Sailing East to West out of Gosport is easy enough as it is possible to use the English coastline to determine where you are. But passing through the Needles Fairway means losing sight of the land on the West of the Isle of Wight. However, even that far out to sea it is possible to spy the town of Bournemouth. This means always have some form of bearing. The problem emerges when the mist comes down or night sailing and this demands that sailors rely on navigational techniques. For hundreds of years this meant using a sextant, a compass and charts. The margin of error in using such techniques was huge and a testament to this is shown by the number of graves around the coast of sailors who met their end in shipwrecks. So the ability to precisely determine a location was literally a lifeline for mariners when then President Reagan allowed with the introduction of the Global Positioning System (GPS) for civilian use.

A network of 24 orbiting satellites help to pinpoint latitude, longitude and even altitude and was credited in the successful rescue of Captain Thomas “TO” Hanford in 1995, after he was shot down by a Serbian missile. The GPS system helped US Marines locate Hanford and extract him just four hours later, even though up until that point, the USAF had been unable to even determine if Hanford was alive behind enemy lines.

Such technology would seem incredible to mariners of past – say in the 1970s but nowadays everybody has access to GPS. In 2008, Nokia said that half of all mobile-phone sales were attributable to their capacity as navigation devices. This year, the global market for GPS-enabled gadgets is expected to top £15bn and Google’s stand alone Google Earth has been downloaded more than 500 million times since being launched in 2005. Instead of using the GPS with Google maps to extract shot-down personnel in military conflict zones, most people in Hendon are using the technology just to find their way to a restaurant or through a Tom Tom to find their way to a friend in Harrow or Hampstead. While I won’t belittle the introduction of such technology, maybe just its everyday use, we should remember a famous figure from history whose very early work was pivotal in allowing the GPS to be invented. This man, John Harrison, retains a link with our area and if it was not for him many people would still be looking for that branch of Pizza Express on the Finchley Road.

John Harrison was not the inventor of longitude and early sailors were well aware of the principle behind calculating longitude. They knew that for every 15 degrees travelled eastward, the local time moves forward one hour. Likewise, they knew it moves back one hour for every 15 degrees travelled west. So, they understood that if they had the local time at two points on Earth, they could use the difference between them to calculate longitude.

Longitude is the angular distance east or west from a standard meridian, such as Greenwich, to the meridian of any place. On global maps, longitude lines loop from the North Pole to the South in great circles that converge at the ends of the Earth.

When trying to navigate your way across an open ocean, the ability to calculate longitude is vital.

The longitude problem was perhaps the most vexing of the 18th Century. Galileo Galilei, Jean Dominique Cassini, Sir Isaac Newton, and Edmond Halley all tried to solve it, but none of them managed it. The Government of the great maritime nations - Spain, the Netherlands and certain states of Italy periodically offered money for anyone who could come up with a practical solution to the problem of longitude. It finally took the British Parliament in its famed Longitude Act of 1714 to set the highest prize, equal to a king’s ransom (several million pounds in today’s currency) for a ‘practicable and useful’ means of determining longitude.

Though Harrison, a simple, uneducated man, achieved a solution, it brought him into direct conflict with leading lights of his day. His most infamous enemy was the Reverend Nevil Maskelyne, the fifth astronomer royal and sometime curate of the church at Chipping Barnet. Maskelyne and others thought the answer lay in the ‘clockwork’ of the heavens - in mapping the stars. The commissioners charged with awarding the longitude prize, Maskelyne amongst them, changed the rules of the contest whenever they saw fit so as to favour the chances of astronomers over the likes of Harrison and his fellow ‘mechanics’. But in 1764, after 40 years of work, Harrison proved that a clock could be used to locate a ship's position at sea with extraordinary accuracy. It is down to Harrison – and his son – that we can now all use technology based on the longitude system to get a pizza.

Harrison died on his eighty-third birthday and what makes his story more special is that he was and remains buried in the graveyard of St John's Church in Hampstead, on the left hand side to the entrance.