Greenwich Mean Time: Difference between revisions

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	'''Greenwich Mean Time''' ('''GMT''') is the ] at the ] in ] near ], ], which by convention is at 0 degrees geographic longitude. Theoretically, noon Greenwich Mean Time is the moment when the Sun crosses the Greenwich ] (and reaches its highest point in the sky. Because of the Earth's uneven speed in its elliptic ], this event may be up to 16 minutes off ](known as the ]); but this is ~~averageed~~ out over the ] through the use of the ].		'''Greenwich Mean Time''' ('''GMT''') is the ] at the ] in ] near ], ], which by convention is at 0 degrees geographic ]. Theoretically, noon Greenwich Mean Time is the moment when the ] crosses the Greenwich ] (and reaches its highest point in the sky in Greenwich). Because of the Earth's uneven speed in its elliptic ], this event may be up to 16 minutes off ] (known as the ]); but this is averaged out over the ] through the use of the ].

			The modern continuation of GMT is Universal Time (]).
	GMT, mean solar days, and all clocks based on the rotation of the Earth have been abandoned, because the rotation of the Earth is somewhat irregular (see ]). Nowadays, GMT has been replaced by ], which is measured by ]s, but is kept within 0.9 seconds from GMT.
			It is constantly being measured by observatories around the world.

			The daily rotation of the Earth is somewhat irregular (see ]) and is slowing down.
			Therefore, GMT is not used as official clock time anymore. Nowadays, the official clock time is measured by ]s and is known as ] (UTC). By using ]s, UTC is kept within 0.9 seconds from GMT.

	Hourly time signals from Greenwich Observatory were first broadcasted on ], ].		Hourly time signals from Greenwich Observatory were first broadcasted on ], ].

Revision as of 01:23, 4 March 2003

Greenwich Mean Time (GMT) is the mean solar time at the Royal Greenwich Observatory in Greenwich near London, England, which by convention is at 0 degrees geographic longitude. Theoretically, noon Greenwich Mean Time is the moment when the Sun crosses the Greenwich meridian (and reaches its highest point in the sky in Greenwich). Because of the Earth's uneven speed in its elliptic orbit, this event may be up to 16 minutes off apparent solar time (known as the analemma); but this is averaged out over the year through the use of the mean sun.

The modern continuation of GMT is Universal Time (UT). It is constantly being measured by observatories around the world.

The daily rotation of the Earth is somewhat irregular (see Delta-T) and is slowing down. Therefore, GMT is not used as official clock time anymore. Nowadays, the official clock time is measured by atomic clocks and is known as Coordinated Universal Time (UTC). By using leap seconds, UTC is kept within 0.9 seconds from GMT.

Hourly time signals from Greenwich Observatory were first broadcasted on February 5, 1924.