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Libration

What is libration?

Libration is a small wobble in the Moon’s facing. We still have one near side, but over time we see about 59% of the surface, not 50%. The extra is a peek around the edges.

A locked Moon on an ellipse seems to nod: the spin is steady, the orbital speed is not, so we look slightly around the east or west limb.

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

Related questions

What is tidal locking? The Moon keeps the same face toward Earth because it turns once in the same time it goes around us. That match is tidal locking — not a dark side that never sees the Sun.

What is a synodic month? A synodic month is the time from one full moon to the next, about 29.5 days. It is longer than the Moon’s trip around Earth relative to the stars, because Earth has moved and the Moon must catch up to the same angle with the Sun.

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