Spending Measured Time Around Greenwich

Marcus Aurelius Antoninus once said 'Time is like a river made up of the events which happen? no sooner does anything appear than it is swept away, and another comes in its place'. Many centuries later, these words could have applied to the problems encountered at sea by captains and their crews where, over and over again, ships were wrecked because of a lack of navigational aids. Until the 17th century, captains could tell the ship's position of latitude by the stars and the sun, but longitude was a real problem: they had no way of telling how far east or west a ship was positioned.

It was Charles II who rescued Greenwich after it fell on hard times under Oliver Cromwell. Previously the birthplace of Henry VIII and his daughters Mary I and Elizabeth I, Greenwich was given a Royal Observatory in 1675 to try to provide seafarers with more accurate charts. The first Astronomer Royal was John Flamsteed who took the job at the age of 28. During his 45 years at the observatory, he made more than 50,000 observations. There was just one problem - in this time he had not arrived at a solution for longitude. The last straw for the government came in 1707 when four Royal Navy ships sunk off the Isles of Scilly, claiming 2,000 lives. Parliament offered a reward of £20,000 for anyone finding a solution to the longitude problem. It was eventually claimed - by a clockmaker.

John Harrison (1693-1776) came down to London in 1730 with the idea that longitude could be worked out by using the time. His solution was based on the fact that for every 15 degrees travelled eastwards, the local time moves one hour ahead. Therefore, if we know the local times at two points on Earth, we can use the difference between them to calculate how far apart those places are in longitude, east or west. This idea was crucial to sailors in the 17th century. The problem was that every minute gained or lost could consequently amount to a navigational error of 15 nautical miles (17¼ miles or 28.9km) so the solution hinged on producing a clock that kept the exact time in turbulent conditions. Harrison made four clocks for this purpose. The first was designed to run consistently, regardless of movement or temperature changes. He made a second clock, then took 19 years to work on the third prototype, the H3. However, it was his fourth, the H4, which clinched the deal. The most compact, it was the forerunner of all precision watches and provided the basis for an accurate sea chronometer which finally enabled sailors to work out their exact position. Ironically, Harrison was well in to his seventies before he received any prize money. Despite the success of his machines, the committee responsible for paying him stalled repeatedly and only the intervention of King George III secured him recognition and his just reward.

In 1884 it was decided, at an international conference in Washington DC, that Greenwich should become the site of the prime meridian, an imaginary line running north to south, denoting the world's longitudinal zero. This means that every position on Earth is defined by its longitude (distance east or west) from Greenwich. With a little help from Charles II and Flamsteed, John Harrison left the world a timeless legacy.

While you're there

Spare a thought for the peasants who, led by Wat Tyler, gathered on Blackheath in revolt against their feudal serfdom in 1381. As labourers were scarce after the ravages of the Black Death, they were in a good position to demand better pay, but a bill was passed to ensure they could earn no more than a decreed amount. The Peasants Revolt saw popular uprisings throughout southern England. The Lord Chancellor and Archbishop of Canterbury were murdered, before Tyler was betrayed and killed near Smithfield in the City.

Where to eat and drink

The Trafalgar Tavern is a large riverside pub with a grand, wood-panelled dining room that was famous for its 'white-bait suppers' in the 19th century. Dickens mentions this room in his novel, Our Mutual Friend. Nowadays, the menu still features fish along with traditional English dishes such as sausages and mash, and lamb with rosemary. On Walk 9 you will pass the Hare and Billet pub, which has a privileged position by the heath. It offers a Sunday roast and a good variety of Belgian beer including Bellevue, Duvel and Hoegaarden.

What to look for

The Greenwich Foot Tunnel was built in 1902 to link Greenwich with the Isle of Dogs, so workers from south London could get to the docks. The wood-panelled lifts that take you underground were replaced in the 1990s. They are enormous and quite a contrast from the starkness of the foot tunnel. The lift attendants also have use of a small electric heater in winter so, if you're not warm after the 400yd (366m) walk along the foot tunnel, you soon will be.