A clock (from the Latin cloca, or “bell”), is an instrument we use for measuring time. There are actually two main qualities that define a clock: First, it must have a regular, constant, or repetitive action (or process) that will effectively mark off equal increments of time. For example, in the old days before our battery-driven, analog and digital clocks and watches, “clocks” included marking candles in even increments, or using a specific amount of sand in an hourglass to measure time.

A sundial, which uses shadows from the sun to mark the passage of time, is one of the oldest timekeeping devices.

Second, there has to be way to keep track of the time increments and easily display the results. This eventually led to the development of watches, large clocks such as Big Ben in London, England, and even the clocks that count down the New Year. The most accurate clocks today are the atomic clocks, which use an atomic frequency standard as the counter.