the phases of the Moon

Sometimes we see the whole Moon lit up (full Moon). Sometimes only half of it is lit (half Moon). Sometimes it's only a thin, curved shape (crescent Moon) or even not there at all (new Moon). What's going on? One thing is for sure, the Moon itself doesn't change shape!

The bright shape of the Moon we can see is just that part of the Moon which is facing Earth and which at the same time is facing the Sun. You can see what we mean in the diagram below. Put your cursor over the 8 Moon phases and get a 'space eye' view of the Sun, Earth and Moon, and an 'Earth eye' view of the Moon.

Only the part of the Moon which is facing the Sun has sunlight falling on it so the rest of the Moon is in shadow and appears dark to us.

The Moon phase we see just happens to be that part of the sunlit Moon which is facing Earth.

One of the magical things about the Moon is that the time it takes to spin on its axis is exactly the same as the time it takes to orbit the Earth. In theory, therefore, on Earth we should always see the same half, and only half, of the Moon. Because Nature is not always that simple, there are three reasons why, over time, we can see more than half of the Moon’s surface.

These effects are called librations and you can see them in this movie, which was created by taking an image of the Moon every day for a month and putting the images in a sequence. This is a great way to see not only the phases of the Moon, but also the wobbles (librations) it suffers during a month when viewed from Earth.