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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 ] (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 |
]]'''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 ]. | ||
| In fact, time was more accurately measured by observations of stars crossing the meridian, which indicates ]. A conventional formula then yields GMT. | |||
| 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 Nowadays, GMT has been replaced by ], which is measured by ]s, but is kept within 0.9 seconds from GMT. | 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 Nowadays, GMT has been replaced by ], which is measured by ]s, but is kept within 0.9 seconds from GMT. | ||
| See also: ] | |||
Revision as of 16:05, 2 November 2002
Greenwich Mean Time (GMT) is the mean solar time at the 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. 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 averageed out over the year through the use of the mean sun.
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 Delta-T). Nowadays, GMT has been replaced by Coordinated Universal Time, which is measured by atomic clocks, but is kept within 0.9 seconds from GMT.
See also: sidereal time