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Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London. GMT was formerly used as the international civil time standard, now superseded in that function by Coordinated Universal Time (UTC). Today GMT is considered equivalent to UTC for UK civil purposes (but this is not formalized) and for navigation is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); these two meanings can differ by up to 0.9 s. Consequently, the term GMT should not be used for precise purposes.〔Hilton and McCarthy 2013, p. 231–2)〕 Because of Earth's uneven speed in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the sun crosses the Greenwich meridian and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy calculated by the equation of time. Noon GMT is the annual average (i.e., "mean") moment of this event, which accounts for the word "mean" in "Greenwich Mean Time". Originally, astronomers considered a GMT day to start at noon while for almost everyone else it started at midnight. To avoid confusion, the name Universal Time was introduced to denote GMT as counted from midnight. Astronomers preferred the old convention to simplify their observational data, so that each night was logged under a single calendar date. Today Universal Time usually refers to UTC or UT1〔''Astronomical Almanac Online'' 2015, Glossary s.v. "(Universal Time )"〕 The term "GMT" is especially used by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, the Met Office and others particularly in Arab countries, such as the Middle East Broadcasting Centre and OSN. It is a term commonly used in the United Kingdom and countries of the Commonwealth, including Australia, New Zealand, South Africa, India, Pakistan and Malaysia; and in many other countries of the eastern hemisphere. In some countries (Britain for example) Greenwich Mean Time is the legal time in the winter and the population uses the term. For an explanation of why this is see "GMT in legislation" below. == History == (詳細はmaritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees, internationally adopted in the International Meridian Conference of 1884. Synchronisation of the chronometer on GMT did not affect shipboard time, which was still solar time. But this practice, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, led to GMT being used worldwide as a standard time independent of location. Most time zones were based upon GMT, as an offset of a number of hours (and possibly a half-hour) "ahead of GMT" or "behind GMT". Greenwich Mean Time was adopted across the island of Great Britain by the Railway Clearing House in 1847, and by almost all railway companies by the following year, from which the term "railway time" is derived. It was gradually adopted for other purposes, but a legal case in 1858 held "local mean time" to be the official time.〔Howse 1997, p. 114〕 On 14 May 1880, a letter signed by 'Clerk to Justices' appeared in 'The Times', stating that 'Greenwich time is now kept almost throughout England, but it appears that Greenwich time is not legal time. For example, our polling booths were opened, say, at 8 13 and closed at 4 13 PM.'〔CLERK TO JUSTICES. ("Time, Actual And Legal" ). Times, London, England, 14 May 1880: 10. The Times Digital Archive. Web. 18 Aug. 2015.〕This was changed later in 1880, when Greenwich Mean Time was legally adopted throughout the island of Great Britain. GMT was adopted on the Isle of Man in 1883, Jersey in 1898 and Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time.〔Myers (2007).〕 Hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924, rendering the time ball at the observatory redundant in the process. The daily rotation of the Earth is somewhat irregular (see ΔT) and is slowing down slightly; atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT was superseded as the international civil time standard by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world. Universal Time (UT), a term introduced in 1928, represents mean time at Greenwich determined in the traditional way to accord with the originally defined universal day; from 1 January 1956 (as decided by the IAU at Dublin, 1955, at the initiative of William Markowitz) this "raw" form of UT was re-labeled UT0 and effectively superseded by refined forms UT1 (UT0 equalised for the effects of polar wandering)〔(UT1 as explained on IERS page )〕 and UT2 (UT1 further equalised for annual seasonal variations in earth rotation rate). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Greenwich Mean Time」の詳細全文を読む スポンサード リンク
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