Why does the Moon
have phases?

Half the Moon is always lit by the Sun. The phases are just our changing view of that lit half as the Moon orbits Earth every 29.5 days — not Earth’s shadow.

Here is the single fact that unlocks it: half of the Moon is always lit by the Sun — just like half of Earth is always in daylight. The Moon has no phases of its own; what changes is how much of that sunlit half happens to face us.

As the Moon orbits Earth once every 29.5 days, our viewing angle sweeps all the way around it. When the Moon sits between Earth and the Sun, its lit side faces away from us and we see nothing — a new moon. A week later we view it side-on and see half of the lit half: first quarter. When Earth is between the Sun and the Moon, the entire lit face points at us — a full moon — and then the cycle unwinds through last quarter back to new.

The most common misconception is that phases are Earth’s shadow falling on the Moon. They aren’t — Earth’s shadow only touches the Moon during a lunar eclipse, a few times a year. The crescent’s dark portion is simply the Moon’s own night side.

A good way to convince yourself: notice that the crescent moon always points toward the Sun. The lit edge is the edge facing the light.

This is one of seven interactive lessons in Moon Academy inside the Moon Explorer app — there, you don’t read the answer, you drag the Moon around its orbit and watch it happen.

← All seven questions

Dates and times are in Universal Time (UT). The free Moon Explorer app for Android converts everything to your local time and your exact location — fully offline, with no ads or accounts.