Understand moon phases with a ball and a lamp

Make moon phases visible

The dark part of a crescent Moon is not Earth's shadow. You are seeing only part of the Moon's sunlit half. A pale, matte ball and an LED lamp let you try this change of viewpoint yourself.

1. Place the lamp in a fixed position, preferably a few steps away, and dim other lights. It represents the Sun.
2. Hold the ball at arm's length, roughly at eye level. It represents the Moon; your eyes are the viewpoint on Earth. Look at the ball, not directly into the bright lamp.
3. Slowly turn on the spot, moving the ball with you. Leave the lamp in place. Watch the visible bright area change into a crescent, a half-circle and an almost full disc.
4. When the lamp is behind you, raise the ball a little so your head does not cast a shadow on it.

Why your view changes

The lamp lights roughly half the ball. Turning does not change that basic rule; it changes how much of the lit half you can see. With the ball to the side of the direction of the lamp, about half its visible disc looks bright: your model of a half Moon.

The real Moon goes through these changes as it orbits Earth. A complete cycle of moon phases takes about 29.5 days.

When Earth's shadow does matter

A lunar eclipse happens when Earth is between the Sun and Moon and its shadow falls on the Moon. This can only happen at full Moon, but not every month: the Moon's orbit is tilted relative to the plane of Earth's orbit.

Your model is not to scale. If your head casts a shadow on the ball, that is not an ordinary moon phase. Keep the ball outside that shadow and use an LED lamp, not a hot, exposed light source.