Why Diurnal Motion Happens

The diurnal (daily) motion of the stars, the Sun, and the Moon is a consequence of the Earth's rotation. This animation shows the Earth as seen from a vantage point above the North Pole. It is day on the side of the Earth that is illuminated by the Sun, and night on the dark, unilluminated side.

At 8:00 P.M. (local time) a person in California sees the constellation Cygnus directly overhead. Four hours later, the Earth has rotated to the east one-sixth of a complete rotation; from the perspective of a person on Earth, the entire sky appears to have rotated to the west one-sixth of a complete rotation. It is now midnight in California, and the constellation directly over California is Andromeda.