A question that popped into my head: if I see a picture of the sun close to the horizon, in an unknown place, can I know if it was taken at sunset or sunrise?
Do sunrises and sunsets look the same in a still image? Can one tell them apart?
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3$\begingroup$ Previously: photo.stackexchange.com/questions/26776/… $\endgroup$– user3067860Nov 9, 2021 at 21:39
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3 Answers
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If you have a sufficiently advanced camera, then you can distinguish a sunrise from a sunset from a still frame. I will assume that the Sun and the horizon are visible.
The Sun is rotating at roughly 2 km/s at the equator.
This rotation imparts Doppler shifts in the light from the Sun, even in a still frame. So with a sufficiently advanced camera which can detect those Doppler shifts (for example, an IFU).
Then you can measure the rotation axis of the Sun. Now, depending on where on Earth you are (latitude and whether this is sunrise or sunset) the rotation axis of the Sun will appear at a different angle relative to the horizon.
So, not only could you theoretically you figure out whether it is sunrise or sunset, but you could also measure the rotation speed of the Sun and your present latitude.
The Sun and the Earth rotate in the same direction (counterclockwise when viewed from above the North Pole). Thus, the top of the Sun relative to the horizon will rotate in the same direction as the observer.
I will use this fact to describe what you would see due to the Sun's rotation:
During sunset, you must be on the side of the Earth moving away from the Sun. From this angle, the top of the Sun (relative to the horizon, not necessarily North!) will appear blueshifted relative to the bottom of the Sun, since the apparent top of the Sun has a velocity towards Earth (same direction as your side of the Earth).
During sunrise, you must be on the side of the Earth moving towards the Sun. From this angle, the top of the Sun will appear redshifted instead.
Side note: I think in a statistical sense you could distinguish between sunrises and sunsets based on effects mentioned in other answers/comments: temperature of the air, stillness of the air, presence of particulates. Statistical meaning you could theoretically do better than a 50/50 guess with a single image, and better if you allow multiple images taken at the same place or with other variables controlled for. So although those answers/comments do not provide a sure way to distinguish sunrise from sunset, I think they suffice to show that the two phenomena are different on some level.
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1$\begingroup$ It's a good answer (+1), but maybe should have stopped after the 5th paragraph, the word 'latitude'. After that it seems flawed as the top and bottom of the sun would both appear to be travelling the same relative to the earth. $\endgroup$ Nov 8, 2021 at 8:45
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1$\begingroup$ Are you sure about this, the rotation of the earth would cause equal doppler shifts to all parts of the Sun, true the Sun's rotation matters, but that's a version of the first part of the answer... $\endgroup$ Nov 8, 2021 at 9:00
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2$\begingroup$ Rotation of the Sun, not the Earth. The 2nd part of the answer is just to spell out exactly what the effect of the rotation of the Sun would have. When I refer to "top" i don't refer to North, but "top" as viewed relative to the horizon. $\endgroup$– AlwinNov 8, 2021 at 9:08
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7$\begingroup$ Also: don't try this at home, at least not with a normal camera. You'd need the correct filters, e.g. an H-alpha. But it's true, with an H-alpha telescope, you do notice the Doppler effect, and it's not possible to "focus" (by selecting the required wavelength) on prominences on both sides of the sun at the same time. $\endgroup$ Nov 8, 2021 at 11:51
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2$\begingroup$ So it's a frequency shift of 2/300,000 ~ 0.66/100,000 that can be measured? Awesome :-). $\endgroup$ Nov 9, 2021 at 14:40
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Yes, the temperature of the air that the sunlight goes through to reach our eyes would be different.
At sunset the air would be warm, at sunrise it's colder and that causes the light to refract differently.
This website shows more about it
answered Nov 7, 2021 at 13:06
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37$\begingroup$ In what way would they look different (can you summarise)? $\endgroup$– ProfRobNov 7, 2021 at 13:59
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3$\begingroup$ Cool, I didn't think of that, though it does assume that I know where the sun is supposed to be, no? $\endgroup$– AlonMlnNov 7, 2021 at 15:04
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7$\begingroup$ @JohnHunter, Re, "more...particles," The average redness might help you to distinguish a large collection of sunrise photos from a large collection of sunset photos; but the amount of dust and whatever in the air depends on more than just the time of day. Compare photos of sunrises taken in the year after a major volcanic eruption to photos of sunsets taken the year before the eruption, and the sunrises probably will be more red. $\endgroup$ Nov 7, 2021 at 17:11
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16$\begingroup$ While this is interesting, I don't think it's an answer $\endgroup$– AlonMlnNov 7, 2021 at 22:51
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12$\begingroup$ @JohnHunter: "if enough data were available": this data must obvously include the time of day, otherwise you wouldn't know where the sun is supposed to be. But then if you know the time of day... $\endgroup$– TonyKNov 7, 2021 at 22:53
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I did not perform any calculations, but if the exposure is long enough, one can see a tiny fragment of sun trajectory. In the northern hemisphere the sun moves from left to right, so the fragments of trajectory will look different at sunrise (left bottom to right top) and sunset (left top to right bottom).
EDIT (11/7/2021): Let me add some calculations. The angular size of the Sun is 0.5 degrees. The visible angular velocity of the Sun is 360 degrees per 24 hours, so the Sun moves by 1/10 of its angular size in 12 seconds. So the direction of the Sun's trajectory can be visible in photos at comparable exposure.
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7$\begingroup$ While this is technically true it does feel a bit like cheating. The question asked for a still image and you are argueing that a real life photograph is not actually a still life but rather an exposition of a small time segment. $\endgroup$– quaragueNov 8, 2021 at 7:27
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19$\begingroup$ @quarague: there is no still image without finite exposure. $\endgroup$ Nov 8, 2021 at 8:22
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9$\begingroup$ Your edit date is 4 months ago ;-) (Please consider using ISO-8601) $\endgroup$– gerritNov 8, 2021 at 13:37
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6$\begingroup$ But really, the very idea that 10% of the world's population simply don't count is, frankly, offensive. $\endgroup$– TonyKNov 8, 2021 at 19:38
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5$\begingroup$ @akhmeteli it's not the European format, it's the basically-everywhere-except-the-US format (if you count YMD and DMY as the same). And it's not distinguished by slashes, I live in Canada where both are unfortunately used and have to disambiguate by using the 3-letter month. You are of course perfectly entitled to use that format as an American resident, but it is ambiguous to an international audience $\endgroup$– llamaNov 9, 2021 at 19:12