A sundial is an ancient timekeeping device that uses the shadow cast by the sun to show what time it is. The basic design includes a flat surface marked with hours and a raised pointer called a gnomon (say “NO-mun”) that casts the shadow. As the sun moves across the sky during the day, the shadow moves around the dial like the hand of a clock.

People invented sundials thousands of years ago, long before mechanical clocks existed. Ancient Egyptians, Greeks, and Romans all used sundials to organize their days. You can still see elaborate stone sundials in old gardens and town squares across Europe.

Sundials have an obvious limitation: they only work when the sun is shining. They're useless at night or on cloudy days. They also need to be positioned carefully to work accurately, since the sun's path changes with the seasons and varies depending on where you are on Earth.

Today, sundials are mostly decorative, but they remind us of an ingenious solution to a practical problem. Before modern clocks, people looked at shadows to know when to meet for dinner or start their afternoon work.