Sundials

The earliest and simplest sundial was the shadow stick, called a gnomen, where the time of day was judged by the length and position of the stick's shadow. This method is still used by some nomadic peoples. As the sun moves through the sky from sunrise to sunset, the shadow of the gnomon rotates, clockwise. The shadow is shortest when the sun is directly in the south, defining local noon.

In 3,500 BC the Egyptians built slender, tapering, four-sided obelisks to tell the time. They had markers arranged on the base to separate the day into divisions and even indicated the longest and shortest days of the year.

However, because the Earth's is a tilt, the sun's path changes slightly from day to day, so the shadow cast by the gnomon is not the same every day. This problem was solved by angling the gnomon towards north. Because this compensates for the Earth's tilt, the hour marks remain the same all year round.

In the quest for accuracy, many types of sundials evolved, including some very complex portable sundials. In about 30 BC. Marcus Vitruvius, a Roman architect, described 13 different sundial designs used in Greece, Asia Minor, and Italy. The invention of more accurate mechanical clocks and the standardisation of time using time zones made sundials obsolete.

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