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I'm trying to solve this task:

During a solar eclipse, an observer noticed that the diminishment of the Sun's disk began directly from below. Where and when could this be?

Concerning "where" question I think it could occur at the equator or somewhere near because only there an observer could see the stars moving "vertically":

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However I have a problem understanding when that could happen. Could somebody help please?

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    $\begingroup$ Yes I think you are basically on the right track. The Moon isn't exactly on the ecliptic with the Sun but it's close, during a solar eclipse the Moon first covers "west" side of the Sun. We know that because after a new Moon, every day the Moon is farther East by about 15 degrees so it stays visible roughly another hour longer after sunset. So you want the west side of the Sun to appear below the center of the Sun. That certainly won't happen at the poles. As you wrote, it will "occur at the equator or somewhere near" where the Sun rises/sets vertically; ecliptic is vertical at the horizon. $\endgroup$
    – uhoh
    Aug 14, 2021 at 21:58
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    $\begingroup$ As to when, you want the west side of the Sun on the bottom, so it won't be at noon! $\endgroup$
    – uhoh
    Aug 14, 2021 at 21:59

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If you regularly observe the moon, you know that it rises later each day (by about 50 minutes) due to the fact that it orbits earth in the same direction as earth's rotation (so earth has to rotate a bit further for the moon to appear in the same position). That is, the moon is moving slower across the sky than the sun. If the eclipse observer sees the moon eclipsing the sun from below, this means therefore it could only be shortly before sunset and at an observing position near the equator.

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