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Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon ; [ 1 ] as a consequence, it cannot be used to specify a particular time unless a context is given.
The astronomical longitude of the Greenwich prime meridian was found to be 0.19″ ± 0.47″ E, i.e. the plane defined by the local vertical on the Greenwich prime meridian and the plane passing through Earth's rotation axis on the ITRF zero meridian are effectively parallel.
A prime meridian is an arbitrarily chosen meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great ellipse.
The subject to discuss was the choice of "a meridian to be employed as a common zero of longitude and standard of time reckoning throughout the world". [1] It resulted in the recommendation of the Greenwich Meridian as the international standard for zero degrees longitude. [1]
Time zones of the world. A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.
Longitude is measured east and west from the Prime Meridian. To determine "longitude by chronometer," a navigator requires a chronometer set to the local time at the Prime Meridian. Local time at the Prime Meridian has historically been called Greenwich Mean Time (GMT), but now, due to international sensitivities, has been renamed as ...
The most important reason for the 5.3 seconds of longitude offset between the IERS Reference Meridian and the Airy transit circle is that the observations with the transit circle were based on the local vertical, while the IERS Reference is a geodetic longitude, that is, the plane of the meridian contains the center of mass of the Earth. [1]
For 1916, DST extended from 21 May to 1 October, with transitions at 02:00 standard time. On 1 October 1916, Greenwich Mean Time was introduced to Ireland. [5] At the beginning of the 20th century, Sandringham Time was used by the royal household. This practice was abolished by King Edward VIII in an effort to reduce confusions over time.