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It appears that the face of the moon "rotates" clockwise from the time the moon rises to the time it sets. Tycho's crater seems more to the left and the "Rabbits head" looks like it's facing down more.

I know that is not actually the case but I am having trouble visualizing why this appears so. Can anyone explain?

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'I know this is not actually the case'... Actually, it is the case. See my answer. – Jeremy Aug 18 '14 at 10:39
up vote 5 down vote accepted

Actually, it is rotating relative to your perspective. (The other answer relating to liberation occurs over a longer period of time).

Here is a simulation from Sky Safari for later tonight (moonrise) and tomorrow (moonset) for my location. Note how the angle of the lit hemisphere rotates between the pictures:





It looks like this for me because I am at 43 south. If I was at the equator, the moon would appear to have rotated 180 as it travelled from horizon to horizon.

It is easier to think about on the equator. Say the moon rises, and you consider the bit at the top to be "up". It continues on up, right over your head, and down to the horizon... But on the way down, the "up" side of the moon is now looking down. It was "up" until the zenith, but then it is "down" all the way to the horizon. It doesn't rotate at all, but the result is a 180 degree flip in appearance from horizon to horizon.

Or consider the perspective at a pole, while the moon is up. The moon travels right around the sky, always facing the same direction, with no apparent rotation (and while it is up, no moonrise or moonset, it is up all day).

Latitudes in between see an in-between effect; some apparent rotation of the moon between moonrise and moonset.

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The equator example makes perfect sense to me now. Very helpful. I am at 40.7N I assume there is some formula matching the apparent rotation to latitude. Thank you – Itumac Aug 18 '14 at 14:07

This effect is called libration - and it is an oscillating motion of orbiting bodies relative to each other (from Wikipedia)

It is down to three factors:

  • The Moon's orbit is not a circle, so it moves ahead and behind where it would be on a true circular orbit
  • Where you stand on the Earth moves during the day relative to the Moon
  • The Moon's orbit is inclined so it moves up and down relative to your viewpoint

A wonderful animation of the effect from that Wikipedia page, based on photographs from the Earth:

enter image description here

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This answers a different issue, the changing orientation of the moon over the period of a lunar month. The question is about the change noticeable between moonrise and moonset, and libration is not really noticeable over that period. – Jeremy Aug 18 '14 at 10:17

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