Here's how it works:
1. The Moon orbits the Earth. As it does, the angle at which sunlight hits the Moon changes from our perspective on Earth.
2. We only see the illuminated part. We see the part of the Moon that is reflecting sunlight directly towards us.
3. The phases we see. The changing angle of sunlight creates the different shapes we see:
* New Moon: The Moon is between the Sun and Earth, so we see no illuminated side.
* Waxing Crescent: A sliver of the Moon becomes visible.
* First Quarter: Half of the Moon is illuminated.
* Waxing Gibbous: More than half of the Moon is illuminated.
* Full Moon: The entire side of the Moon facing Earth is illuminated.
* Waning Gibbous: Less than half of the Moon is illuminated.
* Last Quarter: Half of the Moon is illuminated again, but the other half than the first quarter.
* Waning Crescent: A sliver of the Moon is visible before it disappears again.
It takes about 29.5 days for the Moon to complete a full cycle of phases.
