Rev John Louis Petit, 1801 - 1868
The Royal Observatory, Greenwich
The Royal Observatory, Greenwich
17th October 1868

"17th Oct 1868/Greenwich"

pencil and watercolour
28 x 39 cm. (11 x 15.1/2 in.)


The Royal Observatory, Greenwich, (known as the Royal Greenwich Observatory or RGO when the working institution moved from Greenwich to Herstmonceux after World War II) played a major role in the history of astronomy and navigation, and is best known as the location of the prime meridian. The observatory is situated on a hill in Greenwich Park, overlooking the River Thames.

The observatory was commissioned in 1675 by King Charles II, with the foundation stone being laid on 10 August. The site was chosen by Sir Christopher Wren.[1] 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 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 AR. 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 the observatory was relocated elsewhere in stages in the first half of the 20th century, and the Greenwich site is now maintained as a museum. There had been significant buildings on this land since the reign of William I. Greenwich Palace, near the site of the present-day Maritime Museum, was the birthplace of Henry VIII; the Tudors used Greenwich Castle, which stood on the land that the Observatory presently occupies. 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. The establishment of a Royal Observatory was proposed in 1674 by Sir Jonas Moore who, in his role as Surveyor General at the Ordnance Office, persuaded King Charles II to create the observatory, with John Flamsteed installed as its director.[5] The Ordnance Office was given responsibility for building the Observatory, with Moore providing the key instruments and equipment for 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) out of largely recycled materials on the foundations of Duke Humphrey's Tower, which resulted in the alignment being 13 degrees away from true North, somewhat to Flamsteed's chagrin.

The original observatory at first housed the scientific instruments to be used by Flamsteed in his work on stellar tables, and over time also incorporated additional responsibilities such as marking the official time of day, and housing Her Majesty's Nautical Almanac Office.

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 (3.96 metres) in length mounted above the clock face, giving a period of four seconds and an accuracy, then unparalleled, of seven seconds per day.

British astronomers have long used 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 on 22 October 1884. Subsequently, nations across the world used it as their standard for mapping and timekeeping. The Prime Meridian was marked by a brass strip (presently upgraded to stainless steel) 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 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 was not 0° but 0°00'00.417" (about 8m) 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 2.3m).

This old astronomical prime meridian has been replaced by a more precise prime meridian. When Greenwich 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 the International Terrestrial Reference Frame, use a single oblate spheroid, fixed to the Earth's gravitational centre. The shift from several 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 102.5 metres east of the Greenwich astronomical meridian represented by the stainless steel strip. Thus the strip is now 5.31 arcseconds West. The modern location of the Airy Transit is 51°28'40.1?N 0°0'5.3?W

International time since the end of the 19th century (before 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 coincides in 2013 with a meridian halfway between Airy's circle and the IERS origin : 51°28'40.1247?N 0°0'2.61?W.

Artist biography

John Louis Petit was born at Ashton-under-Lyne, Lancashire, son of John Hayes Petit. He was educated at Eton, and contributed to the "Etonian". He was elected to a scholarship at Trinity College Cambridge in 1822, graduated BA in 1823 and MA in 1826, and on 21st June 1850 he was admitted "ad eundem" at Oxford. He took holy orders in 1824, but undertook no parochial work. Petit showed a taste for sketching in early years. His favourite subject was old churches, and a great part of his life was spent in visiting and sketching them. In 1839 he made his first extensive tour of the continent. The results appeared in his "Remarks on Church Architectire" (1841, 2 vols which had illustratuions. It was followed in 1846 by "Remarks on Church Architectural Character" Royal folio format.In the same year Petit published a lecture which he had delivered on 24th Feb 1846 to the Oxford Society for promoting the study of Gothic Architecture, under the title "Remarks on the Principles of Gothic Architecture as applied to ordinary Parish Churches". It was succeeded by "Architectire of Tewksbury Abbey Church". Royal svo 1846. " Architectural notes in the neighbourhood of Cheltenham"and "Remarks on Wimborne Minster", 1847. "Remarks on Southwell Minster". With numerous good illustrations. 1848. "Architectural Notices relating to Churches in Gloucestershire and Susse". 1849. "Architectural Notices of the Curious Church of Gillingham.Norfolk". And an "Account of Sherborne Minster". 1850. In 1852 Petit published an account of Brinkburn Priory". In 1854 appeared Petit's principal work. "Architectural Studies in France", imperial SVO. It was beautifully illustrated with fine woodcuts and facsimiles of anastic drawings by the author and his companion, Professor Delamotte. It showed much learningand observation, and threw light upon the formation of Gothic in France, and on the differences between English and French Gothic. A new edition , revised by Edward Bell, FSA, with introduction, notes and index, appeared in 1890. The text remained unaltered but the illustrations were reduced in size, and a few added from Petit's unused woodcuts. In 1864-65 he travelled in the East and executed some striking drawings. He died in Lichfield on 2 Dec 1868, from a cold caught while sketching and was buried in St Micheal's Churchyard. Petit was the founder of the British Archealogical Institute at Cambridge in 1844. He was also FSA, an honoury member of the Institute of British Architects, and a governor of Christ's hospital.