Libration

A slight apparent rocking of the Moon, caused by its elliptical, tilted orbit. Although it always keeps the same face towards us, over time we can see about 59% of its surface. Features near the edge are best seen when libration tips them our way.

ToolsThe Moon tonight

Related terms

Moon phases: The Moon’s changing look depending on how much of its sunlit side we see: new Moon, crescent, first quarter, gibbous, full Moon, then back again. The full cycle, the lunation, lasts 29.5 days. For relief, prefer the quarters to the full Moon, which looks flat.
Terminator: The line between day and night on the Moon (or a planet). The Sun is low there and shadows are long, so craters and mountains stand out in relief. That is always where to look in a telescope, and it moves from one evening to the next.

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