Transits of Venus

The 2004 Transit of Venus was special to those who witnessed this once in a lifetime event. History records that in 1769 Captain James Cook was similarly enthralled by a Transit of Venus.

Sea gulls fluttered upward, screeching. City odours drifted in from Plymouth, across the ship, shoving aside the salt air. Sails snapped taut. The wind had changed and it was time to go.

On August 12th, 1768, His Majesty's Bark Endeavour slipped out of harbour, 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 mapmaker's 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.

It was worth the risk, he figured, to observe a transit of Venus.

"At 2 pm got under sail and put to sea having on board 94 persons," Cook noted in his log. The ship's young naturalist Joseph Banks was more romantic: "We took our leave of Europe for heaven alone knows how long, perhaps for Ever," he wrote.

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.

Credit HMB Endeavour Foundation The Endeavour

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.