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.
"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.
The best view of the transit will be 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.
For people on Earth to be able to see Venus transiting the sun, the planet has to cross the plane of Earth's orbit, and that doesn't happen very often: only twice every 100 years or so, in fact.
"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."
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.
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.
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.
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.
Source: CBC
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