How We Use Clocks Today

Clocks are in homes, offices and many other places; smaller ones (watches) are carried on the wrist; larger ones are in public places, e.g. a train stationor church. A small clock is often shown in a corner of computer displays, mobile phonesand many MP3 players.

The purpose of a clock is not always to display the time. It may also be used to control a device according to time, e.g. an alarm clock, a VCR.

Computers depend on an accurate internal clock signalto allow synchronized processing. (A few research projects are developing CPUs based on asynchronous circuits.) Some computers also maintain time and date for all manner of operations whether these be for alarms, event initiation, or just to display the time of day. The internal computer clock is generally kept running by a small battery. Many computers will still function even if the internal clock battery is dead, but the computer clock will need to be reset each time the computer is restarted, since once power is lost, time is also lost.

Ideal clocks

An ideal clockis a scientific principle that measures the ratio of the duration of natural processes, and thus will give the time measure for use in physical theories. Therefore, to define an ideal clock in terms of any physical theory would be circular. An ideal clock is more appropriately defined in relationship to the set of all physical processes. An ideal clock should too measure time in consistent, for example decimalized time units.

Navigation Clocks

Navigationby ships depends on the ability to measure latitudeand longitude. Latitude is fairly easy to determine through celestial navigation, but the measurement of longituderequires accurate measurement of time. This need was a major motivation for the development of accurate mechanical clocks. John Harrisoncreated the first highly accurate marine chronometerin the mid-18th century. The Noon gunin Cape Townstill fires an accurate signal to allow ships to check their chronometers.