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The unit of time in SI, the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom (13th CGPM, 1967).  See atomic time.

As of 2004 the second became the unit whose definition has been most accurately realized (leaving aside units of count), with an absolute uncertainty of about 5 × 10⁻¹⁶, which is equivalent to a second in 60 million years. The world's most accurate clocks, such as NIST-F1 at the National Institute of Standards and Technology, now gain or lose 1 second in 30 million years. NIST-F1 went into service in 1999. The previous clock, NIST-7, which began service on April 22, 1993, gained or lost less than a second in a million years, which was about 20 times better than NBS-6, which went into service in 1975.

When the metric system was first established, the meaning of second seemed so obvious that it was not even defined. This oversight was remedied around 1820, as ¹⁄₈₆₄₀₀ of the mean solar day. In 1960 the 11th CGPM abandoned the definition of the second as a fraction of the mean solar day in favor of the ephemeris second, defined in 1956 by the CIPM as “the fraction 1/31 556 925.9747 of the tropical year for 1900 January 0d 12h ephemeris time.”¹ See ephemeris time.  That in turn was replaced by the present definition.

1. Procés-verbeaux des Séances.
Comité Internationale des Poids et Mesures, 2e série, Tome xxv, session de 1956.
Paris, 1957.

Page 77.


= arcsecond, 1/3600th of a degree.


In England, a unit of length = ¹⁄₁₂th of a line. The second is divided into 12 thirds.

Simmonds, page  335. Doursther page 483.

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