It is time to bring to a close this story of the 18th-century transits of Venus and the often amazing expeditions to the ends of the earth that they engendered. The purpose in measuring and timing the passage of Venus across the face of the sun on the very rare occasions it is seen to do this was to establish the scale of the solar system (and eventually the scale of the universe itself). Observers had to be sent to very distant parts of the earth because the longer the baseline between them, the more accurate would be the result, and in the ill-explored world of the 1760s this would cost more than one of them his life. But before we turn to the ultimate results of these undertakings we must look at one more of the expeditions, the most famous of them all, the British expedition to the South Pacific for the 1769 transit.

Early analysis of the 1761 transit observations was not entirely satisfactory, and it was expected that the 1769 transit (the last for more than a century) would offer better results. By 1765 Thomas Hornsby, Savilian Professor of Astronomy at Oxford, was urging the European powers to prepare their expeditions: "Posterity must reflect with infinite regret their negligence or remissness; because the loss cannot be repaired by the united efforts of industry, genius, or power." Calculation showed that the South Pacific, as yet hardly explored by Europeans, would be a desirable station, and in case science should not prove attraction enough, Hornsby noted that it would be a "worthy object of attention to a commercial nation to make a settlement in the great Pacific Ocean." Thus it was that the British expedition to the Pacific would have far more hopes behind it than merely establishing the scale of the solar system. Commerce, politics and empire were not to be denied. The Royal Society of London's estimate that £4,000 would be needed to mount the expedition met with little argument, and an appeal to the 30-year-old King George III was launched. "The Memorialists, attentive to the true end for which they were founded by Your Majesty's Royal Predecessor . . . conceived it to be their duty to lay their sentiments before Your Majesty with all humility, and submit the same to Your Majesty's Royal Consideration." Royal Consideration quickly arrived at acquiescence.

The Society had among its fellows just the man to command such an expedition: Alexander Dalrymple, a former professional sailor with much experience in eastern seas and an adept geographer and navigator. But where to find a ship? Clearly the Royal Navy must be the answer, as it had been for Mason and Dixon years before. And then a major snag. The Admiralty, it seemed, had never forgotten the last time it had allowed an astronomer, Edmond Halley, to command one of its ships on a scientific expedition (see Marginalia, January–February 1986). The result had been mutiny and the near loss of the ship. The First Lord of the Admiralty, Sir Edward Hawke, rather extravagantly announced he would sooner suffer his right hand to be cut off than sign another such commission. So Dalrymple was out. The Admiralty would find its own man. They picked a junior officer, then doing marine survey work on the St. Lawrence River in Quebec. His name was James Cook, the ship he was to command, the Endeavour.