On August 12, 1768, His Majesty’s Bark Endeavour slipped out of harbor, Lt. James Cook in command, bound for Tahiti. The island had been “discovered” by Europeans only a year before in the South Pacific, a part of Earth so poorly explored mapmakers couldn’t agree if there was a giant continent there … or not. Cook might as well have been going to the Moon or Mars. He would have to steer across thousands of miles of open ocean, with nothing like GPS or even a good wristwatch to keep time for navigation, to find a speck of land only 20 miles across. On the way, dangerous storms could (and did) materialize without warning. Unknown life forms waited in the ocean waters. Cook fully expected half the crew to perish.
Their mission was to reach Tahiti before June 1769, establish themselves among the islanders, and construct an astronomical observatory. Cook and his crew would observe Venus gliding across the face of the Sun, and by doing so measure the size of the solar system. Or so hoped England’s Royal Academy, which sponsored the trip.
But there was a problem. Transits of Venus are rare. They come in pairs, 8 years apart, separated by approximately 120 years. Halley himself would never live to see one. An international team did try to time a Venus transit in 1761, but weather and other factors spoiled most of their data. If Cook and others failed in 1769, every astronomer on Earth would be dead before the next opportunity in 1874.