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

The **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 at zero kelvins.

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

## History

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 on its own axis 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 second thus defined is

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

This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960. Reference to the year 1900 does not mean that this is the epoch of a mean solar day of 86,400 seconds. Rather, it is the epoch of the tropical year of 31,556,925.9747 seconds of ephemeris time. Ephemeris Time (ET) was defined as the measure of time that brings the observed positions of the celestial bodies into accord with the Newtonian dynamical theory of motion.

With the development of the atomic clock, it was decided to use atomic clocks as the basis of the definition of the second, rather than the rotation of the earth.

Following several years of work, two astronomers at the United States Naval Observatory (USNO) and two astronomers at the National Physical Laboratory (Teddington, England) determined the relationship between the hyperfine transition frequency of the caesium atom and the ephemeris second. Using a common-view measurement method based on the received signals from radio station WWV, 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.*

The ground state is defined at zero magnetic field. The second thus defined is equivalent to the ephemeris second.

The definition of the second was later refined at the 1997 meeting of the BIPM to include the statement

*This definition refers to a caesium atom at rest at a temperature of* 0 K.

In practice, this means that high-precision realizations of the second should compensate for the effects of ambient radiation to try to extrapolate to the value of the second as defined above.

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