HOW SUNDIALS TELL TIME

These ancient time-keeping devices keep track of Earth's movements around the Sun

- SCOTT DUTFIELD

Humans have used the Sun to monitor the passing of time for thousands of years. For example, Stonehenge is believed to have been erected around 2,500 BCE, constructed in such a way that each stone aligns with the Sun’s rise during the summer solstice and the sunsets of the winter solstice. However, like so many other technological inventions, the timekeeping sundial seems to have emerged from the minds of ancient Egyptians. The first evidence of a sundial dates back to around 1,500 BCE and was composed in two parts of an L-shaped shadow-casting device to determine the time of day. Over time, different versions of the sundial emerged, such as the ancient Greek hemispherical sundial around 280 BCE and the portable sundials created in Rome around 164 BCE. By the 8th century BCE, more accurate shadow-clocks that you might recognise today were commonly used.

As Earth rotates on its axis, the Sun appears to move across the sky, rising in the east and setting in the west. Along its journey, the Sun’s light beats down over the world and casts shadows in places it can’t reach. Taking advantage of light’s inability to pass through solid objects, sundials use the shadow of a central stick, called a gnomon, to pan across a series of numbers, like a clock face, to indicate what time it is based on the Sun’s position in the sky.