Sundial The sundial is probably the world’s first timekeeper. It is a horological instrument that shows the time by using shadows cast by the sun. It works when the sun shines down upon the dial, typically a plate marked with the hours of the day. In the centre of the plate is a pointer (the gnomon), which casts a shadow over one of the hours depending on the sun’s position in the sky. The time indicated on the dial represents solar time. There are limits to the timekeeping abilities of sundials, as they work only during the day, and only on clear sunny days at that! Sundials first appeared in Egypt some 3500 years ago and were relied upon as the primary source for accurate timekeeping for thousands of years. When turret clocks (also known as tower or Great clocks) began to appear in the medieval period (around AD 1300), cathedrals and town halls used sundials to correct the time. Later, people in the countryside set their clocks locally using a sundial in their gardens. Even after the invention of the domestic pendulum clock by Christiaan Huygens in 1656, all clocks still had to be set locally by reference to a sundial. You can read more about sundials and other early timekeepers in the Clocktime article The first timekeepers: Telling time before the pendulum clock.
