What causes the phases of the moon?

You’ve probably noticed that the moon seems to change shape over time. This is because the sun only lights up one side of the moon, the other side is dark. As the moon orbits the Earth different parts of it are lit, which is why it seems to change shape. These different shapes are known as phases of the moon.

The moon itself doesn’t emit any light, what we’re actually seeing is sunlight reflected off the moon. When we see the whole of the moon, this is called a full moon, when none of the moon is lit up this is a new moon. As we start to see more of the moon after a new moon, this is called waxing. As we see less of the moon after a full moon, we say the moon is waning.

The moon takes 27.32 days to orbit the Earth and in that time we see all the phases of the moon. New Moon to New Moon is 29.5 days.

Phases of the Moon shown with the Earth in the middle — Phases of the Moon Diagram

Phases of the Moon Activity Ideas

Chalk Academy has some lovely moon phase water colour paintings.

A popular way to demonstrate moon phases is with oreos.

This moon phase learning toy from Happy Tots Shelf is just genius.

These moon phase cupcakes were made using chocolate cupcakes and ready to roll icing. The first and third quarter should have been a bit neater, but for a first try they worked pretty well!

What is the lunar calendar?

The lunar calendar uses the phases of the moon to measure time, whereas the solar calendar uses the orbit of the Earth around the sun. A lunar month is shorter than a solar month and so the lunar calendar doesn’t fit our traditional year, leaving us 12 days short.

Why do you never see the far side of the Moon?

Only one side of the Moon can be seen from Earth as the Moon rotates at the same rate that it orbits the Earth, this is called tidal locking.

Activities for learning about the moon

One fun way to really learn about the phases of the moon is to keep a moon phase log over a few weeks and watch how the shape of the moon changes.

Make a note of the date, time and sky condition and sketch how the moon looks each day. Try to make your observations at a similar time.