Sundial

An instrument that indicates solar time by casting a shadow from a pointer onto a marked surface as the Sun moves across the sky.

Gnomon: The part of a sundial that casts a shadow to indicate time. Its angle and orientation determine how the shadow tracks the Sun.
Celestial sphere: An imaginary sphere centered on Earth onto which celestial objects are projected. It provides the geometric model for locating the Sun relative to a sundial.
Ancient Egyptian shadow clock: An ancient Egyptian timekeeping device that used a shadow’s changing length to mark daylight hours. It is among the early known instruments that used sunlight to divide the day.
Equatorial sundial: A sundial whose dial plane is parallel to Earth’s equator and whose gnomon is parallel to Earth’s axis. Its geometry makes the hour lines evenly spaced.
Water clock: A timekeeping device that measures time through the regulated flow of water. Unlike a sundial, it can operate without direct sunlight.
Hour line: A marked line on a sundial corresponding to a particular solar time. The shadow’s position against these lines provides the time reading.
Latitude: Angular distance north or south of Earth’s equator. The gnomon’s inclination and hour-line geometry depend on the sundial’s latitude.
Vitruvius: A Roman architect and author of the architectural treatise De architectura. His account describes sundial types and their design in the ancient world.
Vertical sundial: A sundial mounted on a vertical wall, with hour lines calculated for its orientation and latitude. It adapts solar timekeeping to building facades.
Mechanical clock: A clock that measures time using a regulated mechanical oscillation and a gear train. It tracks uniform time rather than the Sun’s apparent daily motion.

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