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On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). More simply, 1→2 is a complete ...
Sidereal time vs solar time. Above left: a distant star (the small orange star) and the Sun are at culmination, on the local meridian m. Centre: only the distant star is at culmination (a mean sidereal day). Right: a few minutes later the Sun is on the local meridian again. A solar day is complete.
Starting in 1847, Britain established Greenwich Mean Time, the mean solar time at Greenwich, England, to solve this problem: all clocks in Great Britain were set to this time regardless of local solar noon. [a] Using telescopes, GMT was calibrated to the mean solar time at the prime meridian through the Royal Observatory, Greenwich.
EOT, the time difference between apparent solar time and mean solar time; GHA, the Greenwich Hour Angle of the apparent (actual) Sun; GMHA = Universal Time − Offset, the Greenwich Mean Hour Angle of the mean (fictitious) Sun. Here time and angle are quantities that are related by factors such as: 2 π radians = 360° = 1 day = 24 hours.
Ruth Belville – "the Greenwich Time Lady", daughter of John Henry Belville, who was in the business of daily personal distribution of Greenwich Mean Time via a watch; Coordinated Universal Time – Primary time standard; Greenwich Time Signal – Series of six pips broadcast by the BBC; Marine chronometer – Clock used on ships to aid in ...
The Royal Observatory with the time ball atop the Octagon Room. Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich, which overlooks the River Thames from a hill in Greenwich Park.
The Royal Observatory, Greenwich, founded in 1675, established Greenwich Mean Time (GMT), the mean solar time at that location, as an aid to mariners to determine longitude at sea, providing a standard reference time while each location in England kept a different time.
Newcomb's tables formed the basis of all astronomical ephemerides of the Sun from 1900 through 1983: they were originally expressed (and published) in terms of Greenwich Mean Time and the mean solar day, [10] [11] but later, in respect of the period 1960–1983, they were treated as expressed in terms of ET, [12] in accordance with the adopted ...