The Moon travels around (orbits) the Earth. It does not make its own light. Like a mirror, it reflects light from the Sun. At any time, the half of the Moon facing the Sun is lit up and the other half is dark.

As the Moon orbits the Earth, we see different amounts of its lit half. This makes the Moon seem to change shape. These shapes are called the phases of the Moon. One full cycle of phases takes about one month (about 29 and a half days).

The phases, in order, are: new moon (the lit side faces away from us, so we cannot see the Moon), waxing crescent, first quarter (half of the disc looks lit), waxing gibbous, full moon (the whole disc is lit), waning gibbous, third quarter (also called last quarter) and waning crescent. Then it is new moon again.

Waxing means the lit part we see is growing. Waning means it is shrinking. Gibbous means more than half of the disc is lit. The Moon always keeps the same side facing the Earth.

A common mistake is thinking the Earth's shadow causes the phases. It does not. The Earth's shadow only falls on the Moon during a lunar eclipse, which does not happen every month.