The regular recurrences of the seasons, the motions of the sun, moon and stars were noted and tabulated for millennia, before the laws of physics were formulated. In order to measure time, one can record the number of occurrences (events) of some periodic phenomenon. Clocks based on these techniques have been developed, but are not yet in use as primary reference standards. As further advances occurred, atomic clock research has progressed to ever-higher frequencies, which can provide higher accuracy and higher precision. The caesium atomic clock became practical after 1950, when advances in electronics enabled reliable measurement of the microwave frequencies it generates. The smallest time step considered theoretically observable is called the Planck time, which is approximately 5.391×10 −44 seconds - many orders of magnitude below the resolution of current time standards. The relative accuracy of such a time standard is currently on the order of 10 −15 (corresponding to 1 second in approximately 30 million years). The UTC timestamp in use worldwide is an atomic time standard. The state of the art in timekeeping Prerequisites These clocks became practical for use as primary reference standards after about 1955, and have been in use ever since. This definition is based on the operation of a caesium atomic clock. It is a SI base unit, and has been defined since 1967 as "the duration of 9,192,631,770 of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". In the International System of Units (SI), the unit of time is the second (symbol: s ). The unit of measurement of time: the second Simultaneously, our conception of time has evolved, as shown below. the length of the shadow cast by a gnomon Įventually, it became possible to characterize the passage of time with instrumentation, using operational definitions.the marking of the moment of noontime during the day.the position on the horizon of the first appearance of the sun at dawn.the periodic succession of night and day, seemingly eternally. ![]() ![]() the first appearance (see: heliacal rising) of Sirius to mark the flooding of the Nile each year.Main article: History of timekeeping devicesīefore there were clocks, time was measured by those physical processes which were understandable to each epoch of civilization:
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