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Longitude, the Greatest Scientific Problem of the 18th Century

© Wesley Colley

Despite being an enlightening time for many fields, the 18th Century was something of a time-out in astronomy. After the revolutionary work of Copernicus in the 16th Century and Galileo, Tycho, Kepler, and Newton in the 17th Century, the 18th and 19th Centuries almost pale by comparision.

The lack of progress in astronomy can probably be blamed principally upon the single tedious enterprise to which nearly every astronomer was dedicating his career, and that was to "discover the longitude." Despite the best efforts by astronomers, however, it was an English watchmaker who would accomplish the greatest scientific feat of his day. For this report, I have widely consulted the very excellent bestseller on this subject by Dava Sobel (Longitude, 1995, Penguin, New York).

Any sailor worth his salt can tell his latitude by the stars. Unfortunately, the stars don't tell him his longitude unless he knows what time it is at some standard place such as Greenwich (the prime meridian). While today anyone with a wristwatch, a straight stick and a level can tell his longitude, a rather remarkable fact is that neither a wristwatch nor any reliable and portable timepiece existed until the 18th century.

First, here's how to determine your longitude. When you depart, note your longitude and erect a perfect vertical (use the level). Wait until the shadow of the stick is shortest. That's local noon, so set your watch to noon. Now you can set off on your ship. After days at sea, you can check your longitude by repeating the experiment to determine local noon and cross-checking your watch. Multiply the difference (in hours) of the two times by 15 degrees, and that's the difference in the longitude from where you set out. Trouble is, in 1700, who had a wristwatch?

Enter John Harrison, English clockmaker.

In 1707, 2000 sailors were killed when four ships smashed into the island Scilly long before the captain of the fleet had any idea they were close. So great was the problem that the British Parliament offered up £20,000 (millions of 1999 dollars) for a person who could produce a "Practicable and Useful" method of determining the longitude. This seemed so impossible that the current idiom at the time included the expression "discover the longitude" much as those in the first half of this century might have said "put a man on the moon." While building a clock would have solved the problem, astronomers and mathematicians of the time were convinced that charting the timing of certain astronomical events would be accomplished sooner than anyone could build a mechanical gizmo that could stand the harsh extremes of sea-faring.