If you happen to glance at the sun in the early evening Tuesday and notice a black dot moving across it, fear not, that's not dust in your eye or an early sign of glaucoma — it's Venus.

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

"The view is best the farther west you go, but, actually, it's pretty good for most of Canada," said Mike Reid, a lecturer in the department of astronomy at the University of Toronto and a public outreach co-ordinator at the university's Dunlap Institute for Astronomy and Astrophysics.

Public viewings underway across Canada

To mark this year's transit of Venus, events are being held across Canada at which the public can observe the transit safely and learn more about the phenomenon.

In what is believed to be the largest gathering in Canada, more than 5,000 people have gathered at Varsity Stadium at the University of Toronto, using special sunglasses distributed by the Dunlap Institute and telescopes to look at the transit safely.

The Slooh Space Camera, a robotic telescope that transmits live images of space online, has real-time feeds of the transit from solar telescopes in Australia, Japan, New Zealand, Hawaii, Norway, Arizona and New Mexico. Other sites, including those of the San Francisco-based Exploratorium museum and the group Astronomers Without Borders, are also carrying live webcasts of the event.

The Dunlap Institute has compiled a list of some live feeds by location.

Children in Bombay watch the June 8, 2004, transit of Venus. Special eye protection must be worn when looking directly at the sun to avoid eye damage. (Arko Datta/Reuters)

Users of Apple and Android smartphones can download a free transit of Venus app created by Astronomers Without Borders and ESRI, a company that works with geographic information system, or GIS, technologies. The app, available at iTunes App Store and Android Market, allows users to find out when and where the transit will be visible, follow transit-related tweets, download photos and videos and learn about the history of the transit.

The Dunlap Institute has distributed another 43,000 pairs of "transit glasses" to universities and Royal Astronomical Society of Canada chapters across the country, and similar glasses are being sold through Sky News magazine and on various websites.

Eye protection needed

For those who can't get their hands on a pair of glasses, Reid suggests crafting a simple device like a pinhole projector to project an image of the sun onto the ground or another surface. (The Exploratorium science museum in San Francisco suggests a variation on this using binoculars.)

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.

Eastern Australia, Asia will have best vantage points

The best view of the transit is in the middle of the Pacific Ocean, but people in eastern Australia, New Zealand, Japan, Philippines, Korea, parts of China and Russia and Pacific nations like Papua New Guinea will also be able to see Venus's entire journey across the sun — although for them it will be occurring the morning of June 6.

Europeans, and people in eastern Africa, the Middle East, India and places in between will have to get up very early to catch a glimpse of the transit. West Africa, Portugal and parts of Latin America and Spain will miss it altogether.

'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.'— Mike Reid, Dunlap Institute for Astronomy and Astrophysics

For people on Earth to be able to see Venus transiting the sun, the planet has to cross the plane of Earth's orbit at the precise time that Earth, Venus and the sun are lined up.

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."

Jordanian Bedouins in the town of Azraq look at the 2004 transit of Venus. This year, people in the Middle East will see only the tail end of the transit at sunrise. (Ali Jarekji/Reuters)

When transits do occur, they occur in pairs, with each transit in a pairing separated by eight years, and then not again until 105.5 or 121.5 years later. The last transit was in June 2004, and after this year's transit, the next one won't occur until December 2117. (Learn why the timing of transits follows this pattern.)

"It's very similar to the reason why we don't see a solar eclipse every time there is a new moon," Reid said. "You'd think that every time the moon comes between Earth and the sun, it would block out the sun, but sometimes, it's a bit above the sun, sometimes it's a bit below the sun."

Mercury also occasionally aligns with the Earth and the sun, and does so more frequently than Venus, but its transits can't be seen with the naked eye, because Mercury is much smaller and farther away from Earth than Venus.

Transits used to measure distance to sun

This will be the eighth transit of Venus since the invention of the telescope in 1609. The earliest record of a transit we have is from 1639 (the transit prior to that, in 1631, was known about but not visible in Europe).

The 1639 transit of Venus was documented by a young amateur astronomer in Lancashire, England, named Jeremiah Horrocks (sometimes spelled Horrox) and his friend William Crabtree. Horrocks used his observations to measure the diameter of Venus and to obtain a crude estimate of the distance between the Earth and the sun.

In later years, astronomers, most notably Edmond Halley, refined the method of using transits to estimate the distance to the sun by measuring the differences in how long the transit appeared to take depending on where the observer was located on Earth and applying the principles of the displacement effect known as parallax.

Getting a precise measurement of the distance preoccupied many researchers in the 18th and 19th centuries and was in part the motivation behind Captain James Cook's first expedition to the South Pacific. Cook was tasked with observing the 1769 transit of Venus by Britain's Royal Society and did so from Tahiti.

Children in Vienna watch an image of the sun and Venus projected onto a cardboard box on June 8, 2004. Simple devices like pinhole projectors can be made to view the transit indirectly. (Heinz-Peter Bader/Reuters)

Today, the distance between the Earth and the sun, referred to as the astronomical unit, is well known and can be measured using radar, and transits have instead begun serving another purpose: they are helping astronomers locate planets outside our solar system, known as exoplanets.

When these planets pass between Earth and the star they are orbiting, they block out some of the light the star is emitting, causing it to dim slightly. By observing the nature of this dimming, astronomers can learn about the planet causing it.

"You look for the same pattern of dimming to repeat over and over again, and you can deduce from that what kind of planet it is, how far from the star it is, whether it's a big planet or a small planet, and now, we're getting to the point where we can measure some elements of the atmospheric composition of the planet," Reid said.