### libration

(oscillating motion of orbiting bodies relative to each other)

Libration is relative oscillating motion between two orbiting bodies. A common use of the term is in describing the Moon's rocking motion as viewed from Earth. It is also used for similar situations with other orbiting bodies and disks (e.g., circumstellar disks) and other situations where there is such an oscillation.

The Moon, as viewed from a point on Earth shows four kinds of libration:

• Libration in longitude - the Moon's apparent slight oscillation from side to side due to the Moon rotating so as to keep single face toward the Earth, but due to the orbit's eccentricity, sometimes during the month, the rotation is slightly ahead or behind this position.
• Libration in latitude - the Moon's axis is slightly different than the axis of its orbit around Earth, so sometimes during the month its north or south pole is slightly visible.
• Diurnal libration - as the Earth rotates, a person on the Earth is effectively moving in relation to the Moon, seeing the Moon's face from slightly different angles as the day progresses, so the Moon appears to wobble.
• Physical libration or forced libration - this is an actual motion of the Moon and is small: the Moon's mass's slight asymmetry, with gravity, determine which part faces the Earth, and the Moon can swing back and forth like a pendulum through that direction, with very little dampening. Generally it happens north-to-south, which is the case for the Moon.

A study of Enceladus's physical libration has claimed evidence of a disconnection between its surface and much of its mass, implying a global ocean under its ice.

Lagrangian points, are also known as libration points. I imagine this is because an object can reside in a small orbit around a stable Lagrangian point, the small orbit constituting an oscillation within its movement around the larger body.

(orbits,kinematics)