🌙 Why the Moon Has Phases

Sometimes the Moon is a thin crescent, and sometimes a big round full moon. Does the Moon really change shape? Let’s find out together!

The Moon makes no light of its own. It reflects the light it gets from the Sun. The moonlight we see at night is really sunlight.

The Sun always lights up half of the Moon: the half that faces the Sun. This never changes, even as the Moon orbits the Earth.

On the left, the view from space. On the right, the view from Earth. The blue line is the half of the Moon that faces us. As the Moon orbits, we see more or less of the lit half. The Moon’s shape never changes, we just see it from different angles.

First the Moon waxes, which means it grows: New Moon, Crescent, First Quarter, Gibbous, Full Moon. At New Moon the lit half faces away from us, so we can’t see the Moon. From the Northern Hemisphere, including the UK, the right-hand side of a waxing Moon is lit.

Then the Moon wanes, which means it shrinks: Full Moon, Gibbous, Last Quarter, Crescent and back to New Moon. The left-hand side of a waning Moon is lit.

One full cycle takes about 29.5 days, roughly one month. There is about 1 week between New Moon, First Quarter, Full Moon and Last Quarter.

A very common mix-up: “Earth’s shadow makes the phases.” Wrong! At First Quarter the Moon is nowhere near the shadow, yet half of it looks dark. Earth’s shadow only falls on the Moon during a lunar eclipse.

Summary: the Moon makes no light of its own, and the Sun always lights half of it. As the Moon orbits, we see that half from different angles. One cycle takes about one month. Now try it in Moon Observatory!