By how much time the day would have been shorter if the earth had no atmosphere?
Due to the earth's atmosphere, light coming from the sun gets refracted and it appears two minutes before it is actually above the horizon (advanced sunrise). Also, it can be seen about two minutes even after it is actually below the horizon (delayed sunset). Thus, the time from sunrise to sunset is increased by about four minutes because of the refraction of sunlight through the earth's atmosphere.
So, if the earth had no atmosphere, then the length of the day would have been shorter by about four minutes.
Explanation
Due to the refraction of sunlight caused by the atmosphere, we can see the sun 2 minutes before it's actually above the horizon (advanced sunrise), and 2 minutes even after the actual sunset (delayed sunset).
This happens because when the sun is slightly below the horizon, then the sun's light coming from the less dense air to more dense air is refracted downwards as it passes through the atmosphere. Thus, due to this atmospheric refraction, the sun appears to be raised above the horizon when actually it is slightly below the horizon.
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