Stars appear to move across the sky because of Earth's rotation about its axis. As the Earth rotates, the horizon appears to move under the fixed stars. This apparent movement of the stars across the sky is similar to how objects in the foreground appear to move when viewed from a moving car.

As the Earth rotates on its axis from west to east, different parts of the Earth's surface move at different speeds depending on their latitude. The closer a point is to the equator, the faster it moves. This means that the stars appear to move faster across the sky near the equator than they do near the poles.

In addition to the rotation of the Earth, the stars also move across the sky because of the Earth's orbit around the Sun. As the Earth moves in its orbit, different parts of the sky become visible at different times of year. This is why we see different constellations in the sky at different times of the year.