Sundials

A sundial is a device that tells the time of day by the position of the Sun. It consists of a flat or curved dial plate and a gnomon, a rod or thin plate that casts a shadow on the dial. As the Sun moves across the sky, the shadow traverses marked hour lines, indicating solar time.

The geometry depends on the dial's orientation and the latitude. The gnomon is aligned with the Earth's axis, with its angle equal to the local latitude (for a typical horizontal or vertical dial). The dial must be oriented to true north in the Northern Hemisphere or true south in the Southern Hemisphere.

Common types include horizontal dials (dial plate lies flat), vertical dials (mounted on a wall), and equatorial dials (the dial lies on a plane parallel to the equator with the gnomon along the pole). Analemmatic dials position the gnomon along a movable line depending on the date. Portable sundials exist for travelers.

Sundials are among the oldest timekeeping instruments. They appear in ancient Egypt and Mesopotamia, were developed by the Greeks and Romans, refined in the Islamic world, and influential again during the Renaissance, preceding mechanical clocks.

Sundials measure apparent solar time, which varies with the equation of time and longitude. They require clear skies and correct calibration for latitude. They do not function at night or during overcast periods, and daylight saving time requires offset.

Today sundials are primarily educational tools and decorative objects, appreciated for historical value and for illustrating the movement of the Sun.