The sun was man’s first clock. Long ago men guessed at the time of day by watching the sun as it moved across the sky. It was easy to recognize sunrise and sunset, but harder to know when it was noon, the time when the sun is highest above the horizon. In between these times, it was difficult to tell time by the position of the sun.

Then men noticed that the shadow changed in length and moved during the day. They found they could tell time more accurately by watching shadows than by looking at the sun.

CLEOPATRA’S NEEDLE