A sundial is the most primitive form of a clock. The simplest sundial consists of a stick set in the ground, which casts a shadow indicating the time of day.
In mankind's early history, the most basic division of time was night and day. Beyond that, it was obvious that the sun rose over one horizon, and set in the other. It's a sure bet that the shadows cast while the sun made its trip across the sky, did not escape the notice of primitive humans. Morning shadows were long, gradually becoming shortened to disappear at noon, then lengthening again in the opposite direction, trailing the sunset. Shadow casting would have been a natural tool for telling time.
No one can be sure when the first sundial appeared, but we do know that as early as 3500 B.C. Egyptians were building sundials that not only kept time but indicated the longest and shortest day of the year.
Sundials are generally made with a bottom circular disk that represents the face of a clock, upon which are segmented lines indicating hours. The sundial might also track tides, seasons or the equinoxes. An obelisk or gnomon stands at the center, serving as the element that throws the shadow-hand, which marks the time. In the northern hemisphere as the sun travels across the sky the shadow will move clockwise. At local noon, the shadow disappears.
To compensate for the earth's tilt on its axis, sundials can be angled north. Otherwise the sundial won't compensate for the slightly different path the sun takes through the sky each day as the earth moves through the seasons. Sundials that are angled north, are called styles. Because their alignment compensates for the earth's tilt, the hour marks can remain the same all year round.
The very language we use to keep time refers us back to the sundial. Hour and hora (Greek and Latin for same), come from har or hor, the ancient Egyptian word for "day" or "the sun's path." The Egyptian god Horus, with the head of a hawk and body of a man, was the god of dawn.
Sundials endured as the most accurate form of time-keeping, even after the invention of clocks. In fact people used sundials to reset stopped clocks! With the advent of the handy wristwatch, however, sundials were relegated to lovely garden decorations.
The world's largest sundial is located in the Jantar Mantar observatory built by Jai Singh in northern India. It is called the Samrat Yantra, and the imposing stone structure towers over 89 feet (27 meters) high. More surprisingly, it is said to be accurate within a fraction of a second!