From the Globe and Mail, Toronto

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Venus ascending

'For most people, it'll be about as exciting as watching paint dry,' but astronomers are in a tizzy over the chance to see the planet pass between the sun and us next month. DAN FALK explains the transit fever

By DAN FALK

Saturday, May. 15, 2004

Why would anyone travel thousands of kilometres to see a small black dot move slowly across a big yellow circle?

For amateur astronomers around the world, the question is just the opposite: How could anyone think of missing a phenomenon so exotic that no living person has seen it? You might as well ask a birdwatcher why he would bother tracking down the black paradise flycatcher or a stamp collector why he would want a specimen with an upside-down airplane on it.

"It's one of these rare things," says Ralph Chou, a professor of optometry at the University of Waterloo and an avid amateur astronomer.

Driven by "sheer curiosity," Prof. Chou will go to Egypt next month for the chance to see the black dot travel across the yellow circle, an event otherwise known as a transit of Venus. At that spot, a combination of geography and climate should guarantee him a front-row view.

A transit of Venus occurs when the planet passes directly between the Earth and the sun. Such transits occur in pairs eight years apart -- but the pairs are separated by either 105 or 122 years.

In other words, if you are born at the wrong time, you won't see it, period. Transits of Venus are so infrequent that they have been recorded as being seen by human beings on only five previous occasions, beginning in 1639; the last one was in 1882.

But the dry spell is nearly over: The next transit of Venus will take place on June 8.

"There's nobody alive on the face of this Earth that saw the last transit of Venus," Prof. Chou says. "And we are just fortunate to be alive at a time when we're going to have the possibility of seeing two transits - the one in June of this year and another one in 2012."

A transit is not as spectacular as an eclipse; indeed, without a telescope and the proper filter needed to observe it safely, you would never know it was happening. "For most people, it'll be about as exciting as watching paint dry," Prof. Chou says.

But the rarity itself is enough to give sky watchers a case of transit fever.

Some enthusiasts will be flying to exotic locations; in Egypt, Prof. Chou will be leading a tour on behalf of the Toronto Centre of the Royal Astronomical Society of Canada, a nationwide association of astronomy clubs.

Others will stick closer to home. "I'm going to try to observe it locally, here in the city -- if the weather is suitable," says Geoff Gaherty, a Toronto-based computer consultant. "But I'm prepared to drive at least a few hours if it looks more promising somewhere else."

Observers in Central and Eastern Canada can indeed see the transit -- but only a portion of it: It will already be under way as the sun rises. Viewers in Europe, eastern Africa and western Asia will be able to see the entire event, which lasts just over six hours.

The first transit of Venus known to have been seen by human eyes occurred in 1639, and only two people glimpsed it: a young Englishman named Jeremiah Horrocks and his friend, William Crabtree.

By the time of the next transit, in 1761, the situation was quite different. Transits could be predicted with great accuracy, and astronomers knew that data from a transit could be used to calculate the distance to the sun (and, from that, the distance to the planets).

It boils down to a problem of triangulation. As seen from different locations on the Earth's surface, Venus takes a slightly different path across the face of the sun. If you compare the path as seen from location A with the path recorded at location B, and you know the distance separating A and B, along with the duration of the transit as seen from the two locations, then -- with a bit of geometry -- you can work out both the distance to Venus and the distance to the sun.

Thus, accurate transit observations became a top scientific priority, and about 70 expeditions were mounted to locations around the world to record the 1761 event.

The first person to see a transit of Venus from North America, and the only one to view the 1761 transit from this continent, happened to be standing on what is now Canadian soil -- a hill in St. John's. The observer was John Winthrop, a professor of mathematics and natural philosophy at Harvard College (now Harvard University). The transit was not visible from Massachusetts, so he persuaded Harvard and the governor of the colony to support his expedition -- and he set sail for Newfoundland.

The trip was a great success. "The morning of the 6th of June was serene and calm," he wrote in his log. "And at 4h 18m we had the high satisfaction of seeing that most agreeable Sight, VENUS ON THE SUN, and of shewing it in our telescopes to the Gentlemen of the place, who had assembled very early on the hill to behold so curious a spectacle."

This year's transit will again be visible to early risers in St. John's, where keen amateur astronomers are hoping to experience the same thrill that captivated Prof. Winthrop nearly 21⁄2 centuries ago. The transit is of "enormous historic interest," says Fred Smith, a professor in the faculty of science at Memorial University of Newfoundland. "It does allow us to recreate a little bit of history."

In fact, Prof. Smith believes that he has figured out precisely where Prof. Winthrop was stationed. Based on the Harvard professor's log, which gives the latitude of the observing site but not the longitude, he believes that Prof. Winthrop observed the transit from Kenmount Hill on the west side of the city.

The transit eight years later, in 1769, motivated one of the more famous of all sea voyages: the expedition of James Cook to the South Pacific. But there were also four observing stations in Canada that year, including a remarkable expedition by two English astronomers, William Wales and John Dymond, to Fort Prince of Wales on Hudson Bay, the site of present-day Churchill, Man.

Dr. Wales and Dr. Diamond spent 13 months at the fort, observing the local flora, fauna and geology, complaining of mosquitoes, and were paid £200 by the Royal Society of London for their efforts.

Astronomers would eventually combine the data from the 1761 and 1769 transits and work out the Earth-sun distance to within a few per cent of the modern value of 149.6 million kilometres.

The next transit, in 1874, was visible from Asia, and European and American teams observed the event from Russia, Japan, China and Mauritius. Despite the advent of photography, the results were disappointing; a number of factors, including hazy weather and turbulence in the Earth's atmosphere, hampered the observations.

More important, astronomers were now developing other methods for determining the Earth-sun distance. One method involved tracking the position of Mars against the background stars and turned out to be more accurate (and easier to do) than the transit method. (Today, astronomers have more sophisticated ways of determining distances in the solar system, including radar, and the average distance separating the Earth and sun is now known to within a few dozen metres.)

By the time the next transit rolled around, on Dec. 6, 1882, the event was no longer of great scientific importance -- though members of the public were certainly excited about it. Newspapers of the day record a high level of interest, although bad weather hampered the viewing in many locations. "Cold and miserable, the amateur astronomers stuck to the roof of The Mail until the transit was over," the Toronto newspaper reported; observers in Halifax experienced "very dark and rainy" conditions and "made no observations," a local paper said.

Still, the 1882 event triggered a surge of interest in astronomy and science. The Royal Society of Canada was founded the same year, historians note. "The prospect of the transit, and the hope that even Canadians could make some useful observations, was, I think, the beginning of professional astronomy in Canada," says Peter Broughton, a retired Toronto teacher who has written extensively on the history of Canadian astronomy.

These days, of course, Venus is hardly a stranger. We now recognize it as our sister planet, with a diameter just a bit smaller than Earth's and a mass four-fifths of Earth's. We know it has a thick (albeit poisonous) atmosphere, and no moons. By now, 26 space missions, including eight landers, have explored the cloud-covered world.

The June 8 transit will tell astronomers little that is new about either Venus or the sun -- although there is still scientific interest in the "black drop" effect, a complex phenomenon in which the disc of Venus appears stretched out into the shape of a raindrop as it first passes in front of the sun and later as it appears to exit the solar disc.

Jay Pasachoff, an astronomer at Williams College in Massachusetts, will observe the transit from northern Greece, together with a group of students, in order to study the black drop effect in more detail.

But he admits that transits are now primarily of educational and historical value. This spring's event "is a great time for public education, and making people appreciate the value of science in general and astronomy in particular," Prof. Pasachoff says.

On June 8, people will again look up at that black dot inching across the sun, and ponder the nature of these distant spheres -- at once unfathomably remote and yet strangely familiar. Many will muse on the rarity of the event, just as astronomer William Harkness of the U.S. Naval Observatory did the last time around.

"What will be the state of science when the next transit season arrives God only knows," he told an audience in 1882. He could hardly imagine that far-off day when "the 21st century of our era has dawned upon the Earth, and the June flowers are blooming in 2004."

Dan Falk is a Toronto science writer, broadcaster and amateur astronomer. He plans to view the June 8 transit from somewhere in southern Europe.

Our sister planet

Venus is named for the Roman goddess of love and beauty. It is known as the morning star as well as the evening star, depending on its position in the sky.

Orbit: 108,200,000 kilometres from the sun, about two-thirds the size of Earth's orbit.

Diameter: 12,100 kilometres, about 95 per cent size of Earth.

Mass: 4.87 x 10{+2}{+4} kilograms, about 80 per cent of the mass of Earth.

Rotation: in an opposite direction to that of Earth.

Length of year: 225 Earth days (but its day is 243 Earth days because it rotates so slowly).

Surface: craters and volcanoes.

Atmosphere: mainly carbon dioxide, with some nitrogen and virtually no water vapour; several layers of clouds many kilometres thick composed of sulphuric acid.

Atmospheric pressure: At the surface, 92 times that of the Earth's at sea level.

Surface temperature: about 475 degrees Celsius.

Times to watch

For all locations except the High Arctic, the transit will already be under way as the sun rises on June 8. Canada's Far West is outside the zone of visibility. To view the sun safely, you can use No. 14 welder's glass. All times are local.

Location Sunrise Transit ends

Iqaluit 2:22 a.m. 7:23 a.m.

St. John's 5:04 a.m. 8:55 a.m.

Halifax 5:30 a.m. 8:26 a.m.

Quebec 4:51 a.m. 7:25 a.m.

Montreal 5:06 a.m. 7:25 a.m.

Toronto 5:35 a.m. 7:25 a.m.

Winnipeg 5:19 a.m. 6:24 a.m.

Regina 4:43 a.m. 5:23 a.m.

For more locations, see the website: http://www.planetarium.montreal.qc.ca/Information/Actualite (dead link)

Venus2004/tableau_a.html

Source: Montreal Planetarium