The apparent movement of stars across the sky each night is a phenomenon called the diurnal motion. This motion is due to the Earth's rotation on its axis, and not the stars themselves actually moving.

As the Earth rotates from west to east, different parts of the sky come into view from our perspective on the Earth's surface. The stars that are visible in the sky at a given time depend on the time of night and the observer's location on Earth.

The diurnal motion of stars can be observed by looking at the night sky at different times throughout the night. For example, if you look at the eastern part of the sky at sunset, you will see a different set of stars than if you look at the same part of the sky at midnight. This is because the Earth has rotated during that time, bringing new stars into view from the eastern horizon.

The apparent motion of stars across the sky is a fundamental concept of astronomy and has been known since ancient times. It is one of the ways that astronomers measure time and determine the positions of celestial objects.