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2 - Time from the Earth’s Rotation

Published online by Cambridge University Press:  01 October 2018

Dennis D. McCarthy
United States Naval Observatory
P. Kenneth Seidelmann
University of Virginia
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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.
Publisher: Cambridge University Press
Print publication year: 2018

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