The 8 phases of the Moon explained

The Moon goes through eight phases in a cycle of about 29.5 days. Here we explain what each one is, in what order they occur and why the Moon appears to change shape night after night.

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✦ What are the phases of the Moon?

The Moon does not shine on its own: it reflects the Sun's light. As it orbits the Earth, the angle between the Sun, our planet and the Moon changes, and from here we see a different portion of its face lit up. That visible portion is what we call the lunar phase. The full cycle is called the synodic month and lasts about 29.5 days.

✦ The 8 phases, one by one

✦ Why does the Moon change shape?

Imagine a ball lit by a lamp: it always has one half lit and the other in shadow. What changes is the angle we look at it from. When the Moon is between us and the Sun, we see its dark side (new moon); when it is on the opposite side, we see its fully lit face (full moon). In between, we see intermediate portions: the quarters and the gibbous and crescent phases.

✦ How long does each phase last?

The four main phases (new, first quarter, full and last quarter) are specific moments, but about 7.4 days pass between them. The intermediate phases —crescent, gibbous— describe the days of transition. The full moon "looks full" for 2 or 3 nights, though the exact moment of fullness lasts only an instant.

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✦ Frequently asked questions