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A clock (from the Latin cloca, "bell") is an instrument for measuring time. The clock in its modern form (24
hour clock) has been in use since at least the 15th century.
The display can be analog, with hands, or digital, expressing the time in digits. The former usually has a circular scale of 12 hours, which also serves as a scale of 60 minutes, and often also as a scale
of 60 seconds; the latter has an hour range of 1-12, with an indication am/pm, or 0-23.
Clocks are in homes and offices; smaller ones (watches) are carried along; big ones
are in public places, e.g. a train station or church.
A small clock is also often permanently shown in a corner of computer displays or mobile phones.
The main 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. a VCR and a time bomb. See: counter. For an alarm clock both are important.
A clock, by measuring time (e.g. in seconds). supplies a numerical comparison between the durations of different time
intervals. For example, a clock will provide the ratio of the duration of one day to the duration of a different day (for
example, the earth is spinning slower today than it did a billion years ago. If the earth's spin is used as a clock, each
rotation will take exactly one day, by definition.)
A clock can be a physical instrument (an especially accurate one is called a chronometer) or refer to an abstract system of time measurement (see calendar).
Modern clocks define constant units of time: an hour is always sixty minutes, of sixty seconds each.
The medieval canonical hours, however, were the intervals between
set times of prayer: they differed in length, and varied as the times of sunrise and sunset shifted.
Navigation
Accurate navigation by ships beyond the sight of land depends on the ability
to measure latitude and longitude.
Latitude is fairly easy to determine through celestial
navigation, but the measurement of longitude requires accurate measurement of time. This need was a major motivation for the
development of accurate mechanical clocks.
The notion of an ideal clock
An ideal clock appropriately measures the ratio of the duration of natural processes, and thus will give the appropriate 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. This leads to the following
definitions:
- A clock is a recurrent, periodic
process and a counter.
- A good clock is one which, when used to measure other recurrent processes, finds many of them to be periodic.
- An ideal clock is a clock (i.e., recurrent process) that makes the most other recurrent processes periodic.
This definition can be further improved by the consideration of successive levels of smaller and smaller error tolerances.
While not all physical processes can be surveyed, the definition should be based on the set of physical processes which
includes all individual physical processes which are proposed for consideration. Since atoms are so numerous and since, within
current measurement tolerances, they all beat in a manner such that if one is chosen as periodic then the others are all deemed
to be periodic also, it follows that atomic clocks represent ideal clocks
to within present measurement tolerances and in relation to all presently known physical processes. However, they are not so
designated by fiat. Rather, they are designated as the current ideal clock because they are currently the best instantiation of
the definition.
Notable clocks
Types of clock
See also
The British band Coldplay also has a popular song called "Clocks".
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