Royal Observatory Greenwich: The Story Behind the Prime Meridian and Greenwich Mean Time

A French astronomer’s failed longitude claim, aired at court in December 1674, was enough to convince King Charles II that England needed its own answer. On 4 March 1675 the king signed a warrant appointing John Flamsteed as the first Astronomer Royal, charged with correcting the star tables so ships could finally fix their position at sea. Christopher Wren, helped by Robert Hooke, designed the building on the ruined foundations of Duke Humphrey’s Tower, an old fort on the same hill; Flamsteed laid the foundation stone on 10 August 1675 and moved in on 10 July 1676.

The whole project ran on a shoestring: the same warrant ordered the building paid for by selling off old, decayed gunpowder from the Board of Ordnance’s stores, capped at £500. The final bill came to £520 9s 1d, slightly over budget even with salvaged materials and secondhand fittings doing most of the work.

Wren’s showpiece room sits above Flamsteed’s old quarters: eight sides, tall windows, and, in his own words, built “a little for pompe” as well as for astronomy. Because the building reused an old fort’s foundations rather than a true north-south layout, none of the Octagon Room’s walls line up with the meridian, so Flamsteed did his real star-charting from a small brick shed in the garden outside instead. The room still earned its keep: Sir Jonas Moore, the Board of Ordnance official who backed Flamsteed, commissioned clockmaker Thomas Tompion to build two year-going clocks for it in 1676.

Each had a 13-foot pendulum, hidden away behind the wainscot in the corner rather than swinging in view, and kept time far more evenly than anything before it. Flamsteed used that precision to establish something no one had proven: that the Earth’s own rotation was constant, the basic assumption every later time-and-longitude calculation from this hill would rest on.

Flamsteed spent four decades cataloguing more than 50,000 star positions, work published as the Historia Coelestis Britannicae and far more precise than anything before it. His astronomical method was one answer to the longitude problem; John Harrison’s marine clocks, on display inside Flamsteed House today, were the other. A later Astronomer Royal, Nevil Maskelyne, turned Flamsteed’s approach into something a ship’s officer could actually use: the Nautical Almanac, first published in 1767, with the Observatory’s own tables built in.

By the 1880s, roughly two-thirds of the world’s shipping already navigated by charts fixed to this hill, so when the International Meridian Conference chose a Prime Meridian in 1884, Greenwich was the practical choice. The vote in Washington was not unanimous: San Domingo voted against it, and France and Brazil abstained rather than back a British line.