Moon Essentials: Libration in Latitude

Libration in latitude is an apparent north-south, up and down rotation of the Moon, as if the Moon were slowly nodding its head "yes." This motion slightly changes the Moon's face as seen from Earth, revealing parts of the north and south poles. This repeats in a cycle lasting about 27.32 days, the average time it takes the Moon to complete an orbit relative to the stars (the sidereal month).

The visualization on this page shows how libration in latitude is mostly an effect of the tilt of the Moon's orbit relative to its equator. The angle between these two planes is 6.68°. In the first half of the animation, the Moon's orbit is tilted while its equator is nearly horizontal, supporting the notion that we are slightly above or below the Moon. But this is a matter of perspective. In the second half of the animation, the orbit is horizontal while the Moon's equator is tilted, giving the impression that the Moon's orientation and not our relative position is responsible for the polar reveals.

Libration in latitude is defined by an angle, the latitude component of the sub-Earth point. This is the lunar latitude at which the Earth is directly overhead, or the latitude line passing through the center of the Moon's disk as seen from Earth. This point is marked with a blue dot on the lunar globe in the lower half of the animation.