It is being billed as one of the rarest astronomical events of the century.
On Tuesday June 5th, the Earth, Venus and the Sun will be in perfect alignment. You will be able to witness the silhouette of Venus slowly moving across the face of the sun – a six-hour journey known as the Transit of Venus.
For professional astronomers and amateur sky watchers, it is special treat that’s packed with historical significance and a valuable source of scientific data.
In earlier centuries, observations made during previous transits provided a better understanding of the vast size of the solar system. And now, astronomers are getting ready to take detailed measurements that could aid in the detection of earth-like planets around distant stars.
Because the orbits of the Earth and Venus are slightly tilted, they seldom line up in a straight line with the sun. Transits occur in pairs eight years apart, but the pairs are separated by more than a century. The last transit was in 2004, the first since 1882. When tomorrow’s transit is over, there won’t be another one until 2117.
“So if you ever want to see a Transit of Venus this is your chance,” said Jay Pasachoff, a professor of astronomy at Williams College in Massachusetts.
MAKING USE OF TRANSITS
Although Transits of Venus have occurred for eons, they escaped attention until the 16th century Polish astronomer Nicolaus Copernicus postulated that the sun – and not the earth – is the centre of the solar system.
Based on the motions of the planets in the new sun-centric cosmos, the German astronomer Johannes Kepler predicted there would be a transit of Venus in 1631 – but no one saw it. Kepler was dead by that time and the transit wasn’t actually visible from Europe.
A little-known English astronomer and church cleric, Jeremiah Horrocks, used Kelper’s figures to determine there would be another transit in 1639. Horrocks and a friend, William Crabtree, were the first people to see Venus on the face of the sun.
In 1716, Edmond Halley, the English astronomer best known for predicting the return of the comet that bears his name, proposed that Transits of Venus could be used for settling one of the most challenging scientific questions of his era – the size of the solar system.
If a transit were witnessed from two locations widely separated in latitude, Venus would appear to cut across the Sun on slightly different paths – an optical effect known as parallax. By taking very precise measurements of the time it takes Venus to traverse the Sun, combined with the positions of the observers on Earth, it is possible to calculate the distance between Earth and Venus using geometry.
“You are basically setting up a triangle in space,” explained Roy Bishop, a retired physics professor at Acadia University. Once the distance between Earth and Venus is known, the distances to the other planets can be calculated using Johannes Kepler's third law of planetary motion involving how long the planets take to orbit the Sun.
The next pair of transits would occur in 1761 and 1769. Halley, born in 1656, realized he would not live long enough to witness them. So he urged the next generation to organize scientific expeditions that would gather transit observations from far-flung spots around the globe.
THE FIRST SPACE RACE
The 18th century marked what could be considered the first space race.
The major European powers, at the urging of Halley, financed dozens of scientific expeditions to distant parts of the globe to observe the pair of Venus transits in 1761 and 1769. From these observations, astronomers hoped to gain a better understanding of the size of the solar system.
Beneath this lofty goal, the individual states advanced their own political ambitions. And, in one case, the transit was used as a cover to grab more territory.
Still, many of those who ventured abroad suffered great personal hardship in the pursuit of science.
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.
The upcoming transit of Venus provides a unique opportunity for scientists to double-check this planet-hunting technique.
“Venus is similar in size to the earth. And so it is a perfect example of an earth-like planet travelling in front of a sun-like star – which is just what we are looking for out there,” said Terence Dickinson, editor of SkyNews, a Canadian-based astronomy magazine.
However, Hubble can’t be pointed directly at the sun which would fry its instruments. Instead, it will focus on the moon which will act as a mirror to collect reflected sun light. A tiny fraction of this light would have passed through Venus’s atmosphere. By analysis these rays, scientists hope to see the chemical signatures of Venus’s atmosphere which is well known from the robotic space probes. This approach mimics how scientists are trying to sample the atmospheres of worlds outside the solar system.
“It will be a great test case to understand whether we are doing it correctly,” noted James Graham, director of the Dunlap Institute for Astronomy and Astrophysics at the University of Toronto.
What’s truly amazing is the sensitivity of today’s telescopes, which can detect the slightest changes in light. The Transit of Venus will dim the sun by less than 1 per cent.
HOW TO WATCH THE TRANSIT
For sky watchers in Canada, the Transit of Venus will start late on Tuesday afternoon. But the sun will set while Venus’s six-hour trek is still in progress. Only those in the far northwest will have a chance to witness the entire celestial show.
You’ll need a solar filter or specialized glasses – the same type used for viewing an eclipse – to protect your eyes from the sun’s potentially damaging rays. A pair of transit glasses is available with each newsstand copy of the May/June issue of SkyNews.
Many educational institutions and local branches of the Royal Astronomical Society of Canada will be hosting viewing sites where the public can look at the sun through special solar telescopes. For instance, the Astronomy and Astrophysics Department of the University of Toronto will be staging a show at Varsity Stadium, starting at 5:30 p.m. (Pre-register at universe.utoronto.ca/transit2012/)
Of course, you’ll also need relatively clear skies. Dr. Roy Bishop, a retired professor of physics at Acadia University, had hoped to see the transit in 2004. He set up his telescope on the shores of the Bay of Fundy, not far from his home in Avonport, N.S. But thick clouds blocked the view.
“I was certainly disappointed, although I thought there was a good chance I would be alive in eight years and have another crack at it,” said Prof. Bishop. He is now monitoring weather forecasts. And, if need be, he’s prepared to travel to a spot that’s likely to have sunny skies.
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