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Sunday, June 03, 2012

Captain Cook and the Transit of Venus

James Cook (Endeavor, 200px)\Timothy Birdnow

NASA has a very interesting historical perspective on the mission of James Cook to Tahiti. Cook went to the island to observe a transit of Venus across the Sun - and to help calculate the size of the solar system.

From the article:

"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.

The Endeavour. Credit: HMB Endeavour Foundation. 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.

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 spots. 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."

End excerpt.

It's a terrific piece for those interested in the history of science. Read it all at

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