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

From this it was an easy step to inventing the sundial, which is really a shadow clock. Instead of trying to guess the position of the sun and thus the time of day, the shadow gave a more accurate idea of the sun’s position.

The first sundials were probably poles stuck into the ‘ground. Stones placed around a pole marked the positions of the shadow as it moved during the day. Thus men could measure the passing of time. Later, huge stone columns were used. Cleopatra’s Needle, now on the Thames Embankment in London, was once part of a sundial. Smaller sundials were used too. One small Egyptian sundial, about 3,500 years old, is shaped like an L. It lays flat on its longer leg, on which marks show six periods of time.

About 300 B.C. a Chaldean astronomer invented a new kind of sundial, shaped like a bowl. A shadow thrown by a pointer moved along and marked 12 hours of the day. This sort of sundial was very accurate and continued to be used for many centuries.

Today sundials are built in gardens for their beauty rather than for their usefulness. However, on the walls and window sills of old houses one sometimes sees crude sundials. They are so arranged that a nail or the edge of the window casing will cast the shadow. In an accurate sundial, the pointer must be slanted at an angle equal to the latitude of the place where it is to be used. A vertical pointer will show the correct time only at one latitude and at one season. If the dial is flat, the hour marks must be spaced unequally on it.