Why Does the Moon Have Phases? | The Schools' Observatory

The Moon is the brightest object in the night sky when its whole face is showing. This only happens at full Moon. So why isn't it always a full Moon?

The Moon is constantly orbiting the Earth. We only see the part of the Moon that is lit up by the Sun. During its orbit, different amounts of the lit-up side of the Moon are visible from Earth. This makes it look like the Moon is changing shape. We call these changes the phases of the Moon.

It takes 29.5 days to go from one full Moon to the next. We call this a lunar month or a lunar phase cycle.

Approx Duration

< 15 mins

Watch the animation on this page to see how the orbit of the Moon around the Earth affects how much of it we see.

The box on the left hand side shows the Moon orbiting the Earth.
The box at the top-right shows how we see the Moon in the sky.
The box at the bottom-right shows the date changing with time.

Top tip!

You can click on a date in the calendar to skip to that part of the lunar cycle.

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