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. Most time zones were based upon GMT, as an offset of a number of hours (and possibly half or quarter hours) 'ahead of GMT' or 'behind GMT'. 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. Synchronisation of the chronometer on GMT did not affect shipboard time, which was still solar time. Main article: History of longitude The Shepherd Gate Clock at the gates of the Royal Observatory, Greenwich is permanently kept on Greenwich Mean Time.Īs the United Kingdom developed into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was considered to have longitude zero degrees, by a convention adopted in the International Meridian Conference of 1884. The term GMT is especially used by United Kingdom bodies, such as the BBC World Service, the Royal Navy, and the Met Office and others particularly in Arab countries, such as the Middle East Broadcasting Centre and OSN. The term GMT should thus not be used for purposes that require precision.

For navigation, it is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude) but this meaning can differ from UTC by up to 0.9 s. Today, Universal Time usually refers to Coordinated Universal Time (UTC) or UT1 English speakers often use GMT as a synonym for UTC. To avoid confusion, the name Universal Time was introduced in 1928 to denote GMT as counted from midnight. Originally, astronomers considered a GMT day to start at noon, while for almost everyone else it started at midnight. Noon GMT is the annual average (the arithmetic mean) moment of this event, which accounts for the word 'mean' in 'Greenwich Mean Time'. This event may occur up to 16 minutes before or after noon GMT, a discrepancy described by the equation of time. īecause of Earth's uneven angular velocity 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. The term GMT is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom. At different times in the past, it has been calculated in different ways, including being calculated from noon as a consequence, it cannot be used to specify a particular time unless a context is given. Greenwich Mean Time ( GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. A The islands of Cape Verde are to the west of the African mainland.ī Mauritius and the Seychelles are to the east and north-east of Madagascar respectively.

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