A clock is a system of measurement of the Temps.
In the absence of absolute reference (impossible as well theoretically as practically), one builds a clock by observing a physical system equipped with a number finished (and equal to or higher than two) of states, of which one is regarded as the origin. One measures time by counting the number of passages at this origin, by considering conventionally that between two passages, the past duration is fixed.
Various clocksThere exist clocks on all the scales of the Univers, smaller with largest, for example:
- atomic clocks
- crystallographic clocks (with quartz)
- clock of Processor
- Mechanical clock S (pendulum, Clock of bell-tower)
- portable clocks (watches)
- clock with water (Clepsydre)
- weather clock S (Monsoon),
- cosmic clocks: lunar clock (lunation), clock Ground stress (diurnal rotation and annual revolution around the Sun)
- chemical clocks and in particular biological clocks (cardiac pulsation , beat of Lash S, breathing, Sleep, Menstruation S, etc)
Why and howThe clocks in general use the Oscillation S of a system to its Eigen frequency. The stability of this clean Frequency and the maintenance of its oscillations are fundamental.
The absence of absolute, legitimates the use of different clocks according to the field. This absence makes also necessary the choice (purely arbitrary) of a clock of reference.
However, it is observed that there exist almost synchronized different clocks and, this same on very different scales: it is then possible to consider that a clock easy to build is representative of the clock of reference, or of another useful clock (in other words: one can make use of a watch fixed sufficiently well over the time of the Atomic clock of reference to envisage the moment of the rising of the sun, terrestrial clock).
This is of paramount importance in Cosmologie, since according to the theories in force, the evolution of the universe results in the progressive appearance of new clocks, of which those which are useful to us and, the disappearance of the relevant clocks in previous times, which raises the question of the extrapolation our current clocks at times when they did not exist.
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