![]() ![]() Whatever the mechanism is, it is clear that its effects cannot be predicted with the current state of knowledge. However, climatological changes and variations in sea-level may also play significant roles since they should alter Earth's moment of inertia. It is believed that these fluctuations may be due to fluid motions in Earth's core which interact with and disturb the rotation of the mantle. If we compare the rotation of the actual Earth with a fictional Earth turning at a constant rate, we would find that solar time on the real Earth would fall about one minute behind the fictional Earth after just one century.Įarth's rotation on its axis is also subject to short term fluctuations for periods of up to several decades. Such a trivially small amount may seem insignificant, but it has very measurable cummulative effects. In any case, the current value implies an increase in the length of the day by about 0.002 seconds per day per century. The secular acceleration of the Moon is very poorly known and may not be constant.Ĭareful records for its derivation only go back as far as 100 years or so.īefore then, spurious and often incomplete eclipse and occultation observations from medieval and ancient manuscripts comprise the data base. This secular acceleration gradually transfers angular momentum from Earth to the Moon.Īs Earth loses energy and slows down, the Moon gains this energy and its orbital period and distance from Earth increase. Unfortunately, Universal Time is not a uniform time scale because Earth's rotational period is gradually increasing.Īs Earth rotates on its axis, tidal friction is imposed on it through the gravitational attraction with the Moon and, to a lesser extent, the Sun. Universal Time or UT (colloquially called Greenwich Mean Time or GMT) is based on mean solar time from Greenwich, England. Coordinated Universal Time ( q.v.), the international basis of civil and scientific time, is obtained from an atomic clock that is adjusted in epoch so as to remain close to UT1 in this way, the solar time that is indicated by Universal Time is kept in close coordination with atomic time.Delta T (ΔT) and Universal Time Fred Espenakįor many centuries, the fundamental unit of time was the rotational period of the Earth with respect to the Sun. ![]() Finally, an empirical correction to take account of annual changes in the Earth’s speed of rotation is added to UT1 to convert it into UT2. UT1, which gives the precise angular coordinate of the Earth about its spin axis, is obtained by correcting UT0 for the effects of polar motion. These values differ slightly from each other because of the effects of polar motion ( q.v.). UT0 represents the initial values of Universal Time obtained by optical observations of star transits at various astronomical observatories. In 1955 the International Astronomical Union defined several categories of Universal Time of successively increasing accuracy. Universal Time replaced the designation Greenwich Mean Time in 1928 it is now used to denote the solar time ( q.v.) when an accuracy of about one second suffices. Universal Time (UT), the mean solar time of the Greenwich meridian (0° longitude).
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