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We can tell the time of day from the Sun's position in the sky.

As the Earth rotates, the Sun moves from east to west across the sky.

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).