Sundial, instrument for measuring time, by means of the location on a plane of the shadow cast by a marker exposed to sunlight. A sundial consists of two parts: a gnomon and a dial plane. The gnomon is the shadow-producing device, usually a metal plate set parallel to the earth's axis and pointing toward the celestial pole. The dial plane is the flat surface marked with the times of day. A sundial is specially designed for its location on the earth, and tables are used to calculate standard time from solar time, because solar time changes daily with the position of the sun. Sundials were used for judging time before clocks and watches became more available in the 18th century.

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