Encyclopedia of Database Systems

2018 Edition
| Editors: Ling Liu, M. Tamer Özsu

Physical Clock

  • Curtis E. DyresonEmail author
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_1063




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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Recommended Reading

  1. 1.
    Dyreson CE, Snodgrass RT. Timestamp semantics and representation. Inf Syst. 1993;18(3):143–66.CrossRefGoogle Scholar
  2. 2.
    Dyreson CE, Snodgrass RT. The baseline clock. The TSQL2 temporal query language. Kluwer; 1987. p. 73–92.Google Scholar
  3. 3.
    Fraser JT. Time: the familiar stranger. Amherst: University of Massachusetts Press; 1987.Google Scholar

Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Utah State UniversityLoganUSA

Section editors and affiliations

  • Richard T. Snodgrass
    • 1
  • Christian S. Jensen
    • 2
  1. 1.University of ArizonaTucsonUSA
  2. 2.Aalborg UniversityAalborg ØstDenmark