Phases of the Moon

Lunar cycle
Moon Phases Diagram
Credit: with permission.

The Moon is the brightest thing in the night sky when its whole face is showing, but this only happens at full Moon. Why do we see different phases of the Moon?

The Moon makes a complete orbit around the Earth every 27.3 days. We can only see the part of the Moon that is lit up by the Sun. Because the Sun shines on the Moon from different angles during this time, it appears that it is changing shape. We call these changes the phases of the Moon. 

The Earth-Moon-Sun diagram on the left gives a clearer picture of the different lunar phases we can see.

A lunar month takes around 29.5 days. This is the time it takes for the Moon to get back to exactly the same phase. It is a little longer than the time it takes the Moon to orbit the Earth because the Earth is also moving around the Sun at the same time.

On the animation above you can see the Earth and Moon as seen from above, to the left, where the light from the Sun is coming from the right. On the right panel you can see what the Moon would look like from the surface of the Earth, and a "calendar" showing the days go by. To go to a particular time, move the mouse pointer over the day you want to see.