Gnomon

A projecting or upright object whose shadow indicates the Sun’s position. Gnomons serve as the shadow-casting part of sundials and as tools for observing solar motion.

Sundial: A timekeeping device that indicates time from the position of a shadow cast by sunlight. A sundial uses a gnomon to cast the moving shadow read against its marked face.
Ancient Greek astronomy: The study of celestial motions and structures developed by Greek-speaking scholars in antiquity. Greek astronomers used gnomons to measure solar motion and develop geometrical accounts of the heavens.
Solar time: Time measured by the Sun’s apparent daily motion across the sky. A gnomon indicates apparent solar time through the changing direction of its shadow.
Astrolabe: An instrument for measuring celestial angles and solving problems involving time and the stars. Unlike a gnomon, an astrolabe measures angles through sighting and graduated scales rather than a cast shadow.
Solar declination: The angular distance of the Sun north or south of Earth’s celestial equator. The gnomon’s shadow length and direction change as solar declination changes through the year.
Anaximander: A sixth-century BCE Greek philosopher associated with early cosmology and natural explanations of the world. Ancient accounts credit Anaximander with introducing the gnomon to Sparta.
Eratosthenes' experiment: A geometric estimate of Earth’s circumference based on differing solar angles at two locations. It demonstrates how gnomon-based solar angles can measure a planetary scale.
Armillary sphere: A model of celestial circles made from rings arranged around a central sphere. It represents celestial coordinates with rings, whereas a gnomon observes the Sun through its shadow.
Hour angle: The angle between an observer’s meridian and the hour circle passing through a celestial object. A gnomon’s shadow direction tracks the Sun’s changing hour angle during the day.
Eratosthenes: A Greek scholar who estimated Earth’s circumference by comparing solar angles at two locations. His measurement depended on comparing the Sun’s angle, inferred from shadows, at Syene and Alexandria.

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