TOP of the best seller list in Britain these days is a book called Longitude by David Sobel. It tells the story of the search over many centuries for a method to determine longitude at sea, a search which culminated in the invention of the ship chronometer by John Harrison in the middle of the 18th century.
Lines of latitude and longitude began to crisscross maps at least three centuries before the birth of Christ. The line of zero latitude, the equator, was astronomically fixed, but cartographers were free to choose the location of the prime meridian. Ptolemy, for example, chose it to run through the Canary Islands, while later mapmakers moved it to the Azores, to Rome, Copenhagen, Paris and Jerusalem, before it settled down eventually at Greenwich. But wherever it happened to be, the determination of the angular distance east or west of the zero line was a problem. The solution remained, for many centuries, elusive.
Latitude at any particular spot was easy to determine it could be evaluated by a simple calculation based on the elevation of the sun at local noon. But finding the longitude was more intractable. In principle it could be found by comparing local time with the time at the prime meridian, since it was known that each hour of the time difference accounted for a 15 degree difference in longitude. But in practice it was very difficult, because of lack of accuracy in measuring time.
Clocks of the day were regulated by a pendulum, a device unsuitable for use at sea. However, John Harrison, a Yorkshireman who was born in 1693, designed a timepiece that was governed by a complex system of balance wheels and springs, all carefully assembled to eliminate the effects of changing temperatures and to minimise those of friction. He completed his first version in 1735; it is known today as H-1 and still keeps almost perfect time more than two and a half centuries after it was built.
H-1, however, although more accurate than anything designed before, was bulky and was not portable. Harrison went on to build several revised versions, and 25 years later had completed H-4, a timepiece no bigger than a large pocket watch and of a design we now call a chronometer. On its trial run, H-4 lost only five seconds on an 81 day outward voyage to Jamaica and less than two minutes on the round trip back to Portsmouth and so "the longitude problem" had been solved. The original H 4 can still be seen today in the Royal Maritime Museum at Greenwich, a precision instrument proudly inscribed "John Harrison & Son, AD 1759" and sometimes described as the Mona Lisa of horology.