Synonyms
Clock
Definition
A physical clock is a physical process coupled with a method of measuring that process to record the passage of time. For instance, the rotation of the Earth measured in solar days is a physical clock. Most physical clocks are based on cyclic processes (such as a celestial rotation). One or more physical clocks are used to establish a time-line clock for a temporal database.
Key Points
Every concrete time is a measurement of some physical clock. For instance a windup watch provides the time “now” by measuring the rate at which a coiled, wound spring unwinds. A physical clock is limited by the durability and regularity of the underlying physical process that it measures, so to provide a clock for every instance in a time line for a temporal database, several physical clocks might be needed. For instance, a clock based on the rotation of the Earth will (likely) be limited as current models predict the Earth will eventually be gravitationally drawn into the Sun....
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Dyreson CE, Snodgrass RT. Timestamp semantics and representation. Inf Syst. 1993;18(3):143–66.
Dyreson CE, Snodgrass RT. The baseline clock. The TSQL2 temporal query language. Kluwer; 1987. p. 73–92.
Fraser JT. Time: the familiar stranger. Amherst: University of Massachusetts Press; 1987.
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Dyreson, C.E. (2018). Physical Clock. In: Liu, L., Özsu, M.T. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8265-9_1063
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DOI: https://doi.org/10.1007/978-1-4614-8265-9_1063
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