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As the Moon moves around the Earth, we see different fractions of its illuminated side.

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.


Try your hand at Example #3 now. Remember that it takes the Moon a full month to orbit once around the Earth, and work through Example #4.