Telling Time With the Sun

The sundial is the oldest known device for the measurement of time. It is based on the fact that the shadow of an object will move from one side of the object to the other as the sun moves from east to west during the day. The sundial is believed to have been used in Babylon at least as early as 2000 B.C. The first recorded use of a sundial is in the Book of Isaiah, chapter 38, which dates back to about 700 B.C.

A sundial consists of the plane (dial place) and the gnomon (style). The dial face is divided into hours and sometimes half and quarter hours. The gnomon is a flat piece of metal set in the center of the dial. It points toward the North Pole in the Northern Hemisphere and toward the South Pole in the Southern Hemisphere. The upper edge of the gnomon must slant upward from the dial face at an angle equal to the latitude of the location of the sundial. However, as the following illustrations will show, as long as the correct "parts" are present, a sundial does not have to look anything at all like the one above in order to "do its job."

Some time, way back in prehistoric times, some "cave man" noticed that the shadow of a cliff or a tree reached almost the same place every day when the sun was high in the heavens. It was easy to place a small stone at the point reached by the shadow. So now there were three points in the day-- sunrise, midday, and sunset. The next step was probably to trace the curve of the shadow and mark other spots upon it. This would have been the world's first sundial.
	An Egyptian shadow-clock of about three thousand years ago. In the morning the clock was placed so that the short bar (1) faced the east and the shadow cast on the long bar (2) indicated the hour. At noon the clock was turned so the short bar faced the west and the shadow the long bar told the afternoon time.
This great pointed shaft told time for the pharaohs in Egypt sixteen hundred years before the birth of Christ. It now stands in New York City's Central Park; it has a twin in London. Popularly known as "Cleopatra's Needles," the king who set up these two great obelisks as pointers for his sundials lived long before Cleopatra's day.
	Berosus, a Chaldean astronomer who lived beteen 350 and 320 B.C., is credited with the invention of a bowl-shaped sundial, called a hemispherium. A pin (A) placed in the center of the bowl and pointing upward cast a shadow (B) on the inner surface of the bowl. The hour of the day could be read--when the sun was shining--from lines marked on the inner portions of the board.
The portable sundial can be held up so that the sunlight shines through a tiny hole in the straight pice of metal, and lights up one of the figures engraved inside the circle, which is placed a right angle to the straight piece.
	This primitive watch was always held in one position, and the sun, shining through the little hole, fell upon one of the numbers engraved inside the circle.

"How We Learned to Tell the Time." Richards Topical Encyclopedia (vol. 10, pg. 459-473). New York:The Richards Company, Inc., 1961.
"Watches and Clocks." The Book of Knowledge (vol. 7, pg. 2383-2391). New York:Grolier Incorporated, 1962.

Nordhoff, Helga. "Fun in the Sun, A Sundial Tutorial." Helga's Sundial Pages. 2003. www.qwerty.co.za/sundials/index.htm