Royal Observatory, Greenwich (1675): Charles II's Observatory

The Royal Observatory at Greenwich was founded by King Charles II, who signed the warrant for its construction on 22 June 1675. Built on a hill in Greenwich Park overlooking the Thames, it was created to chart the stars and the Moon so accurately that sailors could find their longitude at sea. Over the following two centuries its work made Greenwich the starting point for the world’s longitude and time zones.

Background: the longitude problem

By the 17th century sailors could find their latitude, their position north or south, by measuring the height of the Sun or the Pole Star. Longitude, their position east or west, was far harder. It required knowing the time at a reference place while at sea, and no clock of the period could keep accurate time on a rolling ship.

One proposed solution was the “lunar distance” method. The Moon moves steadily against the background stars, so by measuring its angle from a known star, a navigator could in principle read the time as if from a clock in the sky. But this needed very accurate star positions and tables predicting the Moon’s motion, which did not yet exist.

In 1674 a Frenchman calling himself the Sieur de St Pierre proposed to Charles II a method of finding longitude by the Moon. The king referred it to a commission, which consulted the young astronomer John Flamsteed. Flamsteed reported that the idea could not work because existing star catalogues and lunar tables were too inaccurate, and that the only remedy was a programme of new observations.

Founding the observatory

Charles II, who had been restored to the throne in 1660 and had already chartered the Royal Society, responded by creating a new post. A royal warrant of 4 March 1675 appointed Flamsteed as “astronomical observator” with a salary of £100 a year, charged with correcting the tables of the heavens and the places of the fixed stars in order to find the longitude for the improvement of navigation.

The warrant of 22 June 1675 ordered an observatory to be built in Greenwich Park, on the foundations of an old castle known as Duke Humphrey’s Tower. Sir Christopher Wren, the architect of the new St Paul’s Cathedral and a former professor of astronomy, chose the site and designed the building. Wren later remarked that it was built partly for the observer’s living quarters and partly for show.

The foundation stone was laid on 10 August 1675. Money was tight: the cost of around £520 was met partly by selling spoilt gunpowder, and bricks, timber and iron were reused from other royal buildings, including a gatehouse at the Tower of London. The building, now called Flamsteed House, was finished in 1676, and Flamsteed moved in that July.

Flamsteed’s work

The observatory was poorly equipped by the crown. Flamsteed relied on instruments he bought or borrowed, helped by the mathematician and Surveyor of the Ordnance Sir Jonas Moore, who paid for two fine clocks by Thomas Tompion. These clocks, with 13-foot pendulums, helped Flamsteed show that the Earth rotates at a steady rate.

Over more than forty years Flamsteed made tens of thousands of careful observations of stars, planets and the Moon. He was a perfectionist who was reluctant to publish before his work was complete. This led to a bitter quarrel with Isaac Newton, then President of the Royal Society, who needed lunar data for his theory of the Moon and the Principia. In 1712 Newton and Edmond Halley published an unauthorised edition of Flamsteed’s observations. Flamsteed later obtained around 300 of the 400 printed copies and burned them.

His own catalogue, the Historia Coelestis Britannica, was published in 1725, six years after his death. It listed nearly 3,000 stars with unprecedented accuracy and became a standard reference.

Instruments and daily routine

Flamsteed’s most important instrument was a mural arc, a large graduated arc fixed to a wall running north to south. By timing the moment a star crossed the meridian and measuring its height, he could fix its position precisely. After an early instrument proved unsatisfactory, he had a new mural arc of about seven feet radius built in 1689 by the young instrument maker Abraham Sharp, largely at his own expense. With it he reached an accuracy of around ten to fifteen seconds of arc, far better than the catalogue of Tycho Brahe a century earlier.

The work was relentless. Flamsteed observed on clear nights for decades, often with only a single assistant, and spent his days reducing the observations by hand. To supplement his small salary he took private pupils and later became rector of Burstow in Surrey. His health was poor, and he suffered from headaches and other ailments throughout his life.

The main room of Flamsteed House, the tall Octagon Room, was designed for viewing comets and eclipses through its high windows and housed Tompion’s clocks. It was less useful for precise work, which was done in a smaller building in the garden. The Octagon Room survives today as one of the best-preserved interiors by Wren.

Later Astronomers Royal

Flamsteed was followed as Astronomer Royal by Edmond Halley (1720), famous for his comet prediction, and then by James Bradley (1742), who discovered the aberration of starlight and nutation of the Earth’s axis. Nevil Maskelyne, the fifth Astronomer Royal, launched the Nautical Almanac with tables for 1767, making the lunar distance method practical for ordinary navigators.

At the same time John Harrison’s marine timekeepers offered another solution, and the two methods were used side by side. In 1833 a red time ball was installed on the roof of Flamsteed House; it dropped at 1 p.m. each day so that ships in the Thames could set their chronometers.

Greenwich Mean Time and the prime meridian

In 1851 the seventh Astronomer Royal, George Biddell Airy, installed a new transit circle telescope. The line through this instrument became the Greenwich meridian. Railways adopted Greenwich time across Britain from the 1840s, and in 1880 it became legal time in Great Britain.

Because most of the world’s shipping used charts based on Greenwich, the International Meridian Conference in Washington, D.C., voted in October 1884 to adopt the Greenwich meridian as the prime meridian, longitude zero, for the world. Greenwich Mean Time became the basis of the world’s time zones.

Later history

Light pollution and smoke from London made Greenwich a poor place for astronomy in the 20th century. After the Second World War the Royal Greenwich Observatory moved to Herstmonceux Castle in Sussex between 1948 and 1957, and in 1990 to Cambridge, where it closed in 1998. The original site is now a museum and planetarium, where visitors stand astride the meridian line.

Timeline

• 1675, 4 March: John Flamsteed appointed by royal warrant.
• 1675, 22 June: Charles II signs the warrant to build the observatory.
• 1675, 10 August: Foundation stone laid.
• 1676, July: Flamsteed moves into the completed building.
• 1725: Flamsteed’s Historia Coelestis Britannica published.
• 1767: First tables of the Nautical Almanac produced at Greenwich.
• 1851: Airy’s transit circle defines the Greenwich meridian.
• 1884, October: International Meridian Conference adopts Greenwich as the prime meridian.
• 1998: The Royal Greenwich Observatory closes in Cambridge.

Why it matters

The Royal Observatory was one of the first state-funded scientific institutions in Britain and was founded for a practical goal: safer navigation. Its observations improved maps and charts, and its meridian still defines the world’s longitude and time zones.