How does a sundial tell time?

For man, the sun was his first clock. By watching its movements as it made its way across the sky, he could guess the time. The sundial is a shadow clock for it helped to tell time by observing shadows. The first sundials were poles stuck into the ground in all likelihood. Stones were placed around the pole to mark the positions of the shadow as it moved during the day. In this manner people could measure the passage of time.