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
2 - Time from the Earth’s Rotation
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
Apparent solar time is the time given by sundials and varies by location and time of year. Mean solar time provides a more uniform version of solar time based on solar theories or tables. The difference between them is the equation of time. Sidereal time is the hour angle of the equinox of the celestial frame, so it measures the rotation of the Earth with respect to the stars. In 1884, the Greenwich meridian was established as the international prime meridian, and Greenwich Mean Time (GMT) was defined to be mean solar time measured from noon on the Greenwich meridian. In 1935, Universal Time was defined to be the mean solar time on the meridian of Greenwich reckoned from midnight.
UT1, the description of the rotation angle of the Earth, agrees with mean solar time within 0.2 seconds. The current international standard Coordinated Universal Time (UTC) is based on International Atomic Time (TAI) and kept within 0.9 second of UT1 by leap seconds. Time zones and Daylight Savings Time provide local times. In 2000, the concept of the Earth rotation angle (ERA) was used to develop a new definition of UT1.
- Type
- Chapter
- Information
- Time: From Earth Rotation to Atomic Physics , pp. 10 - 24Publisher: Cambridge University PressPrint publication year: 2018
References
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