The French Academy of Sciences sent Guillaume Le Gentil to Pondicherry, (now called Puducherry), India for the transit of 1761. By the time he arrived, the town had fallen into British hands and was forced to make his observations from a ship in the Indian Ocean. The rolling waves rendered his measurements useless. Le Gentil decided to stay and wait for the next transit in 1769, at which point Pondicherry was back under French control. But on the day of the transit it was cloudy and his efforts were foiled once again.
During his time abroad, he endured bouts of sickness and near disasters at sea. When Le Gentil finally got back to France, he discovered his family presumed he had died. Le Gentil had to fight to regain his possessions including his position with the French Academy.
Others faired much better. British seaman James Cook was dispatched to Tahiti aboard the Endeavour. After observing the transit of 1769 under clear skies, Cook opened up secret orders to search for new territories in the name of the British crown. He circumnavigated New Zealand and mapped the east coast Australia. That part of the world would remain under British influence for generations to come.
The stories of exploration and hardship continue to be a source of inspiration. In 1992, Winnipeg writer Maureen Hunter created a play called the Transit of Venus based on the life of Le Gentil. It was then turned into an opera by Victor Davies and performed by Manitoba Opera in 2007. “I was just fascinated by the price he had to pay for what he set out to accomplish,” said, Ms. Hunter.
THE BLACK-DROP EFFECT
From many different locations on Earth, 18th century astronomers had hoped to take very precise measurements of the time it took Venus to travel across the solar disc. But there was an unexpected complication.
The image of Venus became distorted once it entered the sun. The black silhouette took on a slightly tear-drop shape and briefly seemed to cling to the edge of the sun. Some observers reported seeing a “black drop” or thin filament connecting Venus to outer rim of the sun.
“It was almost like taffy that was pulled out and stretched – and after a minute, popped,” explained Prof. Pasachoff .
Venus continued its journey as a perfectly round silhouette, but it became distorted once again when it approached the opposite side of the sun.
The so-called “black drop” effect was partly caused by the normal limitations of the telescopic equipment which slightly blurred the image, said Dr. Pasachoff. It also reflected the fact that the sun is a big ball of gas and does not have a crisp edge. In particular, it’s less bright towards the apparent edge.
These visual distortions led to less than perfect measurements. Still, some diligent individuals used the figures to create relatively accurate estimates of the vast distances separating the solar system’s heavenly bodies.
James Short, an English mathematician and telescope maker, concluded the Earth was 152.1 million kilometres from the sun – an over estimate of only about 2.5 million kilometres compared with modern calculations. “He enjoyed some luck in arriving so close, given that he ‘cherry-picked’ the results he liked, but nothing will alter the fact that he did arrive at a good figure,” said Randall Rosenfeld, the archivist for the Royal Astronomical Society of Canada.
THE MODERN APPROACH
Some of the finest instruments of modern astronomy – including the famed Hubble Space Telescope – will be used to study Venus as it glides across the solar disc.
Scientists hope their observations will aid in the quest for Earth-like planets orbiting distant stars.
Ground and space-based telescopes have already identified hundreds of planets outside our solar system, but most of them are massive bodies the likes of Jupiter.
The U.S. launched the Kepler spacecraft in 2009 to search for smaller planets that could potentially harbour life. Its sensors continuously monitor the light from more than 145,000 stars. If the light of one star dims briefly, it could be a sign a planet has passed in front of it.
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