TimeAndSpace.Science

Moon phases

Why does the Moon change shape?

The Moon does not change shape. Sunlight lights half of it. As the Moon goes around Earth, we see different amounts of that bright half. These changing views are called phases.

Try the question

Why does the Moon change shape?

Move the Moon around Earth. Watch the Moon itself stay half lit while the phase seen from Earth changes.

Moon’s angle from the Sun 90°

What changed First quarter: 50% of the Earth-facing disc is lit, although half of the Moon is still in sunlight.

Watch for: The dark part is lunar night, not Earth’s shadow.

Open this experiment in the full Moon lab

The Sun is off to the left. The Moon is a ball, half bright, half dark. Walk around it and the bright part points a different way relative to you. That is a phase.

What to look for

Move the Moon around Earth in the model. Compare the Moon in space with the view from Earth. When do you see the most light?

At new moon, the bright side faces mostly away from Earth. At full moon, it faces us. The shapes in between include crescents and half moons. Learn the names of the phases.

Questions answered

Is Earth’s shadow covering the dark part? Usually, no. The dark part is the Moon’s own night side. Earth’s shadow causes a lunar eclipse only when the three bodies line up.

Get more on this

Why does the cycle take about a month?

A full cycle of phases takes about 29.5 days. One orbit compared with the stars takes about 27.3 days. Earth moves around the Sun too, so the Moon needs extra time to reach the same lighting angle. Compare the two kinds of month.

See it live

Change the Moon’s angle and make every phase

On the hub: Moon · Sun–Earth–Moon simulator · Glossary