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.
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Related terms
- 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.
- Earthshine
The faint glow on the dark part of the Moon when it is a thin crescent. It is sunlight reflected by the Earth onto the Moon, then back to us. Leonardo da Vinci had already explained it.
- 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.
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