Free The Planets by Dava Sobel

monsoons, dysentery, floods, severe cold—to cover prime observing sites in Africa, India, Russia, and Canada, as well as several European cities. Clouds foil most of the expeditions, however, and astronomers’ indeterminate results focus even greater attention on the next opportunity, in 1769, which dispatches 151 official observers to 77 locations around the world.
    Each group must time the four crucial moments of the transit, called “contacts,” when Venus and the Sun touch rim to rim. The first occurs as Venus appears to attach herself to the outside of the Sun’s circle. Second contact soon follows, as Venus enters fully inside the Sun’s embrace, but it takes hours for her to achieve third contact on thefar side of the solar disk. By fourth contact she has already exited the Sun, and stands on the brink of separation.
    Responsibility for the Royal Society’s all-important observations at King George III Island (Tahiti) falls to Lieutenant James Cook. He sets out from England the year before, in August of 1768, so as to arrive in time to make preparations that include the building of a secure observatory, Fort Venus.
    “Saturday 3 rd June [1769]. 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. D r Solander observed as well as M r Green and my self, and we differ’d from one another in observing the times of the Contacts much more than could be expected. M r Greens Telescope and mine were of the same Mag[n]ifying power but that of the D r was greater then ours.”
    Through no one’s fault, astronomers everywhereencounter the same difficulties as Cook’s men in judging the exact moments of Venus’s entry into and exit from the Sun’s disk. The limitations of even the best available optics undermine everyone’s results, and the international astronomical community must be content with merely narrowing the earth-Sun distance to something between 92 and 96 million miles.
    Cook turns his attention from Venus to the second, secret part of his instructions—a sortie through the icy sea in search of the great southern Terra Incognita. Failing to find it on this quest, he returns home, but mounts a second discovery attempt in 1772. Through three cold years of effort Cook, now made Captain, becomes adept at turning his ship frequently into the wind to shake the snow from her sails.
    “Monday 6 th February [1775]. We continued to steer to the South and SE till noon at which time we were in the Latitude of 58° 15′ S Longitude 21′ 34′ West and seeing neither land nor signs of any, I concluded that what we had seen which I named Sandwich Land was either a group of Isles &c a or else a point of the Continent, for I firmly believe that there is a tract of land near the Pole, which is the source of most of the ice which is spread overthis vast Southern Ocean….I mean a land of some considerable extent….It is however true that the greatest part of this Southern Continent (supposing there is one) must lay within the Polar Circle where the Sea is so pestered with ice that the land is thereby inaccessible.”
    Cook’s reckoning of latitude and longitude surpasses the accuracy of all who preceded him in such pursuits. By tracking the motion of the Moon against the stars—a method Halley helped to develop—and with the aid of a new timekeeper that keeps up with the master clock back home at the Greenwich Observatory, Cook knows exactly where he is. His maps show others the way from Success Bay in Tierra del Fuego, his source of wood and water, to Botany Bay in Australia, which he named for its abundance of new plant species, and Poverty