Which ancient timekeeping device is noted for being in use prior to mechanical clocks?

The ancient timekeeping device that is noted for being in use prior to mechanical clocks is the gnomon. A gnomon is essentially a vertical stick or object placed in a horizontal surface that casts a shadow. By observing the position and length of the shadow at different times of the day, people could estimate the passage of time. This method of timekeeping was foundational in the development of sundials, which utilized the same principle but were more refined.

While hourglasses, water clocks, and sundials also served as early timekeeping devices, the gnomon specifically represents one of the earliest forms of measuring time based on the position of the sun. It exemplifies how people sought to use natural phenomena to track the passage of time long before the advent of mechanical clocks.