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Royal Observatory
Greenwich, Greater London, UK
associated engineer
Not known
date  1675
UK era  Stuart  |  category  Building  |  reference  TQ387773
The Royal Observatory, designed by Sir Christopher Wren, was built on the site of Greenwich Castle. It is one of the most important historical scientific sites in the world, lying on the Prime Meridian. It gained World Heritage status in 1997.
The original building was constructed for the first Astronomer Royal, John Flamsteed (1649-1719), with a budget of just £500. To save money it was built onto the foundations of the castle’s tower, once owned by Humphrey, Duke of Gloucester (brother of King Henry V).
Wren (1632-1723), a keen astronomer himself, designed the first building (later known as Flamsteed House) on the orders of King Charles II. Flamsteed House contains London's only public camera obscura. The first floor Octagon Room has large windows for observing the sky, with living quarters for the Astronomer Royal on the ground floor.
The observatory's purpose was to improve navigation at sea by finding a method to measure longitude by astronomical means, and in the process to measure time accurately. However, none of the walls of Flamsteed House were aligned with a meridian, so positional observations had to be made from an outbuilding in the grounds.
The Octagon Room also had the two best clocks of the era, accurate to +/- 7 seconds per day. They were made by Thomas Tompion (1639-1713) and had pendulums almost 4m long suspended above them, which enabled Flamsteed to prove that the Earth has a uniform rate of axial spin (one hour to travel 15 degrees of longitude). The clocks, and other scientific apparatus, were removed by Flamsteed's widow after his death and most have been lost, although one of the Tompion clocks was returned to the observatory in 1994.
Flamsteed never solved the problem of longitude, but his book Historia Coelestis Britannica, published posthumously in 1725, helped John Harrison (1693-1776) to calculate longitude with his marine chronometers. The most accurate of them, H4, was made in 1755-59.
The Royal Observatory is the location of the Prime Meridian — the line that divides the Earth's eastern and western hemispheres. The position of the meridian, longitude 0 degrees, is defined by the cross-hairs in the eyepiece of the Transit Circle Telescope in the Meridian Building. The telescope was built by the seventh Astronomer Royal, Sir George Biddell Airy (1801-92), in 1850. Since December 1999, the meridian has been marked by a green laser beam.
Until the Victorian expansion of railways and telegraph, most British towns had their own local time. But global travel and communications dictated the need for an international time standard. In 1884, the Greenwich Meridian was chosen as the Prime Meridian — a decision influenced by the USA already using Greenwich Mean Time as a base for its time zones, and navigational charts being plotted relative to the Greenwich Meridian.
The observatory has a famous '28-inch refracting telescope', the largest of its this type in the UK and the seventh largest in the world. It uses lenses rather than light to collect the illumination required for observations.
The telescope was built by leading Irish optical manufacturer Howard Grubb, to replace an earlier 12¾-inch (324mm) Merz telescope. It was completed in 1893 and first used in 1894, for research into double star systems until its retirement in the late 1960s.
The telescope is 8.5m long and weighs some 1.4 tonnes. Uniquely, the ends of the telescope’s tube are round but rectangular in the centre. It can be kept focused on a fixed point in space as the Earth rotates, by means of the motorised English equatorial mounting. Its objective (top) lens is 711mm (28 inches) in diameter and weighs 102kg.
The refracting telescope is housed in the so-called 'onion' dome, constructed originally in 1859 for the Merz telescope. The dome's maximum radius was increased by 1.5m more than the supporting tower walls beneath in 1893 to accommodate the larger instrument. The dome itself was made from papier maché supported on a riveted iron frame. It was destroyed during World War II (1939-45) by a V1 flying bomb. The telescope was removed in 1947, and the dome was dismantled in 1953.
The Grubb telescope was moved from Greenwich to Herstmonceux in 1957, but was returned to Greenwich in 1971. Its covering dome was replicated in fibreglass in 1974.
The Royal Observatory facility moved to Herstmonceux 1947-57, then to Cambridge in 1990, primarily because of light pollution in London affecting observations. In 1960, the National Maritime Museum took responsibility for the care of Flamsteed House and other buildings on the site — restoring them all by 1967.
The Cambridge facility closed in October 1998, and the Greenwich site is once more known as the Royal Observatory, although it remains part of the National Maritime Museum.
In May 2007, redevelopment of the site was completed at a cost of £15 million and includes a new planetarium, new galleries and an education centre.
Architect: Sir Christopher Wren
Refracting telescope (1885-93): Howard Grubb
Lenses for refracting telescope (1888): Chance Brothers, Birmingham
Dome modifications (1893): T. Cooke & Sons
Research: ECPK

Royal Observatory