If you're outside this evening, look up.
On the evening of Tuesday, June 5th, Venus will pass across the face of the sun. The "rare celestial show" is a twice-in-a-lifetime event. Having already crossed the sun in 2004, the next time the neighbouring planet makes the same journey — known as the Transit of Venus — will be in 2117 and 2125.
"Venus makes the cycle twice every 243 years, so this event that is happening on Tuesday will not be seen again in our lifetime," Jesse Rogerson, a researcher and programmer in astronomy and space sciences at the Ontario Science Centre, told CTV News.
"In terms of rarity, to be here at a time when it's happening, you almost have to look at it," Geoff Chester of the U.S. Naval Observatory, who saw the last transit in 2004, told the Associated Press. "It ain't going to happen again in my lifetime."
Mike Reid, public outreach co-ordinator at University of Toronto's Dunlap Institute for Astronomy and Astrophysics, explains the phenomenon:
"Earth orbits the sun slower than Venus. If they both orbited in the same plane, like runners running around a track, then every time Venus 'lapped' Earth, there would be a transit," Reid told CBC News.
"However, Venus's orbit is 'tilted' relative to Earth's orbit, so the only way Venus and Earth can line up with the sun is if Venus laps Earth exactly when Venus happens to be passing through the plane of Earth's orbit. That only occurs at two points in Venus's orbit (the two points where the ring of Venus's orbit 'punctures' Earth's orbit). The odds of Venus happening to be at one of those two points when it 'laps' Earth are small."
Venus will pass between the Earth and the sun starting at 6:04 p.m. Eastern Time. It will take six hours to cross the sun, although most of Canada will only witness its transit for a few hours before sunset.
The view, visible to the naked eye, is better the further west the viewer is, although most of Canada should have a decent look at the planet.
What can we expect to see?
"When you're looking at the sun, Venus will appear like a little black blemish moving across it. That's what's so remarkable," said Rogerson.
"This planet will literally appear dwarfed by the size of the sun. That's an awesome sight to witness and really hits home just how small we are in this universe."
In Toronto, the Ontario Science Centre is holding a free viewing event in its parking lot, starting at 5:30 p.m. ET.
The Dunlap Institute for Astronomy and Astrophysics will hold its public transit viewing at Varsity Stadium at the same time.
Both viewing sites will provide special viewing glasses. Like the "ring of fire" annular eclipse on May 20th, looking at the stunning scene requires protective eyewear.