Venus will transit between Earth, sun in rare celestial event

If the skies are clear on Tuesday evening, skywatchers should be able to see Venus, the goddess of love and beauty, snuggling up to Helios, the personification of the sun.

In that moment of contact, a small black disc will be visible through telescopes that the University of Alberta, Telus Science Centre and Parks Canada at Elk Island plan to have on hand for public viewing.

Then Venus, like Phaeton on his celestial chariot, will travel slowly across the sky between the Earth and the sun before she arrives at the other end of the sphere, leaving behind for a second the same black teardrop that accompanied her arrival.

For some people, this tiny black disc floating across the sun may not seem like anything to celebrate. But for astronomers such as Doug Hube, professor emeritus at the University of Alberta, this will be one of the greatest shows on Earth — the last chance for everyone to see an astronomical event that occurs in pairs eight years apart and only four times in 243 years.

“Before the last one in 2004, which could not be seen in Edmonton because the sun wasn’t yet over the horizon, the last two transits of Venus occurred in 1874 and 1882,” he said. “The next pairs won’t be seen until 2117 and 2125. So this is it.”

“Those of us living now will never have another opportunity to witness it,” Hube said.

Celestial bodies such as the moon and planets are constantly shifting their positions in relation to fixed stars. But a close encounter between the sun and Venus is one of the rarest of astronomical events that we can see here on Earth.

Johannes Kepler, the mathematician who codified the laws of planetary motion, was the first to discover that both Mercury and Venus would transit the sun in 1631, Mercury in November and Venus in December.

Although Kepler died before he got a chance to see either event, French astronomer Pierre Gasendi witnessed the transit of Mercury that year. He missed the transit of Venus only because it was not visible from Europe.

Although Kepler predicted another transit would not occur until the next century, young British astronomer Jeremiah Horrocks deduced through his calculations a transit would happen in 1639. He and friend William Crabtree, are believed to be the only people to have witnessed the event.

“Nature was giving astronomers more than just a free show back in those early days,” Hube says. “By timing the transit of Venus, astronomers like Edmund Halley, of Halley’s Comet fame, believed that the distance between the Earth and the sun and, hence, the size of the solar system, could be measured.”

The trick, or trigonometry in this case, was possible only if simultaneous expeditions were made to the far corners of the world where the parallax effect on observers enables them to make the calculations.

“A number of scientific expeditions were mounted, including a famous one to Tahiti by explorer James Cook,” Hube says. “But getting an accurate timing of the transits was made difficult by that mysterious black teardrop that occurs when the disk of Venus enters the sun’s disk and when Venus leaves the sun.”

From an observatory in St. Petersburg in 1761, Mikhail Vasilyevich viewed the black drop effect as the first real evidence that Venus had an atmosphere. The planet’s atmosphere may contribute to the black drop effect, but it is not the principal cause.

Others think the black drop is simply the smearing that occurs when Venus touches the disk of the Sun.

Whatever the explanation, Cook’s voyage, which has been likened to a modern-day space mission, made the Transit of Venus famous — so famous, in fact, that in 1883 U.S. composer John Philip Sousa composed a march to celebrate the one that occurred a year earlier.

Knowing beforehand that the Transit of Venus in 2004 could not be seen in Edmonton, Hube and his wife Joan made the trip to Luxor, Egypt, to see it.

Given the region’s climate, they had little fear that weather would sabotage their quest. The Hubes, however, haven’t always been so lucky.

Two years ago, they went to Hikeru atoll near Tahiti to see a total eclipse of the sun, but were clouded out just one or two seconds before totality.

“It happens,” says Hube allowing that he got only a brief glimpse of the most recent transit of Mercury when he made a trip to the Galápagos Islands in November 2006.

The Hubes are planning to drive to Hay River in the Northwest Territories this month to view the Transit of Venus because the last hour will not be visible in Edmonton.

But if the forecast is for rain or cloud in Hay River, they plan to go either to the beach at Astontin Lake in Elk Island National Park, where Parks Canada plans to set up a telescope, or stay closer to home and share the experience at Ezio Faraone Park.

Hube cautions people not to attempt to observe the event on their own because it could result in permanent damage to the eye. He also advises against buying protective eyeware off the shelf.

“Most people will not have, nor know what constitutes, the proper equipment and, therefore, should be directed to observe from local observatories. Both the university’s campus observatory and the Telus World of Science observing deck have dedicated solar telescopes that will be available to the general public.”