Sundial in the grounds on Herstmonceux Castle, East Sussex, England. This was the home of the Royal Greenwich Observatory from 1948-1990. A sundial uses the apparent motion of the Sun across the sky to tell the time. A shadow is cast by a gnomon (white) onto a ring bearing the hours. The shadow is falling between 13 and 14 here, indicating that the time is half past one in the afternoon.