the sun dip below the horizon" $\endgroup$– PM 2RingCommented Jul 10, 2019 at 5:24
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$\begingroup$ Yes the question asks if the photons we see from the sun are from 8 minutes ago, to which the answer is always yes to the photons that we see. Then the comment makes it unclear, as with what I quoted. 'Watching the sun dip below the horizon' might mean only when you can see the actual sun, but for some that might include the time when the sky is red but you don't see the sun? Anyways, to really answer the direct question you could say that refraction is different than reflection so if you can see the sun, it's directly from the sun, not from reflection. $\endgroup$– user234190Commented Jul 10, 2019 at 5:36
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If you can see the sun, you get photons direct and also reflected or scattered photons. There is an infinitesimal delay caused by the fact that light travels more slowly through air than it does in a vacuum, but the delay is so minute that it is hardly worth mentioning. Nobody except for astronauts sees the sun without some atmosphere getting in the way, and it's a good thing it does. At sunset and sunrise, you see it through a maximum thickness of atmosphere. After the sun has gone below the horizon you will only see reflected or scattered photons.
