Historical perspective
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.
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.
The size of the solar system was one of the chief puzzles of 18th century science, much as the nature of dark matter and dark energy are today. In Cook's time astronomers knew that six planets orbited the sun (Uranus, Neptune, and Pluto hadn't been discovered yet), and they knew the relative spacing of those planets. Jupiter, for instance, is 5 times farther from the Sun than Earth. But how far is that … in miles? The absolute distances were unknown.
Venus was the key. Edmund Halley realized this in 1716. As seen from Earth, Venus occasionally crosses the face of the Sun. It looks like a jet-black disk slowly gliding among the Sun's true sunspots. By noting the start- and stop-times of the transit from widely spaced locations on Earth, Halley reasoned, astronomers could calculate the distance to Venus using the principles of parallax. The scale of the rest of the solar system would follow.
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.
Cook's expedition is often likened to a space mission. The Endeavor was not only on a voyage of discovery, it was also a laboratory for testing the latest theories and technologies, much as spaceships are today.
Secret orders from the Navy instructed him to leave the island when the transit was done and "search between Tahiti and New Zealand for a Continent or Land of great extent. For much of the next year Endeavor and her crew scoured the South Pacific, searching for a continent that some 18th century scientists claimed was necessary to balance the great land masses of the northern hemisphere. Along the way Cook met the fierce Maori of New Zealand and the Aborigines of Australia (encounters both races would lament in later years), explored thousands of miles of Kiwi and Aussie coastline, and had a near-disastrous collision with the Great Barrier Reef.
The event will be web cast, broadcast, and targeted by innumerable sidewalk telescopes. In other words, you can't miss it. Look into the inky black disk. It can carry you back to a different place and time: Tahiti, 1769, when much of Earth was still a mystery and the eye at the telescope belonged to a great explorer.
