Mercury transit: How to watch the rare event

A rare daytime celestial show takes place onMonday. The planet Mercury will pass in front of the sun, and Canadians will get an exceptional view.

Such "transits" of Mercury only happen 13 times a century, and not all of them are visible from all parts of the world.

"It's just another kind of neat experience, something that is rare and you don't normally get a chance to do," says Randy Attwood, executive director of the Royal Astronomical Society of Canada. The group's local branches will be holding public viewings of the event across the country.

'We're perfectly placed'

This particular Mercury transithas great timing Mercury will appear as a tiny dot on the east side of the sun at 7:12 a.m. ET and slowly cross it over 7hours, disappearing on the west side of the sun at 2:42 p.m. ET. That will make the entire event visible from most of Canadawith the proper equipment, of course.

"We're perfectly placed," says Paul Delaney, director of the observatory at York University, which will also be holding a public viewing and streaming the event online.

The event lasts so long that you have a good chance of catching part of it even if it's cloudy for part of the day.

Attwood says this will be a great opportunity for people who have never seen Mercury beforeand that's probably most people.

The closest planet to the sun is generally hard to see, even at night, because it only appears close to the horizon just before sunrise or just after sunset, and you really need to know where to look.

Of course, you won't be able to see Monday's Mercury transit with your naked eyes and you shouldn't try, warns Attwood, because looking at the sun directly will damage them.

Nice, safe views

At the public viewing events, people will be able to safely watch the event using telescopes equipped with protective solar filters.

While transits of Venus are visible with ordinary protective solar-filter eclipse glasses,Mercury is so small that you'll need some magnification, Attwood and Delaney predict.

If you can't make it to a public event, the event will also bestreamed onlineon CBC News via NASA.

In past centuries, transits of Mercury gave astronomers the rare opportunity to measure its apparent size and helped them estimate the distance between the Earth and the sun, NASA says. Today, they can help scientists calibrate instruments such as the solar-observing space telescopesSolar and Heliospheric Observatory, Solar Dynamics Observatory and Hinode.

Looking for transits of planets in front of stars in other solar systems is now the main technique used to find such distant planets, Delaney said: "Here you have the opportunity to see exactly in real time in our own solar system what we are doing to find exoplanets around other stars."

Even though Mercury orbits the sun once every 88 days, its orbit is tilted relative to Earth's orbit. That means most of the time, when it passes between the Earth and the sun, it appears to move either above or below the sun rather than in front.

That's why Mercury transits happen so rarely,and only in May and November, Attwood said. This year, we're lucky.

"There hasn't been a transit in May in a while, and of course the weather is so bad in November," he said.

The last Mercury transit was in 2006, and the next one will be in 2019. Then next twoafter that take placeduring our night,so if you miss the 2019 transit, you won't have another chance until 2049.

• Local public viewings of the May 9 Mercury transit across Canada