Libration
What is libration?
Libration is the Moon’s apparent wobble as we watch it from Earth. It lets us see a little around the edges, so over time we can view about 59% of its surface.
Try the question
How can a locked Moon appear to wobble?
Move through the slightly elliptical orbit. Compare the steady spin angle with the changing orbital speed.
Watch for: Near perigee the Moon moves faster but its spin does not speed up to match.
A peek, not a roll
If the orbit were a perfect circle and we sat at the centre, tidal locking would show us exactly half. The orbit is a little stretched, and we are a little off to the side on a turning Earth. Those geometric cheats are libration.
Get more on this
Three small cheats
Longitudinal: the Moon speeds up near perigee while its spin stays even. Latitudinal: the Moon’s equator is tipped a few degrees, so we see a little past the poles. Diurnal: we watch from the sides of a spinning Earth, not from its centre.
See it live
On the hub: Sun–Earth–Moon simulator · Moon