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abstraction > mathematical concept > unit > time unit > second unit

second unit
subject			fact

second unit	is a kind of time unit
time unit	is a unit of time
unit	has historical origin
	represents

Kinds of second unit :

• ephemeris second (3 facts) - The length of a tropical second (1/31556925.97474 of the tropical year) on 1900 January 0.5 ephemeris time.
• leap second (2 facts) - A second (see second, Système International) added between 60^s and 0^s at announced times to keep UTC within 0^s.90 of UT1. Generally, leap seconds are added at the end of June or December.
• mean solar second (3 facts) - 1/86400 of a mean solar day (cf. ephemeris second).
• second (6 facts) (s) - A unit of time defined as the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. In 1967 the General Conference of Weights and Measures (CGPM) adopted this as the tentative definition of the second in SI units, replacing the ephemeris second, which remains in the IAU system of astronomical constants., duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom