Sundial

A sundial is a horological device that tells the time of day from the apparent position of the sun, using a flat dial plate and a gnomon whose shadow moves across marked hour lines as the sun crosses the sky. The earliest known sundials are shadow clocks from around 1500 BC in ancient Egyptian and Babylonian astronomy. Later Greek and Roman scholars, including Eratosthenes, Ptolemy and the architect Vitruvius, developed and documented further dial types, and the device remained in common use for civil timekeeping through the Middle Ages and into the 16th century and beyond.