The daily rising and setting of the sun and stars is due to the earth's rotation about its axis. The apparent motion of the sun, from east to west across the sky during the day, is familiar to us all. But have you ever noticed that the stars complete a similar motion at night? Probably not, unless you have gone out and spotted your favorite constellation then returned a few hours later to look for it again.
To calculate how long it takes the earth to spin once on its axis (its period of rotation), you will measure how fast stars appear to move overhead.