Clock
A
clock (from the
Latin cloca, "
bell") is an instrument for measuring
time. A clock can be a physical instrument (an especially accurate one is called a
chronometer). The clock in its modern form (24 hour clock) has been in use since at least the
15th century. Clock can refer to an abstract system of time measurement (
ed. see
calendar for other measurements).
Purposes
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.
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.)
The main purpose of a clock is not always to display the time. \nIt may also be used to control a device according to time, e.g. a VCR and a time bomb (ed. see: counter). For an alarm clock both are important.
Ideal uses
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.\n*A good clock is one which, when used to measure other recurrent processes, finds many of them to be periodic.\n*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.
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.
Displays
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.
History
The medieval canonical hours were the intervals between set times of prayer: they differed in length, and varied as the times of sunrise and sunset shifted. Modern form clocks have been in has been in use since at least the 15th century. Modern clocks define constant units of time: an hour is always sixty minutes, of sixty seconds each.
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Notable clocks
, reportedly the oldest working clock in the world]]
Types of clock
\n* alarm clock\n* analog clock with digital display\n* astronomical clock\n* atomic clock\n* binary clock\n* chronometer\n* countdown clock\n* cuckoo clock\n* game clock\n* grandfather clock\n* hourglass\n* swinging pendulum clock\n* torsion pendulum clock\n* projection clock\n* quartz clock\n* radio clock\n* stopwatch\n* sundial\n* timer\n* watch\n* water clock\n* watchclock
See also
Miscelleanous
The British band Coldplay also has a popular song called "Clocks".
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