Ancient Egypt water clock

A clock is any free-standing device or instrument for measuring or displaying the current time. The English word “clock” comes from the Celtic words clocca and clogan, both meaning “bell”. A chronometer is an exceptionally precise mechanical timepiece, designed to be accurate in all conditions of temperature, pressure, etc, especially one used at sea. Watches are sometimes distinguished from clocks in general, but really a watch is just a portable clock, usually worn in a pocket or on the wrist.

Really, a clock can be anything that repeats itself in a predictable way. The rotation of the Earth is a good example of something that is both repetitive and predictable, and indeed the turning of the Earth, as measured by the position of the Sun in the sky, was the first method mankind used to estimate the time of day. Other kinds of clock mechanisms, from the passage of water out of a vessel to the swinging of a pendulum to the vibration of a quartz crystal or the oscillation of a microscopic atom, are really just more manageable and more accurate variations on the same idea.

If the principle behind a clock is relatively simple, the technological and engineering challenges involved in building a clock of any accuracy are relatively complex, and much of the history of clocks in the last two millennia has revolved around the invention of more and more complex machines that are able to overcome the intrinsic problems of friction, temperature differences, movement, magnetic fields, size, etc, in search of the ultimate in accurate, reliable and practical timekeeping devices.

Division of Time

We use clocks to divide the day into smaller increments. Although a 10-hour clock was briefly popular during France’s experiment with metric time measurement after the French Revolution, the 12-hour clock – a convention dating back to ancient Egypt and Babylon – has continued to be the norm, along with 24-hour clocks for some (mainly military and astronomical) applications. The standard sexagesimal system of time measurement – 60 minutes in an hour, 60 seconds in a minute – also owes its ancestry to the sexagesimal system used in Sumer and Babylonia from around 2000BCE, in which 60 was the base number for most mathematical and counting purposes.

Our present system of dividing the day and night into 24 equal hours was instituted around the 14th Century. Prior to that, other than for some technical astronomical purposes, the day was usually split into 12 equal hours and the night into 12 equal hours, a practice dating back to ancient Egypt (although in practice, the night was often divided into 3 or 4 “watches” for security reasons). This meant of course that daytime hours were not necessarily of the same length as nighttime hours (except at the equinoxes), and hours at different times of the year would also vary in length. The advent of mechanical clocks institutionalized the use of 24 equal hours per day.