gnomon

A gnomon is the part of a sundial that casts the shadow used to indicate the time of day. In a simple horizontal or vertical dial, the gnomon is the raised edge or stick whose shadow is projected by the Sun onto a marked surface. The term is often extended to refer to the entire device, though strictly the gnomon is the shadow-casting element. The word comes from the Greek gnomon, meaning “indicator” or “one who knows.”

In astronomy and cartography, the term has other uses. In geometry, a gnomon is a figure formed by removing a smaller similar figure from a corner of a larger one. A sundial’s gnomon is typically aligned with the Earth’s axis, pointing toward true north and oriented to the local latitude. For accuracy, the gnomon’s angle relative to the dial surface equals the observer’s latitude, so the shadow traces the hour lines as the Sun moves.

Historically, gnomons appear in ancient timekeeping devices such as Egyptian and Greek sundials. In cartography, the gnomonic projection maps points on a sphere to a plane by projecting from the sphere’s center onto a tangent plane, preserving great-circle routes as straight lines near the center. The term retains relevance in both horology and geometry, and in discussions of astronomical instruments and calendar systems.