Tuesday marks a once-in-a-lifetime opportunity to watch a rare astronomical event high in the skies B次元官网网址� 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.

B次元官网网址淣owadays we have radar and lasers ... and different ways of measuring distance to planets, but in the old days B次元官网网址� by measuring VenusB次元官网网址� 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,B次元官网网址� said Sherry Buttnor, with the Victoria Centre of the Royal Astronomical Society of Canada.

B次元官网网址淭hese were smart people. They didnB次元官网网址檛 have all these computers and fancy stuff, but they were able to do that just by looking at the sun.

B次元官网网址淭hey were actually very, very close with their predictions.B次元官网网址�

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 B次元官网网址� the next is in 2117. The transit has only been viewed seven times since GalileoB次元官网网址檚 invention of the telescope in 1609.

B次元官网网址淚tB次元官网网址檚 neat to watch these things unfold in front of your eyes because it gives you a sense of motion,B次元官网网址� said Buttnor, who will be photographing the transit from Metchosin. B次元官网网址淲e really are orbiting the sun. Things are moving out there.

B次元官网网址�(Viewing this) is a once-in-a-lifetime thing.B次元官网网址�

Watch the transit

Viewing the sun though No. 14 or darker weldersB次元官网网址� 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