If you go out at exactly 9 p. m. each evening over the course of one month, the position of a given star will move westward by tens of degrees. what causes this motion?

The appropriate response is the revolution of the Earth around the Sun. For instance, the Earth finishes one turn about its hub about at regular intervals, yet it finishes one revolution around the Sun about each 365 days. Anyway, the fundamental motivation behind why the planets spin around, or circle, the Sun, is that the gravity of the Sun keeps them in their circles.