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The phases of the Moon

The Moon goes round the Earth once every 28 days. It also takes 28 days to spin once on its axis and so we always see the same side of the Moon from the Earth. However the Moon rocks a little on its axis and so the amount of it that is lit up by the Sun changes from night to night.

In the diagram the inside circle shows that only one half of the Moon is lit up by the Sun. The outer one shows how the Moon would appear in those places when viewed from Earth. The diagram is not to scale.

You should see from the diagram that a full Moon is always seen in the opposite part of the sky from the Sun. In other words if a full Moon and the Sun are visible at the same time one will be rising in the east and the other about to set in the west.

schoolphysics Phases of the Moon animation animation

To see an animation of the phases of the Moon click on the animation link.

© Keith Gibbs 2011