Despite tidal locking, we don't see exactly half the Moon over time. We see about 59%. Those extra 9% appear because of small oscillations called libration.
Despite tidal locking, we don't see exactly half the Moon over time. We see about 59%. Those extra 9% appear because of small oscillations called libration.
Libration in longitude. The Moon's orbit isn't circular, it's an ellipse. It moves faster at perigee (closer to Earth) and slower at apogee (farther). Its rotation, however, is constant. That mismatch makes it "look" a bit east and a bit west across the cycle. This effect alone exposes about 7.9° beyond the average face.
Libration in latitude. The Moon's rotation axis is tilted about 6.7° relative to the orbital plane. So during part of the cycle we see a bit more of the lunar north pole, and during another part a bit more of the south pole. Adds about 6.7° of exposure.
Diurnal libration. The smallest of the three. As Earth rotates, observers at different latitudes see the Moon from slightly different angles. The parallax difference between moonrise and moonset for the same observer is about 1°.
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