Book contents
- Time: From Earth Rotation to Atomic Physics
- Time: From Earth Rotation to Atomic Physics
- Copyright page
- Dedication
- Contents
- Preface
- 1 Time Before the 20th Century
- 2 Time from the Earth’s Rotation
- 3 Ephemerides
- 4 Variable Earth Rotation
- 5 Earth Orientation
- 6 Ephemeris Time
- 7 Relativity and Time
- 8 Time and Cosmology
- 9 Dynamical and Coordinate Timescales
- 10 Clock Developments
- 11 Microwave Atomic Clocks
- 12 Optical Atomic Standards
- 13 Definition and Role of a Second
- 14 International Atomic Time (TAI)
- 15 Coordinated Universal Time (UTC)
- 16 Time in the Solar System
- 17 Time and Frequency Transfer
- 18 Modern Earth Orientation
- 19 International Activities
- 20 Time Applications
- 21 Future of Timekeeping
- Acronyms
- Glossary
- Index
- References
13 - Definition and Role of a Second
Published online by Cambridge University Press: 01 October 2018
Book contents
- Time: From Earth Rotation to Atomic Physics
- Time: From Earth Rotation to Atomic Physics
- Copyright page
- Dedication
- Contents
- Preface
- 1 Time Before the 20th Century
- 2 Time from the Earth’s Rotation
- 3 Ephemerides
- 4 Variable Earth Rotation
- 5 Earth Orientation
- 6 Ephemeris Time
- 7 Relativity and Time
- 8 Time and Cosmology
- 9 Dynamical and Coordinate Timescales
- 10 Clock Developments
- 11 Microwave Atomic Clocks
- 12 Optical Atomic Standards
- 13 Definition and Role of a Second
- 14 International Atomic Time (TAI)
- 15 Coordinated Universal Time (UTC)
- 16 Time in the Solar System
- 17 Time and Frequency Transfer
- 18 Modern Earth Orientation
- 19 International Activities
- 20 Time Applications
- 21 Future of Timekeeping
- Acronyms
- Glossary
- Index
- References
Summary
Historically the second was considered a 60th of a minute, which was one 60th of an hour. A second based on the Earth’s variable rotation was impractical for modern timekeeping requirements. With the introduction of Ephemeris Time, the ephemeris second was defined in 1954 and revised in 1956 as 1/31 556 925.9747 of the length of the tropical year for 1900.0. The availability of atomic clocks made a more accurate and available second possible. So in 1968, the Système International (SI) second was defined as 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 Caesium 133 atom.
With such an accurate measure of the second, the meter was defined as the length of path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. Other SI units are defined in terms of the second and meter. With improved accuracies of timekeeping based on optical frequency standards, a redefinition of the second is under consideration.
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- Information
- Time: From Earth Rotation to Atomic Physics , pp. 215 - 224Publisher: Cambridge University PressPrint publication year: 2018
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