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Longitude

Dava Sobel

For those navigating at sea, it has long been possible to find one’s latitude. But finding longitude has been another matter. To know the longitude of a ship it was necessary to know the exact time aboard the ship and simultaneously the exact time at the ship’s home port or other place of known longitude. Dead reckoning was not always successful and in 1707, four British warships ran aground near the Scilly Isles near the southwestern tip of England with the loss of more than 2000 lives. The problem became so severe that in 1714 parliament passed the Longitude Act with a prize of £20,000 for whoever first devised a method for determining longitude within very narrow specified limits. The act also established a Longitude Committee to judge any submitted methods and to award the prize if it was merited.

Most of those working on the problem looked to the sky and the predicted regularity of the heavenly bodies, but John Harrison chose to try building a very accurate clock that could remain accurate even in a shipboard environment. Harrison was without formal education. He taught himself to read and write. He had never been apprenticed to a clock maker, yet he set a goal of making a clock that would meet all the requirements.

One of the members of the Longitude Committee was the Royal Astronomer who was also working to win the prize. He believed that the prize should not be won by an uneducated man-a real scientist should win the prize. The result was that when Harrison appeared to have won the prize the committee changed the rules, requiring Harrison to do more work to meet the new conditions. The Royal Astronomer developed his own method using the distance between the moon and stars or the moon and the sun-known as measuring lunars. One of the problems with the lunar method was that after the distances were measured it required four hours of calculating to get the answer. All of this took forty or fifty years and Harrison became an old man. He felt aggrieved and appealed to King George III who granted him a personal interview. The king agreed that Harrison had been wronged and ordered the committee to award Harrison the prize.

I am now reading a book in which the author points out that for a trip across the Atlantic Harrison’s chronometer was fine, but for eighteenth century explorers who might be out for a year or more, the chronometer was too inaccurate and the method of “lunars” was more satisfactory.

— Warren Langdon

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