What is Greenwich Mean Time (GMT)?

What is Greenwich Mean Time?

Greenwich Mean Time is the mean solar time on the prime meridian — the imaginary north–south line that passes through the transit instrument at the Royal Observatory in Greenwich, on the southern bank of the Thames. Mean matters because the apparent solar day, measured from one noon to the next, varies through the year by up to about half a minute either side of 24 hours, mostly because the Earth's orbit is slightly elliptical and its axis is tilted relative to the orbital plane (see equation of time). Mean solar time averages the solar day to a uniform 24 hours, which is what a clock can keep without continuously retuning.¹

The Royal Observatory was founded in 1675, and from the late 1670s its first Astronomer Royal, John Flamsteed, kept its pendulum clocks on the local mean time at Greenwich. That clock time is the original Greenwich Mean Time. Britain made GMT the legal basis for time in statutes and contracts in 1880, and the rest of the world agreed to reckon civil time from the Greenwich meridian at the International Meridian Conference in Washington four years later.¹⁶⁷

For most of the twentieth century GMT was both an astronomical scale and a civil one. Today the astronomical scale is called UT1 — a modern realisation of mean solar time on the Greenwich meridian, computed from observed star transits with corrections for the wobble of the Earth's spin axis — and the civil one is UTC, which ticks at the steady rate of atomic clocks and is kept within 0.9 seconds of UT1 by occasional one-second adjustments called leap seconds.⁸⁹

How does GMT differ from UTC?

For civil purposes GMT and UTC are the same thing. The U.S. measurement standards body — the National Institute of Standards and Technology — sums it up directly: "either use of GMT can be considered equivalent to Coordinated Universal Time (UTC) when fractions of a second are not important."² London local time, a Boeing flight plan, a BBC time pip, and a UTC timestamp on a financial trade all refer to the same instant when they say "12:00 GMT" — the difference is below a second, and below the resolution of anything outside a national timing laboratory. For a side-by-side comparison of where the two agree and where they differ, see GMT vs UTC: are they the same?.

Technically the two are still distinct. UTC is an atomic timescale produced by averaging more than 300 caesium clocks across some 80 national time laboratories; GMT, in its original sense, is an astronomical timescale defined by the Earth's rotation. UTC was introduced precisely because the Earth's rotation is too irregular to keep a stable clock — tides slow it gradually, the seasonal redistribution of air and water across the planet wobbles it by milliseconds within a year, and large earthquakes nudge it by smaller amounts still. UTC reconciles atomic uniformity with astronomical reality by ticking at the atomic-clock rate and inserting a leap second whenever the gap to UT1 is about to exceed 0.9 seconds.⁹¹⁰

The naming changed in stages. In 1928 the international body of professional astronomers — the International Astronomical Union — recommended replacing the name "Greenwich Mean Time" in astronomical almanacs with Universal Time, partly because civil GMT and astronomical GMT had been counted from different reference points (midnight and noon respectively) and the resulting confusion was costly.¹⁰ The name UTC followed in 1967, when broadcast time signals had been coordinated across national observatories for several years. UTC formally replaced GMT as the international civil standard on 1 January 1972, when the modern leap-second system began.¹⁰

What is the difference between GMT and BST?

British Summer Time (BST) is the United Kingdom's daylight saving time — clock time exactly one hour ahead of GMT, observed from the last Sunday of March to the last Sunday of October. The Summer Time Act 1972 sets the dates and the offset directly: "the time for general purposes in Great Britain shall, during the period of summer time, be one hour in advance of Greenwich mean time", with the period beginning at "one o'clock, Greenwich mean time, in the morning of the last Sunday in March and ending at one o'clock, Greenwich mean time, in the morning of the last Sunday in October."⁴

The pattern is the same one in force across the European Union under its summer-time directive — the United Kingdom kept the dates after leaving the EU — so a Londoner's wall clock reads UTC+0 for about five months of the year and UTC+1 for the remaining seven. Formally, "BST" is the time-zone abbreviation that replaces "GMT" on the London clock during the summer period; both refer to a fixed offset from UTC and neither is its own astronomical scale.⁴

Where did GMT come from?

Greenwich Mean Time grew out of the practical needs of marine navigation. A ship at sea can determine its latitude from the noon altitude of the Sun with little more than a sextant; longitude requires knowing the time at a reference meridian back home, then comparing it with the local time observed on board. The eighteenth-century British solution — pioneered by John Harrison's marine chronometers, kept on the time of the Royal Observatory at Greenwich — made GMT the de facto reference clock of the Royal Navy and, in turn, of much of the world's merchant shipping.¹

Britain put GMT on a statutory footing with the Statutes (Definition of Time) Act 1880, which provided that wherever an expression of time appeared in any Act of Parliament or legal instrument, it was to be interpreted as Greenwich Mean Time across Great Britain (and Dublin Mean Time across Ireland) unless the document said otherwise.⁶ The motivation was railway scheduling: the spread of telegraph-synchronised station clocks had made a single national time more convenient than the patchwork of local solar times that preceded it, and the courts had recently held that "local mean time" was still the legal default.¹⁶

Four years later the world followed. The International Meridian Conference convened in Washington in October 1884, on the invitation of U.S. President Chester Arthur, to choose a single prime meridian for the whole of geography and a single reference for civil time. The conference adopted the meridian of the transit instrument at Greenwich as the prime meridian for longitude, and resolved that a "Universal Day" be reckoned from mean midnight at Greenwich.⁷ From then until 1972, GMT was the international reference from which every other civil time zone was defined as a fixed offset.¹

The transition from GMT to UTC was gradual. The astronomers' rename to "Universal Time" came in 1928; the formal redefinition of the international civil standard as an atomic-derived scale came on 1 January 1972, with the introduction of the modern leap-second mechanism. The name "GMT" was kept as the time-zone label at the prime meridian — the local reading of UTC+0 — and as the legal default in the United Kingdom.¹⁰⁵

Is GMT still a legal time?

In the United Kingdom, yes. The Interpretation Act 1978, the statute that supplies the default rules for reading every other Act of Parliament, provides at section 9: "whenever an expression of time occurs in an Act, the time referred to shall, unless it is otherwise specifically stated, be held to be Greenwich mean time."⁵ The same default applies to deeds and other legal instruments, and to references to time in subordinate legislation, by virtue of the same statute.

The default is overridden during the summer-time period by the Summer Time Act 1972, which provides that during summer time references to time in any enactment, deed, or other document are read as the British Summer Time hour rather than the GMT hour — except for purposes specifically tied to astronomy, meteorology, or navigation, where GMT continues to apply.⁴ The carve-out preserves the convention that, for instance, the times of high tide in tide tables and the times of solar transits in astronomical almanacs are quoted in GMT year-round.

Why do aviation, broadcasting, and weather services still use "GMT"?

Convention. When GMT was the international standard, every regulation, training manual, and operating procedure in industries that worked across national borders was written in GMT; the cost of rewriting all of them after 1972 was much greater than the cost of treating "GMT" and "UTC" as synonyms in everyday speech, since the two agree to within a fraction of a second.

The pragmatic effect is that an air traffic controller's flight plan, a shipping forecast, a BBC News broadcast headline, and a marine weather bulletin will all happily quote a time as "GMT" while the underlying clock the system reads is UTC. The IANA Time Zone Database — the operational source of truth for time-zone rules used by every major operating system and programming language — encodes the legacy as a set of Etc/GMT zones whose offsets from UTC are zero (and whose UT-family alias names match those of Etc/UTC); for any moment after 1972 a timestamp labelled "GMT" and the same timestamp labelled "UTC" denote the same instant.³²

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