second

second, abbr. sec or s, fundamental unit of time in all systems of measurement. In practical terms, the second is 1/60 of a minute, 1/3,600 of an hour, or 1/86,400 of a day. Since the length of the day varies, however, the second must be defined in more precise terms. For many years it was defined as 1/86,400 of the mean solar day (see solar time), thus eliminating seasonal variations. Because the rotation of the earth itself is not constant, the second was redefined (1956) in terms of ephemeris time (ET), which is calculated from the motions of celestial bodies in accordance with the laws of motion; 1 sec is 1/31,556,925.9747 of the length of the tropical year for 1900. In 1967 the second was redefined to be 9,192,631,770 periods of vibration of the radiation emitted at a specific wavelength by an atom of cesium-133.

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

More on second from Fact Monster:

  • Second Lateran Council - Lateran Council, Second Lateran Council, Second, 1139, 10th ecumenical council of the Roman ...
  • Second Council of Nicaea - Nicaea, Second Council of Nicaea, Second Council of, 787, 7th ecumenical council, convened by ...
  • Second Council of Constantinople - Constantinople, Second Council of Constantinople, Second Council of, 553, regarded generally as the ...
  • Second Council of Lyons - Lyons, Second Council of Lyons, Second Council of, 1274, 14th ecumenical council of the Roman ...
  • Second Vatican Council - Vatican Council, Second Vatican Council, Second, popularly called Vatican II,1962–65, the ...

See more Encyclopedia articles on: Weights and Measures