The moon orbits the Earth in a little under one month (unless it's February). Because its lit face is always pointed directly at the Sun but not Earth, the Moon displays various phases as seen from Earth, going from new (completely black) through full (completely lit) and back through new.
Unfortunately, the lunar phases do not repeat exactly from month-to-month. Additionally, the moon looks as if it wobbles back and forth and up and down. The wobbling back and forth (East and West) is known as "libration." The wobbling up and down (North and South) is known as "nutation." With libration and nutation, we can actually see up to about 60% of the lunar surface from Earth, not 50%. The reason this is unfortunate for my purposes is that it means that I can't substitute images from one month into a montage from a previous month. But, it is why if you watched the movie, the moon wobbles.
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