K1 chronometer, courtesy National Maritime Museum UK
Yesterday afternoon (Tuesday 24 July 2007) Sydney Observatory staff farewelled the Director of the Director of the Powerhouse Museum, Dr Kevin Fewster. Dr Fewster has been appointed the Director of the National Maritime Museum at Greenwich UK that includes the Royal Observatory, Greenwich, the centre of time and space on Earth.
Naturally, the talk turned to time measurement and the wonderful Harrison clocks on show at Greenwich. Dava Sobel has ably told the story of the clocks and their maker John Harrison in the best-selling book Longitude.
Interestingly Captain James Cook on his first voyage did not have a chronometer on board his ship the Endeavour. H4 was still the only one then in existence and that was far too precious to send on a highly risky voyage. So how did Cook find the longitude, that is how far he was east or west of Greenwich? He used the method of lunar distances. In one of the great coincidences in the history of science two techniques of determining longitude at sea were developed at the same time, lunar distances and chronometers.
Lunar distances involve measuring the angular distances of stars from the Moon. More precisely three observations are made, of a star’s elevation above the horizon, of the Moon’s elevation of the horizon and of the angular separation between the star and the Moon. This is then repeated with another star. The measurements of angular distance are made with a sextant, an instrument based on the previously invented octant, but modified to be able to measure larger angles for use with lunar distances.
Sextant, image Nick Lomb
After taking the observations Cook had to do long and laborious calculations to establish the ship’s position. To assist he used the Nautical Almanac published at Greenwich. Unfortunately, at the start of the three year voyage in 1768 only the almanacs for 1768 and 1769 were available so they were the only ones he could take on the voyage.
For his subsequent voyages Cook did take chronometers. Kenneth Slessor has a wonderful poem, Five Visions of Captain Cook. The short extract below refers to two chronometers including K1, Larcum Kendall’s famous replica of H4, which is pictured at the top of this post:
Two chronometers the captain had,
One by Arnold that ran like mad,
One by Kendal in a walnut case,
Poor devoted creature with a hangdog face.
Arnold always hurried with a crazed click-click
Dancing over Greenwich like a lunatic,
Kendal panted faithfully his watch-dog beat,
Climbing out of Yesterday with sticky little feet.