49-590: Greenwich Mean Time ( GMT ) is the local mean time at the Royal Observatory in Greenwich , London , counted from midnight . 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. The term "GMT" is also used as one of the names for the time zone UTC+00:00 and, in UK law,
98-679: A "Noon Gun" is fired daily from the citadels in Halifax and Quebec City and from Signal Hill in St. John's, Newfoundland and Labrador . In the same manner, a Noon Gun has been fired in Cape Town , since 1806. The gun is fired daily from the Lion Battery at Signal Hill . The Noonday Gun serves a similar purpose in Hong Kong . The tradition, which started in the 1860s under British colonial rule, has become
147-568: A "clang" that originates from the Nauvoo Bell on Temple Square, in Salt Lake City, which has been a staple on the station since the early 1960s. In Canada, the national English-language non-commercial CBC Radio One network broadcast the daily National Research Council Time Signal from 5 November 1939 until 9 October 2023. The simulcast would occur daily at 1pm Eastern Time . Its French-language counterpart, Radio-Canada , broadcasts
196-435: A national or regional longwave digital signal; for example, station WWVB in the U.S. . The audio portions of the shortwave WWV and WWVH broadcasts can also be heard by telephone. The time announcements are normally delayed by less than 30 ms when using land lines from within the continental United States, and the stability (delay variation) is generally less than 1 ms. However, when mobile phones are used,
245-506: A signal that allows automatic synchronization of clocks, and commercial broadcasters still include time signals in their programming. Today, global navigation satellite systems ( GNSS ) radio signals are used to precisely distribute time signals over much of the world. There are many commercially available radio controlled clocks available to accurately indicate the local time, both for business and residential use. Computers often set their time from an Internet atomic clock source . Where this
294-660: A similar signal at noon. Vancouver radio station CKNW also broadcasts time signals, using a chime every half-hour. Time signals on CBC broadcasts may be delayed up to 3 seconds due to network processing delays between the local radio transmitter and the time signal origin in Ottawa. The CBC's predecessor, the Canadian National Railways Radio network , broadcast the time signal over its Ottawa station , CNRO (originally CKCH), at 9 pm daily and also on its Moncton station, CNRA, beginning in 1923. CNRA closed in 1931 but
343-626: A single 24-hour clock for the entire world. At a meeting of the Royal Canadian Institute on 8 February 1879 he linked it to the anti-meridian of Greenwich (now 180°). He suggested that standard time zones could be used locally, but they were subordinate to his single world time. Standard time came into existence in the United States on 18 November 1883. Earlier, on 11 October 1883, the General Time Convention, forerunner to
392-507: A source of time signals: The telegraphic distribution of time signals was made obsolete by the use of AM, FM, shortwave radio , Internet Network Time Protocol servers as well as atomic clocks in satellite navigation systems. Time signals have been transmitted by radio since 1905. There are dedicated radio time signal stations around the world. Time stations operating in the longwave radio band have highly predictable radio propagation characteristics, which gives low uncertainty in
441-572: A synonym for UTC. 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. The term "GMT" should thus not be used for purposes that require precision. The term "GMT" is especially used by institutional bodies within the United Kingdom , such as the BBC World Service , the Royal Navy , and
490-530: A term introduced in 1928, initially represented 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 International Astronomical Union in Dublin in 1955, at the initiative of William Markowitz ) this "raw" form of UT was re-labelled UT0 and effectively superseded by refined forms UT1 (UT0 equalised for
539-647: A tourist attraction in recent times. A cannon was fired at one o'clock every weekday at Liverpool , at the Castle in Edinburgh , and also at Perth to establish the time. The Edinburgh " One O'Clock Gun " is still in operation. A cannon located at the top of Santa Lucia Hill, in Santiago , is shot every noon. In Rome , on the Janiculum , a hill west of the Tiber since 1904 a cannon
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#1732844906903588-415: A visual signal, the dropping of a ball, to allow mariners to check the chronometers used for navigation. The advent of electrical telegraphs allowed widespread and precise distribution of time signals from central observatories. Railways were among the first customers for time signals, which allowed synchronization of their operations over wide geographic areas. Dedicated radio time signal stations transmit
637-549: Is fired daily at noon towards the river as a time signal. This was introduced in 1847 by Pope Pius IX to synchronise all the church bells of Rome. It was situated in Castel Sant'Angelo until 1903 when it was moved to Monte Mario for a few months until it was placed in its current position. The cannon was silenced from the start of WWII for about twenty years until 21 April 1959, the 2712th anniversary of Rome's founding, and has been in use since then. For many years an old cannon
686-411: Is not available, a locally connected GNSS receiver can precisely set the time using one of several software applications. One sort of public time signal is a striking clock . These clocks are only as good as the clockwork that activates them, but they have improved substantially since the first clocks from the 14th century. Until modern times, a public clock such as Big Ben was the only time standard
735-572: Is not strictly defined in material form but from a statistical solution resulting from observations of all time-determination stations which the BIPM takes into account when co-ordinating the world's time signals. Nevertheless, the line in the old observatory's courtyard today differs no more than a few metres from that imaginary line which is now the prime meridian of the world. Historically, GMT has been used with two different conventions for numbering hours. The long-standing astronomical convention, dating from
784-781: 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 p.m." This was changed later in 1880, when Greenwich Mean Time was legally adopted throughout the island of Great Britain. GMT was adopted in the Isle of Man in 1883, in Jersey in 1898 and in Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time . Hourly time signals from Greenwich Observatory were first broadcast by shortwave radio on 5 February 1924 at 17:30:00 UTC, rendering
833-506: Is produced and distributed in a similar manner), though unlike program content which is on a broadcast delay for content concerns, the time signal airs as-is over-the-air, meaning it can sometimes be talked over during a live news event or sports play-by-play. KYW-AM in Philadelphia broadcasts a time signal at the top of the hour along with its jingle . Bonneville International -owned news/talk station KSL (AM-FM) in Salt Lake City uses
882-522: Is still shot every night at 9 pm. (This gun was brought to Stanley Park in 1894 by the Department of Fisheries originally to warn fishermen of the 6:00 pm Sunday closing of fishing.) The 9:00 pm firing was later established as a time signal for the general population. Until a time gun was installed, the nearby Brockton Point lighthouse keeper detonated a stick of dynamite. Elsewhere in Canada,
931-519: Is the equation of time . Local mean time was used from the early 19th century, when local solar time or sundial time was last used until standard time was adopted on various dates in the several countries. Each town or city kept its own meridian , so locations one degree of longitude apart had times four minutes apart. This became a problem in the mid 19th century when railways needed clocks for railway time that were synchronized between stations, while local people needed to match their clock (or
980-474: Is the annual average (the arithmetic 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 in 1928 to denote GMT as counted from midnight. Today, Universal Time usually refers to Coordinated Universal Time (UTC) or UT1; English speakers often use GMT as
1029-570: Is the basis for civil time in the United Kingdom. Because 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. This event may occur up to 16 minutes before or after noon GMT, a discrepancy described by the equation of time . Noon GMT
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#17328449069031078-622: The American Railway Association , approved a plan that divided the United States into several time zones . On that November day, the US Naval Observatory telegraphed a signal that coordinated noon at Eastern standard time with 11 am Central, 10 am Mountain, and 9 am Pacific standard time. A March 1905 issue of The Technical World describes the role of the United States Naval Observatory as
1127-560: The CBS Radio Network , of which WCBS is the flagship, air a "bong" (at a frequency of 440 Hz – the international standard for the musical note A ) that immediately precedes each top-of-the-hour network newscast. (The same bong could be heard on the CBS Television Network, at the top of the hour immediately before the beginning of any televised program, in the 1960s and 1970s.) An automated "chirp" at one second before
1176-753: The Met Office ; and others particularly in Arab countries, such as the Middle East Broadcasting Centre and OSN . As 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. Synchronisation of
1225-590: The Nelson Monument, Edinburgh ; the sailors' home Broomielaw , Glasgow; Liverpool and one at Deal, Kent , installed by the Admiralty . Telegraph signals were used regularly for time coordination by the United States Naval Observatory starting in 1865. By the late 1800s, many U.S. observatories were selling accurate time by offering a regional time signal service. Sandford Fleming proposed
1274-448: The time ball at the observatory redundant. The daily rotation of the Earth is irregular (see ΔT ) and has a slowing trend; therefore atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT as the international civil time standard was superseded by Coordinated Universal Time (UTC) , maintained by an ensemble of atomic clocks around the world. Universal Time ( UT ) ,
1323-403: The announcers use the term "Greenwich Mean Time" consistently throughout the year. Several countries define their local time by reference to Greenwich Mean Time. Some examples are: Greenwich Mean Time is defined in law as standard time in the following countries and areas, which also advance their clocks one hour (GMT+1) in summer. Greenwich Mean Time is used as standard time all year round in
1372-674: The beginning of the hour. In New York, WCBS and WINS have distinctive beginning-of-the-hour tones, though the WINS signal is only approximate (several seconds error). WINS also has a tone at 30 minutes past the hour for those setting their clocks. WTIC uses the Morse code V for victory to the tune of Beethoven's 5th Symphony at the beginning of the hour continuously, since 1943. Broadcast stations using iBiquity Digital's " HD Radio " system are contractually required to delay their analog broadcast by about eight seconds, so it remains in sync with
1421-624: The broadcasts continued on CNRO when the station was acquired by the Canadian Radio Broadcasting Commission in 1933 and by the CBC in 1936 before going national in 1939. In Australia, many information-based radio stations broadcast time signals at the beginning of the hour, and a speaking clock service was also available until October 2019. However, the VNG dedicated time signal service has been discontinued. In Cuba, Radio Reloj
1470-483: 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 occasionally half or quarter hours) "ahead of GMT" or "behind GMT". Greenwich Mean Time
1519-587: The church clock) to the time tables. Standard time means that the same time is used throughout some regional time zone—usually, it is at an offset from Greenwich Mean Time or the local mean time of the capital of the region. Time signal A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day . Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks ; however, audible signals (even signal guns) have limited range. Busy seaports used
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1568-626: The delays are often more than 100 ms, due to the multiple access methods used to share cell channels. In rare instances when the telephone connection is made by satellite, the time is delayed by 250–500 ms. The audio from the broadcasts is available by telephone by dialling U.S. numbers (303) 499-7111 for WWV (Colorado), and (808) 335-4363 for WWVH (Hawaii). Calls (which are not toll-free) are disconnected after 2 minutes. Loran-C time signals formerly were also used for radio clock synchronization, by augmenting their highly accurate frequency transmissions with external measurements of
1617-553: The digital stream. Thus, network-generated time signals and service cues will also be delayed by about eight seconds. (Because of the delay, when WBEN-AM in Buffalo, New York was broadcasting time markers, and was simulcast on an FM station that broadcast in HD; the FM signal did not carry the time signal. WBEN does not broadcast in HD.) Local signals may also be delayed. The all-news radio stations of
1666-480: The effects of polar wandering ) and UT2 (UT1 further equalised for annual seasonal variations in Earth rotation rate). Indeed, even the Greenwich meridian itself is not quite what it used to be—defined by "the centre of the transit instrument at the Observatory at Greenwich". Although that instrument still survives in working order, it is no longer in use and now the meridian of origin of the world's longitude and time
1715-490: The following countries and areas: Local mean time Local mean time ( LMT ) is a form of solar time that corrects the variations of local apparent time , forming a uniform time scale at a specific longitude . This measurement of time was used for everyday use during the 19th century before time zones were introduced beginning in the late 19th century; it still has some uses in astronomy and navigation. The difference between local mean time and local apparent time
1764-616: The general public in winter and UTC+01:00 in summer. BBC radio stations broadcast the "six pips" of the Greenwich Time Signal . It is named from its original generation at the Royal Greenwich Observatory . If announced (such as near the start of summer time or of winter time), announcers on domestic channels declare the time as GMT or BST as appropriate. As the BBC World Service is broadcast to all time zones,
1813-404: The general public needed. Accurate knowledge of time of day is essential for navigation , and ships carried the most accurate marine chronometers available, although they did not keep perfect time. A number of accurate audible or visible time signals were established in many seaport cities to enable navigators to set their chronometers. In Vancouver , British Columbia , a " 9 O'Clock Gun "
1862-548: The hour signals a switch to the radio network broadcast. As an example, KNX , the CBS Radio Network all-news station in Los Angeles, broadcasts this "bong" sound on the hour. However, due to buffering of the digital broadcast on some computers, this signal may be delayed as much as 20 seconds from the actual start of the hour (this is presumably the same situation for all CBS Radio stations, as each station's digital stream
1911-405: The number of seconds required for the report of the time gun to reach various locations in the city. Because light travels much faster than sound , visible signals enabled greater precision than audible ones, although audible signals could operate better under conditions of reduced visibility. The first time ball was erected at Portsmouth , England in 1829 by its inventor Robert Wauchope . One
1960-438: The offsets of LORAN navigation signals against time standards. As radio receivers became more widely available, broadcasters included time information in the form of voice announcements or automated tones to accurately indicate the hour. The BBC has included time " pips " in its broadcasts from 1922. In the United States many information-based radio stations (full-service, all-news and news/talk) also broadcast time signals at
2009-458: The received time signals. Stations operating in the shortwave band can cover wider areas with relatively low-power transmitters, but the varying distance that the signal travels increases the uncertainty of the time signal on a scale of milliseconds. Radio time signal stations broadcast the time in both audible and machine-readable time code form that can be used as references for radio clocks and radio-controlled watches . Typically, they use
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2058-473: The time referred to shall (unless otherwise specifically stated) be held to be Greenwich mean time. Under subsection 23, the same rule applies to deeds and other instruments. During the experiment of 1968 to 1971, when the British Isles did not revert to Greenwich Mean Time during the winter, the all-year British Summer Time was called British Standard Time ( BST ). In the UK, UTC+00:00 is disseminated to
2107-546: The tradition of a factory whistle becomes so deeply entrenched in a community that the whistle is maintained long after its original function as a time keeper became obsolete. For example, the University of Iowa 's power plant whistle has been reinstated several times by popular demand after numerous attempts to silence it. In 1861 and 1862, the Edinburgh Post Office Directory published time gun maps relating
2156-558: The work of Ptolemy , was to refer to noon as zero hours (see Julian day ). This contrasted with the civil convention of referring to midnight as zero hours dating from the Roman Empire . The latter convention was adopted on and after 1 January 1925 for astronomical purposes, resulting in a discontinuity of 12 hours, or half a day. The instant that was designated as "December 31.5 GMT" in 1924 almanacs became "January 1.0 GMT" in 1925 almanacs. The term Greenwich Mean Astronomical Time ( GMAT )
2205-716: The world, was initiated in 1852 by the Electric Telegraph Company in collaboration with the Astronomer Royal . Greenwich Mean Time was distributed by telegraph from the Greenwich Observatory . This included a system for synchronising the drop of the time ball at Greenwich with other time balls around the country, one of which was atop the Electric's offices in the Strand . Other synchronised time balls were atop
2254-540: 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. On 14 May 1880, a letter signed by "Clerk to Justices" appeared in The Times , stating that "Greenwich time
2303-460: Was fired "about noon" from a mountain near Kabul . In many Midwestern US cities where tornadoes are a common hazard, the emergency sirens are tested regularly at a specified time (say, noon each Saturday); while not primarily intended to mark the time, local people often check their watches when they hear this signal. In many non-seafaring communities, loud factory whistles served as public time signals before radio made them obsolete. Sometimes,
2352-475: Was installed in 1833 on the roof of the Royal Observatory in Greenwich , London, and the time ball has dropped at 1:00 pm every day since then. The first American time ball went into service in 1845. In New York City, the ceremonial Times Square Ball drop on New Year's Eve in Times Square is a vestige of a visual time signal. The first telegraph distribution of time signal in the United Kingdom, indeed, in
2401-590: Was introduced to unambiguously refer to the previous noon-based astronomical convention for GMT. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours . Legally, the civil time used in the UK is called "Greenwich mean time" (without capitalisation), with an exception made for those periods when the Summer Time Act 1972 orders an hour's shift for daylight saving. The Interpretation Act 1978 , section 9, provides that whenever an expression of time occurs in any Act,
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