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The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s⁻¹.

Bureau international des poids et mesures.
The International System of Units, 9ᵗʰ ed., v 1.08.
Sèvres, Pavilion de Breteuil: BIPM, 2019 .
page 130 of English translation, reproducing resolution

Making the najor constants truly constant be defining thier values..Setting the value of a frequency by definition--any reproducible constant frequency, sets the duration f the unit of time embedded in the definition of the frequency unit. by experminetally determing the value of the frequency (except, of course, that the old, obtained by mesurement sonducted in the pas, ie retained.

This definition, adopted in 20xx, is likely to be shortlived. The proegress in the development of clocks based on has been so great that they are currently orders of magnitude more precise than clocks based on the caesium

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. The weakness in this approach became apparent as it was realized that the rotation of the earth varies.

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 from the astronmers to the physicists.

1. Procés-verbeaux des Séances.
Comité Internationale des Poids et Mesures, 2e série, Tome xxv, session de 1956.
Paris, 1957.
Page 77.



The standard unit of time in all civilized countries is deduced from the time of rotation of the earth about its axis. The sidereal day, or the true period of rotation of the earth, can be ascertained with great exactness by the ordinary observations of astronomers; and the mean solar day can be deduced from this by our knowledge of the length of the year. The unit of time adopted in all physical researches is one second of mean solar time.

J. Clerk Maxwell and Fleeming Jenkin.
On the elementary relations between electrical measurements.
Fleeming Jenkin, editor.
Reports of the Committee on Electrical Standards … reprinted by permission of the Council.
London: E. & F. N. Spon. 1873
Page 61.

All men of science are agreed to use the second of mean solar time as the unit of time.

Second Report — Newcastle-on-Tyne, August 16, 1863.
Page 90 of Reports, above. The 2nd report argued for a meter-gram-second system of units, as did the third.


Definition of the unit of time (second)
Resolution 9 of the 11th CGPM (1960)
The 11th Conference Generale des Poids et Mesures (CGPM),
• the powers given to the Comite International des Poids et Mesures (CIPM) by the 10th CGPM to define the fundamental unit of time,
• the decision taken by the CIPM in 1956,
ratifies the following definition:
"The second is the fraction 1/31 556 925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time."

Comptes Rendus de la 11e CGPM (1960), 1961, p.86.



= 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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