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As the Moon orbits Earth, it rotates at such a rate as to keep the same face aiming our way… but not exactly the same face, as shown in this excellent video from NASA’s Goddard Space Flight Center (lovingly annotated by the Bad Astronomer himself, Dr. Phil Plait.)
The Moon has a slight wobble to its axial rotation, and over the course of a month its orientation shifts slightly — an effect called libration. Think of it like a top or gyroscope spinning on a table; it doesn’t spin perfectly vertically, but rather sways a bit while it spins. Libration is that sway.
In addition to that movement, the Moon also moves closer to and further from the Earth over the course of a year due to its elliptical orbit. This makes it appear to change size slightly.
Except for the Moon’s phases, such effects aren’t immediately obvious from one night to the next. But when assembled into a high-resolution video using images and laser altimetry data maps from the Lunar Reconnaissance Orbiter, the monthly motions of the Moon become incredibly clear!
This video shows all the views of the Moon for the entire year of 2012.
Thanks to Phil Plait of Discover Magazine’s Bad Astronomy blog for adding the music and descriptions to the GSFC’s amazing video. What a marvelous night for a Moon dance!
See the current Moon phase and the original video on the Goddard Space Flight Center’s “Dial-A-Moon” page here.
Video: NASA/Goddard Space Flight Center Visualization Studio. Notations by Phil Plait. Music by Kevin MacLeod/incomptech.com.