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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 kelvin.
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 EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla around the Sun for a particular epochIn astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. In the case of celestial coordinates, the position at other times can be computed by taking into account precession and proper motion. In the cas, 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 SunNewcomb's Tables of the Sun is a work published in the late 19th century by astronomer and mathematician Simon Newcomb. The work contains Newcomb's mathematical development of the position of the Earth in the Solar System, which is constructed from classi, 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
This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960Events January-February January 1 Independence of Cameroon January 9 Aswan High Dam construction begins in Egypt January 11 Chad declares its independence. January 14 Ralph Chubb, the gay poet and printer, dies at Fair Oak Cottage in Hampshire. January 23. Reference to the year 19001900 is the common year starting on Monday. see link for calendar) For the film, see 1900 (film). Events January January 1 Nigeria becomes British protectorate January 2 John Hay announces the Open Door Policy to promote trade with China. January 2 Chicag 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 TimeEphemeris Time (ET was defined by the International Astronomical Union (IAU) in 1952 as the replacement for Universal Time used in the ephemerides beginning in the year 1960 and through the year 1983. It had been known since the late 18th century that the (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 clockAn atomic clock is a type of clock that uses an atomic frequency standard as its counter. Early atomic clocks were masers with attached equipment. Today's best atomic frequency standards (or clocks) are based on more advanced physics involving caesium bea, it was decided to use atomic clocks as the basis of the definition of the second, rather than the orbit of the earth.
Following several years of work, two astronomers at the United States Naval ObservatoryThe United States Naval Observatory USNO is one of the oldest scientific agencies in the United States. It is located in Washington, D. It is one of the very few observatories located in an urban area (when it was initially constructed, there was no conce (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. 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 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
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.