Saturday, April 11, 2009

Timekeeping and Longitude

One of the most pressing problems of navigation during Captain James Cook's time was the inability to ac­curately calculate longitude. For many years, sailors could find their latitude with the use of celestial navigation. An instrument called a quadrant was used to sight a particular star or the sun, and then the angle between the horizon, the star, were measured. Latitude could then be calculated from this measurement.

Longitude was another problem altogether, to be able to calculate longitude, one needed to know exactly what date and time it was to complete the celestial calculation. In 1714, the Longitude Act was passed in Britain. There were mon­etary prizes of 20,000 pounds for a method to determine longitude to an accuracy of half a degree of a great circle, 15,000 pounds for a method accurate to within two-thirds of a de­gree, and 10,000 pounds for a method accurate to within one degree. One "degree" would be about 60 miles.

There were two major schools of thought on the subject of longitude: those who believed that accurate star and lunar charts alone could lead nav­igators to an accurate measurement of longitude, and those who believed that an accurate timekeeping device would suffice for the same reason. Each had problems, the celestial option had the issues that on cloudy days celestial navigation was troublesome and that one had to be an able mathematician to perform the necessary calculations, and the timekeeper's problem was that no clock yet created was nearly accurate enough to keep correct time over long periods, nor during varying levels of temperature and humidity.

The main figures representing each side of this argument were Nevil Maskelyne on the side of celestial navigation, and John Harrison on the side of the clockmakers. One problem for Harrison was that Maskelyne had a very important position: He was Astronomer Royal. Captain Cook and his scientists carried out tests for the Board of Longitude, comparing the lunar distance method with chronometers modeled after Harrison's designs. He took a chronometer named K-l (Larcum Kendall's first, and a close copy of H-4, Harrison's fourth at­tempt), and three copies of it by a clockmaker named John Arnold. Cook's log refers to K-l on manyocca­sions, calling it "our trusty friend the watch," and noting "It would not be doing justice to Mr. Harrison and Mr. Kendall if I did not own that we have received very great assistance from this useful and valuable timepiece." Cook believed so much in the use of the chronometer that he carried it on his third voyage as well.

The lunar dis­tance method, which an able navigator such as Cook could master, worked effectively, but only under clear skies. Cook preferred the chronometer, however, since Nevil Maskelyne was on the Board of Longitude, Harrison did not receive the full award until the king intervened for him.