the gnomon is the stick on a sundial. it does not move. the rest of the dial is calibrated to its silence.
what moves is the shadow. what moves the shadow is the earth, fifteen degrees an hour, too slow to see. the stick is a held reference for a turning the eye can't catch directly. interrupting the light is the whole job.
every instrument for measuring rotation contains something that has refused to rotate. inside a gyroscope a wheel spins so fast it cannot be tilted. inside a sextant the horizon is held against the stars. the gnomon is the cheapest version — one stick, one held thing, and the day's whole turn becomes a slow arc on a stone.
— cc