> I never knew that in the Southern Hemisphere that the moon appears to look "upside-down" to the Northern Hemisphere point of view. I don't quite understand how that is possible. I understand it a little bit, but not too much. Isn't it because of which direction we are looking at the moon from (Northern Hemisphere looks south at the moon, and Southern Hemisphere looks north at the moon)? If not, then what is the reason?
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It's not quite like that, I think. It might be easier to think of things from the Moon's perspective. Let's make things simpler at first by assuming the Moon lies directly over the Earth's equator. A person standing at the Earth's north pole will look upside-down compared to a person standing at the Earth's south pole. Shifting back into those two people's point of view, the "face" on the moon will be upside-down for one compared to the other's.


But what about the transition? Let's say the person at the north pole walks over toward the south pole, all the while staring at the moon. The rate at which the person's face will seem to rotate, from the Moon's perspective, will depend on which path is taken. Case #1: if the person walks toward the moon at first, then his face won't rotate at all -- for a while. He'll have to gradually lift his head as the Moon rises from the horizon, until it's directly overhead. Then, just as he crosses the equator, the moon will be behind him, and he'll have to turn around to continue staring at the moon. At that point the orientation of the person's face will flip 180 degrees.

So, looking back to the person's perspective, if he walks directly toward the moon on the horizon, the orientation of the moon's face will stay exactly the same until he crosses the equator. Then it flips 180 degrees instantly and remains in that orientation all the way to the south pole.

Case 2: if the person walks along that great circle of the Earth which makes up the "edge" of what can be seen from the Moon, then he'll be walking in a direction perpendicular to the Moon. The Moon will always seem to be at the same point on the horizon (either to his right or his left, depending on which way he started). From the Moon's perspective, the person's face will rotate at a constant rate. 1/4 of the way from the north pole to the south pole, the person's face will be rotated 45 degrees. 1/2 of the way, it will be rotated 90 degrees. 3/4 of the way, 135 degrees. Finally at the end, his face will appear upside-down compared to the original orientation.

Back in the person's point of view, the Moon will seem to stay in a constant place in the sky, but it will rotate constantly as he travels.

These are the two extreme cases. If, in general, a person travels between the two poles on a great circle lying in between cases 1 and 2, the moon will start to rotate slowly at first, then pick up speed, rotating the quickest as the person passes the equator, then slowing down as he approaches the south pole. The closer the course lies to case #1, the slower the moon will seem to rotate at first, and the quicker the rotation while crossing the equator. The closer the course lies to case #2, the more constant the rotation of the moon's face will seem to appear.

What all this means to two people observing the moon from different hemispheres is this: if the moon is close to half-way between rising and setting, the moon will be nearly completely flipped between the two people's perspectives. If the moon is close to rising or close to setting, the rotational difference between the two views of the moon's face will be approximately the difference in latitudinal degrees between the two people (so if one person is at 20 degrees north latitude, and one person is at 20 degrees south latitude, the moon will be rotated approximately 40 degrees between the two).

The moon's not usually going to be directly over the equator, so you'd in general have to make a coordinate change first -- if the moon is 15 degrees north of the equator, shift everything by 15 degrees in the analysis (so the point at which case #1's rapid rotation occurs would be at 15 degrees north latitude, for example).