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Transit of Venus

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The transit of Venus will take place on 8 June, 2004. You will see a small, circular black dot gliding across the disc of Sun. This is Venus.


On August 12, 1768, a ship known as Bark Endeavour sailed towards an island called Tahiti. The commander of the ship was Lt. James Cook. The island had been discovered only a year before in the South Pacific. This part of the Earth was so poorly explored that mapmakers couldn't agree if there was a giant continent there or not. The ship 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. On the way, dangerous storms could (and did) materialize without warning. Unknown life forms waited in athe ocean waters. The sailors took this risk to observe a transit of Venus.


The ship started at 2 pm having on board 94 persons. Their mission was to reach Tahiti before June 1769. 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 journey.



The size of the solar system was one of the chief puzzles of 18th century science. 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 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. 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 never lived 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.


By the time Cook reached Tahiti in 1769, he'd been sailing west for 8 months--about as long as modern astronauts would spend en route to Mars. Five crewmen were lost when the ship rounded stormy Cape Horn, and another despairing marine threw himself overboard during the 10-week Pacific passage that followed. Tahiti. There was no contact with "Mission Control," no satellite weather images to warn of approaching storms, no help of any kind. Cook navigated using hourglasses and knotted ropes to measure ship's speed, and a sextant and almanac to estimate Endeavor's position by the stars. It was tricky and dangerous.


Remarkably, they arrived mostly intact on April 13, 1769, almost two months before the transit.


Tahiti was comfortable and well provisioned for human life; the islanders were friendly and eager to deal with Cook's men. Yet the flora, fauna, customs and habits of Tahiti were drastically different from those of England.


The transit finally happened on June 3, 1769. The day proved as favourable to their purpose as they could wish, not a cloud was to be seen and the air was perfectly clear. BUT they very distinctly saw a dusky shade round the body of the Planet which very much disturbed the times of the contacts.


The "dusky shade round the body of the Planet" was a problem. Intense sunlight filtering through Venus' atmosphere fuzzed the edge of the disk and decreased the precision with which Cook could time the transit. For this reason, his measurements disagreed with those of ship's astronomer Charles Green, who observed the transit beside Cook, by as much as 42 seconds.



Cook and Green also observed the "black drop effect." When Venus is near the limb of the sun--the critical moment for transit timing--the black of space beyond the Sun's limb seems to reach in and touch the planet. You can recreate the black drop effect with your thumb and index finger: Hold the two in front of one eye and narrow the distance between them. Just before they touch, a shadowy bridge will spring across the gap. The black drop effect, like the fuzziness of Venus' atmosphere, made it hard to say just when the transit began or ended.


This was a problem for observers elsewhere, too, not only Cook in Tahiti. In fact, when all was said and done, observations of Venus' 1769 transit from 76 points around the globe, including Cook's, were not precise enough to set the scale of the solar system. Astronomers didn't manage that until the 19th century when they used photography to record the next pair of transits.


While returning, during a 10-week stopover in Jakarta for repairs, seven seamen died of malaria. The port city was densely populated by people and diseases. Cook left as quickly as possible, but the damage was done. Ultimately 38 of the Endeavour's original company (and 8 who joined later) perished, including astronomer Charles Green, most from diseases picked up in Jakarta.


On July 11, 1771, Cook returned to England. The surviving crew of the Endeavor had circumnavigated the globe, catalogued thousands of species of plants, insects and animals, encountered new races of people, and hunted for giant continents.



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