# Phase

Also called: lunar phase, waxing, waning, crescent, gibbous, first quarter, last quarter.

How much of the Moon's (or a planet's) sunlit side we can see: crescent, quarter, gibbous or full.

The Moon is always half-lit by the Sun; its phase is how much of that half faces us. From new Moon it waxes through crescent and first quarter (half-lit) to gibbous and full, then wanes back. A full cycle takes 29.5 days.

Venus and Mercury show phases too, because they orbit inside Earth's orbit — Galileo's observation of Venus's phases helped prove that the planets go round the Sun.

**At the eyepiece:** The best lunar views are not at full Moon, when shadows vanish, but a few days either side of the quarters.

**In AstroPilot:** AstroPilot draws the Moon's phase on Tonight, the sky chart and the Moon's page; the planets' pages show Mercury's, Venus's and Mars's.

**Examples:** [moon](https://astropilot.space/object/moon.md), [venus](https://astropilot.space/object/venus.md), [mercury](https://astropilot.space/object/mercury.md)

**Related:** [The Moon](https://astropilot.space/glossary/the-moon.md), [Terminator](https://astropilot.space/glossary/terminator.md), [Elongation](https://astropilot.space/glossary/elongation.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/phase