gnomón

A gnomon is the part of a sundial that casts a shadow onto the dial face to indicate the time. In its simplest form, a gnomon is a rod or a thin flat piece of metal or wood. The shadow cast by the gnomon moves across a marked surface, the dial, which is calibrated to show the hours. The angle of the gnomon is crucial for accurate timekeeping. For a horizontal sundial, the gnomon is typically aligned parallel to the Earth's axis, pointing towards the celestial pole. This means its angle with the horizontal plane is equal to the latitude of the location.

The word "gnomon" comes from the Greek word for "pointer" or "marker." Historically, gnomons were used not only for telling time but also in astronomy for measuring the altitude of celestial bodies and in surveying. Early gnomons were simple vertical sticks, and their shadows were observed at different times of the day. Over centuries, sundial designs evolved, incorporating more sophisticated gnomon shapes and dial layouts to improve accuracy and readability. The design and placement of the gnomon are fundamental to how a sundial functions, directly translating the sun's apparent movement across the sky into a measurement of time.