How does libration allow us to see more than 50% of the Moon? (Advanced)

I have read recently in a science encyclopedia that from a given point on earth, 58% of the surface of the moon in visible, because the moon rotates about 8% and then reverses? I'm sure I'm confused about something...The term 'libration' was used but not in a way I understand.

The Moon's orbit is not a perfect circle, so sometimes it is closer to the Earth and sometimes it is far away. When the Moon is at its closest point (called perigee), it travels a bit faster in its orbit than when it is at its farthest point (apogee.)

The Moon's rotation, however, is constant. The Moon is a big object with lots of momentum, so it spins at the same stately pace no matter where it is in its orbit.

If the Moon's orbit was circular, then its velocity would be constant. Because its spin period exactly matches its orbital period, the Moon would always keep exactly one face pointed at the Earth, so that we would see only 50% of its surface.

However, in real life, the Moon's velocity varies while its rotation rate is constant. When the Moon is near perigee, it is essentially rotating a bit too slow to keep one face pointed toward the Earth, so we get to see a bit more of its western edge. When the Moon is near apogee, its rotation is a bit too fast, so we see an extra slice of eastern real estate.

Thus, from the point of view of a person on Earth (who's not thinking about noncircular orbits and whatnot) the Moon then seems to rock back and forth slightly, once each orbit. This is called libration.

This page was last updated on July 18, 2015.

About the Author

Britt Scharringhausen

Britt studies the rings of Saturn. She got her PhD from Cornell in 2006 and is now a Professor at Beloit College in Wisconson.

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