Astronomy buffs can watch Venus move across the sun on Tuesday afternoon at safe observing stations in Saanich and Victoria. This undated photo in the ultraviolet spectrum from NASA shows Venus transiting the sun.

Seeing a celestial yardstick

Transit of Venus across sun won’t happen again for 105 years

Tuesday marks a once-in-a-lifetime opportunity to watch a rare astronomical event high in the skies – the transit of Venus across the face of the sun.

When Venus travels between the Earth and the sun, visible on Vancouver Island from 3:05 until 9:49 p.m. on Tuesday June 5, it will carry with it historical, as well as celestial, significance.

Beginning in 1639 when the transit of Venus was first observed and recorded, it allowed early astronomers to give a sense of distance and size of objects in the solar system.

“Nowadays we have radar and lasers … and different ways of measuring distance to planets, but in the old days … by measuring Venus’ size against the surface of the sun as it went across, (astronomers) were able to determine the actual size of Venus and of the sun,” said Sherry Buttnor, with the Victoria Centre of the Royal Astronomical Society of Canada.

“These were smart people. They didn’t have all these computers and fancy stuff, but they were able to do that just by looking at the sun.

“They were actually very, very close with their predictions.”

The image of Venus against the sun was used to determine an accurate estimate of the scale of the solar system.

A transit of Venus is rare due to the differing orbital speeds and planes of Earth and Venus – the next is in 2117. The transit has only been viewed seven times since Galileo’s invention of the telescope in 1609.

“It’s neat to watch these things unfold in front of your eyes because it gives you a sense of motion,” said Buttnor, who will be photographing the transit from Metchosin. “We really are orbiting the sun. Things are moving out there.

“(Viewing this) is a once-in-a-lifetime thing.”

Watch the transit

Viewing the sun though No. 14 or darker welders’ glass is generally considered safe, though the glass, if scratched or nicked, opens the viewer to permanent vision loss.

The safest viewing method is pinhole projection, achieved by filtering sunlight through a pinhole in a piece of cardboard onto another blank surface. The projection is remarkably clear and removes the threat of vision damage caused by staring at the sun.

Astronomers from the Victoria Centre of the Royal Astronomical Society will set up transit viewing stations and solar telescopes in front of the Royal B.C. Museum, atop of Mount Tolmie and at Cattle Point in Oak Bay between 3 p.m. and sunset on Tuesday.

The University of Victoria Department of Physics and Astronomy will also host a free viewing of the transit of Venus on the fifth floor of the Bob Wright Centre from 3 to 9 p.m.

nnorth@saanichnews.com