Libration
The Moon's slight rocking, which lets us see 59% of it rather than half.
The Moon keeps one face turned toward us, but not perfectly. Its orbit is elliptical so its speed varies while its rotation does not, letting us peer a little round each limb in turn; its axis is tilted, letting us see over each pole; and the Earth's own size gives observers at different latitudes slightly different views. Over a month those add up to about nine degrees of rocking in each direction, and 59% of the surface becomes visible from Earth at some point.
Related terms
Questions
What is Libration?
The Moon's slight rocking, which lets us see 59% of it rather than half.
Why does it matter?
Its orbit is elliptical so its speed varies while its rotation does not, letting us peer a little round each limb in turn; its axis is tilted, letting us see over each pole; and the Earth's own size gives observers at different latitudes slightly different views. Over a month those add up to about nine degrees of rocking in each direction, and 59% of the surface becomes visible from Earth at some point.