February 2000

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Longitude, the account of how John Harrison came up with a workable method for accurately determining longitude while at sea, was first published without any more illustration than pictures of Harrison's first four chronometers. I'm very happy that I didn't have to try to understand all the descriptions of star movements, lunar eclipses and clock movements without the aid of the diagrams in this illustrated edition. In addition to line drawings and sketches, there are also reproductions of paintings of many seafaring incidents and people involved in the long search for a way to pinpoint a ship's position.

For many years it was thought that, in a similar way to that used to find latitude, the movement of the moon and stars in relation to the sun could be used to calculate longitude. A method which involved the knowledge of local time and time at a known longitude was dismissed as being impractical as clocks taken to sea were notorious for not keeping time.

John Harrison devoted his life to developing a clock which was not affected by the motion of the waves, or changes in temperature or humidity. With such an instrument the difference in local time and that at the prime longitude could be used to calculate local longitude.

The book recounts the events of the many years it took Harrison and his son to persuade the Board of Longitude in England to accept his chronometer as the best solution to the problem of calculating longitude. Though from an early stage in Harrison's endeavours it was already clear that his clock movements were precise and accurate, influential people had vested interests in the astronomical methods and obstructed the rewarding of the 10000 pound prize to him.

I thought this book was great and plan to see Harrison's clocks at the Maritime Museum in London when I'm there next. The Royal Observatory looks like a good place to visit too.