From the April 2008 issue

Does the Moon wobble?

Patricia Willis, Columbus, Ohio
By | Published: April 1, 2008
Full Moon mosaic
Yes. The direction of the Moon’s axis of rotation moves with respect to its north-south axis and with respect to the stars.

If you were on the Moon tracing out the direction of the axis of rotation on the surface near its north pole, you would see an elliptical path of about 0.8 by 1.3 miles (1.3 by 2.1 kilometers) plus much smaller variations. Each cycle of the large elliptical motion takes 6 years.

Other motions also describe the Moon’s three-dimensional orientation. The Moon rotates once every 27.3 days on average, but this rotation rate is not quite uniform. The lunar equator tilts 1.54° to the ecliptic plane and precesses with an 18.6-year period. But again, there are small additional variations in the lunar equator’s orientation. Collectively, these small, non-uniform parts of the rotation and equator orientation are called physical librations.

Because the Moon is not perfectly spherical, Earth’s gravitational pull on the Moon’s lumpy figure twists it. These pulls cause the physical libration variations. — James Williams, Stardust Science Team, NASA’s Jet Propulsion Laboratory, California Institute of Technology, Pasadena