# Second

*This article is about the unit of time. See second (disambiguation) for other uses*

**Second**(symbol

**s**) is a unit for time, and one of seven SI base units. It is

*defined*as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

In common reckoning of time, a second is 1/60 of a minute, and 1/3600 of an hour.

Historically, the second was defined in terms of the rotation of the Earth as 1/86,400 of a mean solar day. In 1956, the International Committee for Weights and Measures, under the authority given it by the Tenth General Conference on Weights and Measures in 1954, defined the second in terms of the period of revolution of the Earth around the Sun for a particular epoch, because by then it had become recognized that the Earth's rotation was not sufficiently uniform as a standard of time. The Earth's motion was described in Newcomb's Tables of the Sun, which provides a formula for the motion of the Sun at the epoch 1900 based on astronomical observations made during the eighteenth and nineteenth centuries. The ephemeris second thus defined is

*the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.*

Following several years of work, two astronomers at the U.S. Naval Observatory (USNO) and two astronomers at the National Physical Laboratory (Teddington, England) determined the relationship between the frequency of the caesium atom (the standard of time) and the ephemeris second. They determined the orbital motion of the Moon about the Earth, from which the apparent motion of the Sun could be inferred, in terms of time as measured by an atomic clock. As a result, in 1967 the Thirteenth General Conference on Weights and Measures defined the second of atomic time in the International System of Units (SI) as

*the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.*

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