Greenwich Mean Time For alternate meanings of "GMT", see GMT (disambiguation). Greenwich Mean Time (GMT) is the mean solar time at the Royal Greenwich Observatory in Greenwich near London in 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 (this discrepancy is known as the equation of time); but this is averaged out over the year through the use of the mean sun. MSK With the growth of Britain as a maritime nation, mariners kept their timepieces on GMT in order to calculate their longtitude "from the Greenwich meridian" although not affecting ship-board time itself which was still solar time. This, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, eventually led to GMT being used world-wide as a reference time independent of location. Although not affecting the local time directly, most time zones are based upon this reference as a number of hours and half-hours "ahead of GMT" or "behind GMT". 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 broadcast on February 5, 1924. Civil time, notably the Greenwich Time Signal, in the United Kingdom has now moved to a UTC-based system, though it is still popularly called GMT it is also called Western European Time (WET). Those countries marked in Dark Blue use European Summer Time and advance their clock one hour in the summer. In the UK this is known as British Summer Time (BST). Those countries marked in light blue keep their clocks on UTC (GMT/WET) all year round. See also Nautical mile sidereal time Swatch Internet Time solar time BPM CHU VNG WWV Central European Time Eastern European Time Moscow Time Sandringham Time Most Popular Articles: Music    Musical Instruments    Guitar    Classical Music    History    Music Definition    Country Music    Rock Music    Pop Music    Hip Hop    Rap    DJ    Video Games    Casino Games    Poker    Blackjack    Casinos    Real Estate    Loan    Auction    eBay    Marketing    Search Engine Optimization    File Sharing    MP3    Music Industry    Kazaa    Internet    Supermodel    Fashion    Fashion Designers    Erotica    Modeling    Clothing    Boots    Shoes    Diamond Engagement Rings    Diamond    Rugs    Gospel Music    Affiliate    E-Commerce    Radio This article is licensed under the GNU Free Documentation License at http://www.gnu.org/copyleft/fdl.html You may copy and modify it as long as the entire work (including additions) remains under this license. You must provide a link to http://www.gnu.org/copyleft/fdl.html To view or edit this article at Wikipedia go to http://www.wikipedia.org/wiki/Greenwich_Mean_Time