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We can tell the time of the lunar month from the Moon's position in the sky.

By comparing the rise and set times of the Sun and Moon, we know how far the Moon has advanced on its monthly circle around the Earth.

Time-lapse film showing the Earth from above the North Pole, rotating through its daily cycle with a 24-hour clockface attached to it. The light from the Sun is shown to arrive in roughly parallel rays from one direction. At any time half of the Earth is illuminated (day) and half is in shadow (night). The Moon is also shown, orbiting slowly around the Earth.


In the new phase the Moon lies near to the Sun in the sky. Two weeks later, in the full phase, it lies on the other side of the sky.

Check your understanding by working through Example #2 now.