71-401: DUT1 is a time correction equal to the difference between Universal Time ( UT1 ), which is defined by Earth's rotation , and Coordinated Universal Time (UTC), which is defined by a network of precision atomic clocks , with a precision of +/- 0.1s. UTC is maintained via leap seconds , such that DUT1 remains within the range −0.9 s < DUT1 < +0.9 s. The reason for this correction
142-599: A Board of Visitors, founded in 1710 and made up of the President and Members of the council of the Royal Society. By the later 18th century it incorporated additional responsibilities such as publishing the Nautical Almanac , advising government on technical matters, disseminating time, making meteorological and magnetic observations and undertaking astrophotography and spectroscopy. The physical site and
213-608: A civil day starting at midnight. As the general public had always begun the day at midnight, the timescale continued to be presented to them as Greenwich Mean Time. When introduced, broadcast time signals were based on UT, and hence on the rotation of the Earth. In 1955 the BIH adopted a proposal by William Markowitz, effective 1 January 1956, dividing UT into UT0 (UT as formerly computed), UT1 (UT0 corrected for polar motion) and UT2 (UT0 corrected for polar motion and seasonal variation). UT1
284-521: A form of atomic time, is now used in the construction of the ephemerides of the planets and other solar system objects, for two main reasons. First, these ephemerides are tied to optical and radar observations of planetary motion, and the TDB time scale is fitted so that Newton's laws of motion , with corrections for general relativity , are followed. Next, the time scales based on Earth's rotation are not uniform and therefore, are not suitable for predicting
355-742: A higher spot with better weather. On 1 December 1967, the Isaac Newton Telescope of the Royal Greenwich Observatory at Herstmonceux was inaugurated by Queen Elizabeth II. The telescope was the biggest telescope by aperture in the British Isles. It was moved to Roque de los Muchachos Observatory in Spain's Canary Islands in 1979. In 1990 the RGO moved to Cambridge . At Herstmonceux, the castle grounds became
426-610: A noted tourist and education attraction in its own right, featuring many old observatory items as exhibits. It was getting 60,000 visitors per year in the early 21st century. In 1990 the Royal Observatory moved from Herstmonceux to a new site at Cambridge , adjacent to the University 's Institute of Astronomy , where it occupied Greenwich House just to the north of the Cambridge Observatory . By now,
497-502: A period of four seconds and an accuracy, then unparalleled, of seven seconds per day. The original observatory housed the astronomer royal, his assistant and his family as well as the scientific instruments to be used by Flamsteed in his work on stellar tables. Over time the institution became a more established institution, thanks to its links to long-lasting government boards (the Board of Ordnance and Board of Longitude ) and oversight by
568-516: A plan to have better instruments at Greenwich observatory. Positional astronomy was one of the primary functions of Greenwich for the Admiralty. The Astronomer Royal Airy was an advocate of this and the transit circle instrument he had installed in 1851 was used for a century for positional astronomy. One of the difficulties with positional astronomy, is accounting for the refraction of light through Earth's atmosphere . Sources of error include
639-498: Is a multiple of half an hour, and in most cases a multiple of an hour. Historically, Universal Time was computed from observing the position of the Sun in the sky. But astronomers found that it was more accurate to measure the rotation of the Earth by observing stars as they crossed the meridian each day. Nowadays, UT in relation to International Atomic Time (TAI) is determined by Very Long Baseline Interferometry (VLBI) observations of
710-506: Is determined by the Earth's rotation, which drifts away from more precise atomic-frequency standards, an adjustment (called a leap second ) to this atomic time is needed since (as of 2019 ) 'broadcast time' remains broadly synchronised with solar time. Thus, the civil broadcast standard for time and frequency usually follows International Atomic Time closely, but occasionally step (or "leap") in order to prevent them from drifting too far from mean solar time. Barycentric Dynamical Time (TDB),
781-713: Is known as DUT1 . The table shows the dates of adoption of time zones based on the Greenwich meridian, including half-hour zones. Apart from Nepal Standard Time (UTC+05:45), the Chatham Standard Time Zone (UTC+12:45) used in New Zealand's Chatham Islands and the officially unsanctioned Central Western Time Zone (UTC+8:45) used in Eucla, Western Australia and surrounding areas, all time zones in use are defined by an offset from UTC that
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#1732766066082852-434: Is normally in a lowered position, then starting at 12:55 pm, the ball begins to rise, then at 12:58 it reaches the top; at 1 pm the ball drops. To help mariners at the port and others in line of sight of the observatory to synchronise their clocks to GMT, Astronomer Royal John Pond installed a very visible time ball that drops precisely at 1 pm (13:00) every day atop the observatory in 1833. Initially it
923-459: Is partly that the rate of rotation of the Earth is not constant, due to tidal braking and the redistribution of mass within the Earth, including its oceans and atmosphere, and partly because the SI second (as now used for UTC ) was, when adopted, a little shorter than the current value of the second of mean solar time. Daily observed values of UT1 - UTC for the past week, and daily forecast values for
994-613: The International Celestial Reference Frame (ICRF), called the Earth Rotation Angle (ERA, which serves as the replacement for Greenwich Mean Sidereal Time ). UT1 is the same everywhere on Earth. UT1 is required to follow the relationship where T u = ( Julian UT1 date − 2451545.0). Prior to the introduction of standard time , each municipality throughout the clock-using world set its official clock, if it had one, according to
1065-675: The International Meridian Conference . At the end of this conference, on 22 October 1884, the recommended base reference for world time, the "universal day", was announced to be the local mean solar time at the Royal Observatory in Greenwich , counted from 0 hours at Greenwich mean midnight. This agreed with the civil Greenwich Mean Time used on the island of Great Britain since 1847. In contrast, astronomical GMT began at mean noon, i.e. astronomical day X began at noon of civil day X . The purpose of this
1136-743: The International Terrestrial Reference Frame , use a single oblate spheroid, fixed to the Earth's gravitational centre. The shift from several local spheroids to one worldwide spheroid caused all geographical coordinates to shift by many metres, sometimes as much as several hundred metres. The Prime Meridian of these modern reference systems is the IERS Reference Meridian , in full the International Earth Rotation and Reference Systems Service Reference Meridian (in short called
1207-972: The UK Astronomy Technology Centre in Edinburgh. The old observatory site at Greenwich returned to its original name – the Royal Observatory, Greenwich – and was made part of the National Maritime Museum . In 2002 the UK joined the European Southern Observatory , building the VISTA infrared telescope at the Paranal Observatory as an in-kind contribution. The Astronomer Royal Martin Rees called PPARC "irresponsible" for how it handled
1278-597: The Earth is somewhat irregular and also is very gradually slowing due to tidal acceleration . Furthermore, the length of the second was determined from observations of the Moon between 1750 and 1890. All of these factors cause the modern mean solar day , on the average, to be slightly longer than the nominal 86,400 SI seconds, the traditional number of seconds per day. As UT is thus slightly irregular in its rate, astronomers introduced Ephemeris Time , which has since been replaced by Terrestrial Time (TT). Because Universal Time
1349-530: The Greenwich meridian, defined by the transit instrument of James Bradley . When the Airy circle (5.79 m to the east) became the reference for the meridian, the difference resulting from the change was considered small enough to be neglected. When a new triangulation was done between 1936 and 1962, scientists determined that in the Ordnance Survey system the longitude of the international Greenwich meridian
1420-705: The IAU retiring GMT for astronomical and chronological purposes, replacing it with Universal Time (UT). In 1929, UT was redefined as a statistical combination of multiple observatories. In 1948, the Office of the Astronomer Royal was moved to Herstmonceux in East Sussex and in 1957, the observatory closed, ceasing time measurement operations. The term "GMT" continues to be promoted by the Observatory and
1491-651: The IRM), which is 102.5 metres east of the Airy Greenwich astronomical meridian represented by the stainless steel strip, which is now 5.31 arcseconds west. The modern location of the Airy Transit is 51°28′40.1″N 0°0′5.3″W / 51.477806°N 0.001472°W / 51.477806; -0.001472 ( Airy Transit ) as the IRM is at 0 degree in longitude nowadays. International time from
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#17327660660821562-604: The Moons Triton (orbiting Neptune) and Hyperion (orbiting Saturn). It was donated to the observatory in the 1880s, but was taken down in the 1890s. The 1890s also saw the construction of the Altazimuth Pavilion, completed in 1896 and designed by William Crisp. In 1898 the Christie Enclosure was established to house sensitive magnetic instruments that had been disrupted by the use of iron at
1633-468: The Octagon room, and there was a catch at the top to hold it. This could then be triggered by hand, while observing the time on an astronomical month clock, that was regulated to the mean solar time . By dropping the ball, the public, mariners, and clock makers could then get a time signal by viewing it from afar. The ball drop would be repeated at 2 pm also if possible. The reason why 12 noon
1704-798: The RGO and the Royal Observatory Edinburgh , the Particle Physics and Astronomy Research Council (PPARC) as the RGO's funding body made the decision to close the institution and the Cambridge site by 1998. When the RGO was closed as an institution, the HM Nautical Almanac Office transferred to the Rutherford Appleton Laboratory ( Harwell Science and Innovation Campus , Chilton , Oxfordshire ), while other work went to
1775-708: The RGO's focus had moved from carrying out observations from the British Isles to providing technical support, acting as a conduit between scientists in British universities and the powerful British-owned telescopes (such as the Isaac Newton Telescope , the Anglo-Dutch Jacobus Kapteyn Telescope , and the William Herschel Telescope ) on the Canary Islands and Hawaii . After abandoning a plan to privatise
1846-564: The RGO. In 2018 the Annie Maunder Astrographic Telescope (AMAT) was installed at the ROG in Greenwich. AMAT is a cluster of four separate instruments, to be used for astronomical research; it had achieved first light by June 2018, and contains: The telescopes and the works at the site required to operate them cost about £150,000, from grants, museum members and patrons, and public donations. The telescope
1917-496: The Royal Observatory as a basis for measurement. Four separate meridians have passed through the buildings, defined by successive instruments. The basis of longitude , the meridian that passes through the Airy transit circle , first used in 1851, was adopted as the world's Prime Meridian at the International Meridian Conference at Washington, DC , on 22 October 1884 (voting took place on 13 October). Subsequently, nations across
1988-615: The Second World War, delaying festivities. One of the ground-swells was to build a 'big better' telescope in honour of the celebrated inventor of the Newtonian reflecting telescope. Some two decades of development led to the commissioning of the Isaac Newton Telescope at Herstmonceux. It proved so successful that the cloudy weather was felt to be a bottleneck to its productivity, and plans were made to get it to
2059-450: The UK in general, despite no longer being measured in any way. Coordinated Universal Time (UTC) forms the basis of modern civil time, and is based on the best attributes of UT1 (the modern form of UT, now measured from extra-galactic radio sources) and International Atomic Time (TAI, time kept by accurate clocks). The red time ball of Greenwich was established in 1833, and is noted as a public time signal. The time ball in modern times
2130-732: The bare minimum. On 15 October 1940, during the Blitz , the Courtyard gates were destroyed by a direct bomb hit. The wall above the Gate Clock collapsed, and the clock's dial was damaged. The damage was repaired after the war. After the Second World War, in 1947, the decision was made to move the Royal Observatory to Herstmonceux Castle and 320 adjacent acres (1.3 km ), 70 km south-southeast of Greenwich near Hailsham in East Sussex, due to light pollution in London. The Observatory
2201-476: The coming year, are published by IERS Bulletin A , with more digits than for DUT1. Final values are published in the monthly IERS Bulletin B . DUT1 forecasts are published in IERS Bulletin D . Several time signal services broadcast values of DUT1. CHU (Canada), HLA (South Korea), MSF (United Kingdom), and WWV (United States) transmit DUT1 with 0.1 s precision. In Russia, RWM , RTZ and
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2272-409: The detonation was intended to occur elsewhere. The novelist Joseph Conrad used the incident in his 1907 novel The Secret Agent . For major parts of the twentieth century, the Royal Greenwich Observatory was not at Greenwich, because it moved to Herstmonceux in East Sussex in 1957. The last time that all departments were in Greenwich was 1924: in that year electrification of the railways affected
2343-568: The differences between UT0, UT1, and UT2. By 1960, the U.S. Naval Observatory, the Royal Greenwich Observatory, and the UK National Physical Laboratory had developed UTC, with a similar stepping approach. The 1960 URSI meeting recommended that all time services should follow the lead of the UK and US and broadcast coordinated time using a frequency offset from cesium aimed to match the predicted progression of UT2 with occasional steps as needed. Starting 1 January 1972, UTC
2414-487: The end of the 19th century until UT1 was based on Simon Newcomb 's equations, giving a mean sun about 0.18 seconds behind UT1 (the equivalent of 2.7 arcseconds) as of 2013; it coincided in 2013 with a meridian halfway between Airy's circle and the IERS origin : 51°28′40.1247″N 0°0′2.61″W / 51.477812417°N 0.0007250°W / 51.477812417; -0.0007250 . A key instrument for determining time
2485-409: The forerunner of Greenwich Castle, which resulted in the alignment being 13 degrees away from true North, somewhat to Flamsteed's chagrin. Moore donated two clocks, built by Thomas Tompion , which were installed in the 20 foot high Octagon Room, the principal room of the building. They were of unusual design, each with a pendulum 13 feet (4.0 metres) in length mounted above the clock face, giving
2556-547: The home of the International Study Centre of Queen's University, Kingston, Canada , and The Observatory Science Centre, which is operated by an educational charity Science Project. The Observatory Science Centre opened in April 1995. Some of the remaining telescopes, which were left behind in the move, have public observation events as part of operations of the centre. The centre has established itself as
2627-545: The library, and observing equipment. The largest telescope at Greenwich at that time, the Yapp telescope 36-inch reflector, was moved out to Herstmonceux in 1958. There it was reconstructed in Dome B of the facility. There it was used for astronomy in the 1960s, 1970s, and 1980s. It was left behind at Herstmonceux in 1990 in its dome when the organization moved once again. The tricentennial of Sir Isaac Newton had passed during
2698-723: The local position of the Sun (see solar time ). This served adequately until the introduction of rail travel in Britain , which made it possible to travel fast enough over long distances to require continuous re-setting of timepieces as a train progressed in its daily run through several towns. Starting in 1847, Britain established Greenwich Mean Time , the mean solar time on the Prime Meridian at Greenwich, England , to solve this problem: all clocks in Britain were set to this time regardless of local solar noon. Using telescopes, GMT
2769-411: The location of the star. The instrument can be used to plot the locations of stars, or alternately, with an accurate star chart, the time at the location of the instrument. The Shuckburgh telescope of the Royal Observatory in Greenwich was used for the 1832 transit of Mercury . It was equipped with a filar micrometer by Peter Dollond and was used to provide a report of the events as seen through
2840-418: The longwave RBU transmit DUT1 with 0.1 s precision and an additional correction dUT1 in 0.02 s increments. Universal Time Universal Time ( UT or UT1 ) is a time standard based on Earth's rotation . While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to
2911-479: The main facility. The Observatory underwent an attempted bombing on 15 February 1894. This was possibly the first "international terrorist" incident in Britain. The bomb was accidentally detonated while being held by 26-year-old French anarchist Martial Bourdin in Greenwich Park , near the Observatory building. Bourdin died about 30 minutes later. It is not known why he chose the observatory, or whether
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2982-457: The most exact care and diligence to the rectifying of the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting of the art of navigation." He appointed John Flamsteed as the first Astronomer Royal. The building was completed in the summer of 1676. The building was often called "Flamsteed House", in reference to its first occupant. The scientific work of
3053-411: The motion of bodies in our solar system. UT1 is the principal form of Universal Time. However, there are also several other infrequently used time standards that are referred to as Universal Time , which agree within 0.03 seconds with UT1: [REDACTED] This article incorporates public domain material from Federal Standard 1037C . General Services Administration . Archived from
3124-576: The north. It played a major role in the history of astronomy and navigation , and because the Prime Meridian passed through it, it gave its name to Greenwich Mean Time , the precursor to today's Coordinated Universal Time (UTC). The ROG has the IAU observatory code of 000, the first in the list. ROG, the National Maritime Museum , the Queen's House and the clipper ship Cutty Sark are collectively designated Royal Museums Greenwich . The observatory
3195-407: The numbers of staff increased over time as a result. When the observatory was founded in 1675, one of the best star catalogues was Tycho Brahe 's 1000-star catalogue from 1598. However, this catalogue was not accurate enough to determine longitudes. One of Flamsteed's first orders of business was creating more accurate charts suitable for this purpose. One of the noted charts made at Greenwich
3266-412: The observatory at his own personal cost. Flamsteed House, the original part of the Observatory, was designed by Sir Christopher Wren , probably assisted by Robert Hooke , and was the first purpose-built scientific research facility in Britain. It was built for a cost of £520 (£20 over budget; equivalent to £99,000 in 2023) out of largely recycled materials on the foundations of Duke Humphrey's Tower ,
3337-456: The observatory was relocated elsewhere in stages in the first half of the 20th century, and the Greenwich site is now maintained almost exclusively as a museum, although the AMAT telescope became operational for astronomical research in 2018. There had been significant buildings on this land since the reign of William I. Greenwich Palace , on the site of the present-day National Maritime Museum,
3408-534: The original on 22 January 2022. Royal Observatory, Greenwich The Royal Observatory, Greenwich ( ROG ; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory , RGO , temporarily moved south from Greenwich to Herstmonceux ) is an observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to
3479-565: The period between 1848 and 1972, all of the major countries adopted time zones based on the Greenwich meridian . In 1928, the term Universal Time ( UT ) was introduced by the International Astronomical Union to refer to GMT, with the day starting at midnight. The term was recommended as a more precise term than Greenwich Mean Time , because GMT could refer to either an astronomical day starting at noon or
3550-637: The positions of distant celestial objects ( stars and quasars ), a method which can determine UT1 to within 15 microseconds or better. The rotation of the Earth and UT are monitored by the International Earth Rotation and Reference Systems Service (IERS). The International Astronomical Union also is involved in setting standards, but the final arbiter of broadcast standards is the International Telecommunication Union or ITU. The rotation of
3621-450: The precision of the instrumentation, and then there has to be accounting for precession , nutation , and aberration . Sources of error in the instrument have to be tracked down and accounted for to produce more accurate results. The transit circle makes two measurements; along with a clock, the time a star passed a certain point in the sky as the Earth rotates , and the vertical angle of
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#17327660660823692-671: The readings of the Magnetic and Meteorological Departments, and the Magnetic Observatory moved to Abinger in Surrey. Prior to this, the observatory had had to insist that the electric trams in the vicinity could not use an earth return for the traction current. After the onset of World War II in 1939, many departments were temporarily evacuated out of range of German bombers, to Abinger, Bradford on Avon , Bristol , and Bath , and activities in Greenwich were reduced to
3763-445: The small refractor. By observing the transit in combination with timing it and taking measures, a diameter for the planet was taken. They also reported the peculiar effects that they compared to pressing a coin into the Sun. The observer remarked: I afterwards observed, that immediately around the planet there was a dusky tinge, making it appear as if, in a small degree sunk below the sun's surface;" British astronomers have long used
3834-480: The world used it as their standard for mapping and timekeeping. The Prime Meridian was marked by a brass (later replaced by stainless steel ) strip in the Observatory's courtyard once the buildings became a museum in 1960, and, since 16 December 1999, has been marked by a powerful green laser shining north across the London night sky. Since the first triangulation of Great Britain in the period 1783–1853, Ordnance Survey maps have been based on an earlier version of
3905-532: Was a petition to have another time ball established in Southampton also. The 1890s marked the addition of a new larger refractor, the 28-inch Grubb in the Great Equatorial Dome. Because the new telescope was longer than the old Great refractor, the new dome had to be bigger; thus the famous "onion dome" that expands beyond the diameter of the turret was established. For the tricentennial, it
3976-473: Was an active observatory, geographical coordinates were referred to a local oblate spheroid called a datum known as a geoid , whose surface closely matched local mean sea level. Several datums were in use around the world, all using different spheroids, because mean sea level undulates by as much as 100 metres worldwide. Modern geodetic reference systems, such as the World Geodetic System and
4047-590: Was by the Astronomer Royal James Bradley , who between 1750 and 1762 charted sixty thousand stars, so accurately his catalogues were used even in the 1940s. Bradley was the third Astronomer Royal , and his tenure started in 1742. In the early 19th century, the main positional devices were the Troughton Transit instrument and a mural circle , but after George Biddell Airy took over as Astronomer Royal in 1835, he embarked on
4118-412: Was calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK. Chronometers or telegraphy were used to synchronize these clocks. As international commerce increased, the need for an international standard of time measurement emerged. Several authors proposed a "universal" or "cosmic" time (see Time zone § Worldwide time zones ). The development of Universal Time began at
4189-440: Was commissioned in 1675 by King Charles II , with the foundation stone being laid on 10 August. The old hilltop site of Greenwich Castle was chosen by Sir Christopher Wren , a former Savilian Professor of Astronomy ; as Greenwich Park was a royal estate, no new land needed to be bought. At that time the king also created the position of Astronomer Royal , to serve as the director of the observatory and to "apply himself with
4260-409: Was completed on Greenwich hill. The establishment of a Royal Observatory was proposed in 1674 by Sir Jonas Moore who, in his role as Surveyor-General of the Ordnance , persuaded King Charles II to create the observatory, with John Flamsteed installed as its director. The Ordnance Office was given responsibility for building the Observatory, with Moore providing the key instruments and equipment for
4331-474: Was defined to follow UT1 within 0.9 seconds rather than UT2, marking the decline of UT2. Modern civil time generally follows UTC. In some countries, the term Greenwich Mean Time persists in common usage to this day in reference to UT1, in civil timekeeping as well as in astronomical almanacs and other references. Whenever a level of accuracy better than one second is not required, UTC can be used as an approximation of UT1. The difference between UT1 and UTC
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#17327660660824402-408: Was determined by marking the time a star of known location would pass through the aimpoint of the telescope. In a reverse case, this type of instrument was also used for making star charts. The stars whose position was known precisely enough for being used for time determination, were called "clock stars". By 1925, confusion about whether GMT was reckoned from noon or from midnight led (in 1928) to
4473-411: Was dropped by an operator; from 1852 it was released automatically via an electric impulse from the Shepherd Master Clock . The ball is still dropped daily at 13:00 (GMT in winter, BST in summer). The original time ball system was built by Messrs Maudslay and Field, and cost £180. The five-foot diameter ball was made of wood and leather. In the original ball system, it was hoisted by a rope up from
4544-412: Was not 0° but 0°00'00.417" (about 8 m) east. Besides the change of the reference line, imperfections of the surveying system added another discrepancy to the definition of the origin, so that the Bradley line itself is now 0°00'00.12" east of the Ordnance Survey Zero Meridian (about 2.3 m). This old astronomical prime meridian has been replaced by a more precise prime meridian. When Greenwich
4615-445: Was not chosen was because astronomers at the observatory would record when the Sun crossed the meridian at that time on that day. In rare occasions where the ball could get stuck due to icing or snow, and if the wind was too high it would not be dropped. In 1852, it was established to distribute a time signal by the telegraph wires also. The time ball was extremely popular with the public, chronometers, railways, mariners, and there
4686-404: Was officially known as the Royal Greenwich Observatory, Herstmonceux . Although the Astronomer Royal Harold Spencer Jones moved to the castle in 1948, the scientific staff did not move until the observatory buildings were completed, in 1957. Shortly thereafter, other previously dispersed departments were reintegrated at Herstmonceux, such as the Nautical Almanac Office , Chronometer Department,
4757-431: Was revitalized with a fibre-glass dome; the old one made of papier-mâché and iron had been taken down. The telescope was installed by 1893, with 28-inch diameter glass doublet lens made by Grubb from Chance of Birmingham glass. The new dome was made by T. Cooke and Sons . This replaced a smaller drum-shaped dome. The Lassell two-foot reflector was a famous metal-mirror telescope that had been used to discover
4828-424: Was the Airy Transit Circle (ATC), which was used primarily from 1851 to 1938. It was agreed that the (Prime) "meridian line marked by the cross-hairs in the Airy Transit Circle eyepiece would indicate 0° longitude and the start of the Universal Day". (Note, however, that this Prime Meridian is obsolete; the ITRF Zero Meridian , which is more than 100 meters east, is the modern standard defining longitude.) The time
4899-441: Was the birthplace of both Henry VIII and his daughters Mary I and Elizabeth I ; the Tudors used Greenwich Castle , which stood on the hilltop that the Observatory presently occupies, as a hunting lodge. Greenwich Castle was reportedly a favourite place for Henry VIII to house his mistresses, so that he could easily travel from the Palace to see them. In 1676 the main building of the observatory, now known as Flamsteed House,
4970-426: Was the version sufficient for "many astronomical and geodetic applications", while UT2 was to be broadcast over radio to the public. UT0 and UT2 soon became irrelevant due to the introduction of Coordinated Universal Time (UTC). Starting in 1956, WWV broadcast an atomic clock signal stepped by 20 ms increments to bring it into agreement with UT1. The up to 20 ms error from UT1 is on the same order of magnitude as
5041-434: Was to keep one night's observations under one date. The civil system was adopted as of 0 hours (civil) 1 January 1925. Nautical GMT began 24 hours before astronomical GMT, at least until 1805 in the Royal Navy , but persisted much later elsewhere because it was mentioned at the 1884 conference. Greenwich was chosen because by 1884 two-thirds of all nautical charts and maps already used it as their prime meridian . During
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