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     It is one of those lifetime events, as Venus rarely crosses the plane of our orbit while in front of the sun.

“Our nearest planetary neighbour will be passing between the Earth and the sun starting at 6:04 p.m. ET on June 5 and will be aligned in such a way that its passage will be visible with the naked eye. The transit, as it’s known, will last about six hours, but in most parts of Canada, it will be visible for only a few hours before sunset.”

That doesn’t happen very often because most of the time when Venus crosses Earth’s orbital plane, Earth is somewhere else in its orbit, so there is no direct sight line from Earth to Venus and the Sun

“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 said.

“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.”

As with the annular eclipse that dazzled millions of people around the world on May 20, the key to viewing the transit of Venus safely is to avoid looking directly at the sun without a protected lens. Sunglasses or ordinary telescope lenses are not enough to protect the eyes.

To be safe, lenses must be treated with something like an aluminized film like Mylar or have a strong filter such as the type found on some welder’s glasses. The Royal Astronomical Society of Canada lists a number of safe lens options in a special section of its website devoted to the transit.

Whatever viewing method you choose, the key, says Reid, is to not miss what will be your last chance to see Venus in transit.

“It will not occur again until 2117, so it’s worth trying to see it,” Reid said.