The explorations of James Cook in the Pacific are well known, but the primary purpose of his first voyage, from 1768 to 1771, is not so extensively recognized. That purpose was to observe the anticipated transit of Venus on June 3, 1769, from Tahiti. In fact there had been a transit in 1761, as noted earlier, and the peculiar tale of that event will be told a little later in the book. For the time being, we pick up the story with Cook having been dispatched to the South Seas on board a small ship named the Endeavour, loaded with men and supplies, but most especially carrying various telescopes, a pendulum clock, English astronomer Charles Green, and Swedish naturalist Dr. Daniel Solander. Also on board was Sir Joseph Banks, who left a long-term mark on British science, serv ing as President of the Royal Society for several decades after his return.
Overcoming mishaps along the way, Cook and his party arrived at Tahiti a couple of months before the transit was due. They needed plenty of time to set up their temporary observatory. Nowadays the location may be denoted Pointe Vénus on a map, one of the few non-Polynesian names thereabouts, but Cook called it Fort Venus, as he had to guard his establishment to stop the indigents from stealing the equipment and supplies. Similarly the island appears in the records of the expedition not as Tahiti, but as King George's Island, for the sovereign. A British party had charted it only a year or so before, just in time for planning Cook's expedition, a good base in the largely unexplored South Pacific being required for the transit timings.
The astronomical observations of the party went well, although with some drawbacks, the significance of which we will come to later. To quote from Cook's personal journal: "Saturday 3rd June. This day prov'd as favourable to our purpose as we could wish, not a Clowd was to be seen the whole day and the Air was perfectly clear, so that we had every advantage we could desire in Observing the whole of the passage of the Planet Venus over the Suns disk: we very distinctly saw an Atmosphere or dusky shade round the body of the Planet which very much disturbed the times of the Contacts particularly the two internal ones." This brings us to the secondary aim of the voyage. Cook took with him a sealed envelope, containing his instructions for the rest of his mission. Although the gist of it seems to have been common knowledge in England, the contingent on board the Endeavour could not have known for sure what those orders were until after the transit. By that time they were already nine months into a voyage that was to last for almost three years.
The secondary task is now well known: explore the southern oceans to search for the hypothesized great southern continent, Terra Australis Incognita, and claim it for the British crown. There is no great landmass in the far southern Pacific, so they sailed that ocean in vain. However, Cook charted and claimed New Zealand and the east coast of Australia, even though the French and especially the Dutch had been there before, with the result that those became British colonies. French Polynesia, from where the transit was observed, is obviously a different story.
Earlier I challenged you to look up "Venus" in a gazetteer, identifying Pointe Venus in that way. If you do the same thing for "Mercury," you will find a small town in Nevada, 60 miles northwest of LasVegas—perhaps because a thermometer's mercury soars in the desert—and also a settlement known as Mercury Bay in New Zealand. This is on the Coromandel Peninsula, east of Auckland, and to the northeast in the Pacific is Great Mercury Island. How did these names come about?
It happens that, like 1631, 1769 was a double transit year. After the one by Venus in June, Cook knew that Mercury would also pass over the face of the Sun in November. This event he planned to put to a different purpose. Concurrent lunar eclipse observations from separate locations allow their difference in longitude to be derived, by comparing the eclipse timings to the local solar time. This method had been used, for example, to determine the distance west from London to the Caribbean and the Americas, certain eclipses being visible both from there and back in Europe. Cook, however, was right around the other side of the planet, meaning that he could not watch an eclipse at the same instant as astronomers viewed it from England.
The instant of the transit of Mercury had been precalculated with some precision. It provided a natural clock in the sky. By comparing the time at which it was observed with the local time according to the Sun, Cook could determine the longitude of New Zealand. For this reason he scouted the North Island, eventually choosing the place now called Mercury Bay because the Maoris there seemed less hostile than elsewhere.
The rival method to astronomical observations for determining longitude at sea was John Harrison's chronometer, which eventually triumphed. On this first voyage Cook had no marine clock, so he needed to rely on astronomy for determining time and longitude. The transit of Mercury in late 1769 provided a particular opportunity. At other places he used both the lunar distance technique, and the eclipses of the four Galilean moons ofJupiter, methods mentioned in Chapter 3. On his subsequent voyages of discovery Cook had an excellent chronometer on board, but still made astronomical observations to verify that clock's accuracy.
Cook eventually arrived back in England in 1771, after many tribulations, with his much-awaited transit timings. The astronomer Green did not make it, having died on ship. Our next port of call is the usage that was made of the transit observations, but to understand that we must step back to 1761.
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