The Age of Sail and Steam - The Science of Navigation - page 173

dangerous waters: even Captain Cook nearly lost his ship on the coral. To
confirm your GPS position, you bring out a sextant and do it the old-
fashioned way (a ''three-point sextant fix'' method that dates from 1775),
making use of those marvelously detailed and accurate charts that earlier
generations of seafarers had risked their lives to create after making ar-
duous journeys and many thousands of survey measurements. Your charts
indicate the distances between three landmarks, A , B , and C , that you see
along the nearby coastline (fig. 7.3). By measuring two angles between
these landmarks, and using some navigator's geometry, you fix your posi-
tion. The method is summarized in figure 7.3. 3
Captain Cook
James Cook lived in a time of scientific enlightenment (which kick-started
the Industrial Revolution). He had been appointed by the British Admiralty
to lead a scientific expedition to Tahiti to observe a rare astronomical event,
the transit of Venus, which was predicted to occur in June 1769. 4 A transit
occurs when a planet passes in front of the sun. In this case the scientific
community wanted three observations from widely spaced locations on
Earth (Tahiti, the North Cape of Norway, and Hudson's Bay). In each
location, scientists would measure the time taken for Venus to cross the
sun, and from these measurements it was hoped that the scale of the solar
system could be estimated—in particular, the distance from the earth to the
sun. (It turned out that the measurements from all three locations were
insu≈ciently accurate to provide a meaningful estimate of this distance.)
Cook and his ship, Endeavour , were both unusual for such an expedition.
The Admiralty was happy to carry out this scientific mission because it had
an ulterior motive, as we will see. Endeavour was not a warship, but a
humble collier, converted for Cook's first expedition (fig. 7.4). 5 However,
she was very strong and an excellent sailor. The 40-year-old Cook was from
equally humble origins, and his maritime credentials had been established
3. The three-point sextant fix method is discussed in, e.g., Mills (1980). Australia had
been discovered by the Dutch and British; later, Cook was prominent among the explorers
who provided detailed surveys of the coastline. Indeed, Cook had earlier made his name in
North America on the basis of his surveying skills.
4. Much has been written by and about James Cook. See, for example, Allen (1980),
Brown (2003), Herman (2005, chap. 13), Holmes (2010), Owen (1979), and Price (1971).
For more on the transit of Venus measurement, see Danson (2006).
5. Endeavour was a bark —a sailing ship with three masts, square-rigged on the front two
and fore-and-aft rigged on the aftermost (mizzen) mast.