How Did the First Clocks Work?
The earliest known clocks are sundials. In the ruins of Egypt, a giant sundial, almost 100 feet tall, has been found. Scientists believe this early clock is 3,000 years old. The sun’s movement from east to west during the day casts a shadow on the dial, and the length or angle of the shadow indicates the time. Sundials continued to be used to monitor the performance of clocks until the modern era.
However, practical limitations, such as that sundials work only when the Sun shines, and never during the night, encouraged the use of other techniques for measuring and displaying time. So other timekeepers, like water clocks and hourglasses, were invented. Clocks with gears and hands came much later. Today, most clocks and watches are run by cells and batteries.
Water clocks, also known as clepsydra, along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day counting tally stick. Given their great antiquity, where and when they first existed is not known and perhaps unknowable.
The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world.
A water clock uses a flow of water to measure time. There are two types of water clocks: inflow and outflow. In an outflow water clock, a container is filled with water, and the water is drained slowly and evenly out of the container. This container has markings that are used to show the passage of time. As the water leaves the container, an observer can see where the water is level with the lines and tell how much time has passed.
An inflow water clock works in basically the same way, except instead of flowing out of the container, the water is filling up the marked container. As the container fills, the observer can see where the water meets the lines and tell how much time has passed.