On June 5th, Canadians will experience a once-in-a-lifetime celestial event. Well, twice-in-a-lifetime if you caught the last one in 2004, and possibly three times if you’re very young and plan to live to be about 130, but it’s still very rare. The event is the transit of Venus, a conjunction of the spheres that has inspired astronomers for over 400 years and which has provided important knowledge about the dimensions of our solar system.

To learn more, I sat down this week with my friend Jesse Rogerson. A dedicated astronomer – the guy has a constellation tattooed on his back – Jesse is working toward his PhD at York University. He’s also a first-rate science communicator, working as a researcher at the Ontario Science Centre and hosting York Universe on astronomy.fm. Here is an edited version of our conversation.

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What is the transit of Venus?

Next to the sun and moon, Venus is the brightest object in the sky. It’s about 100 times brighter than Vega, which is usually used as a ‘reference’ star. In fact, when Venus is as bright as it is now, there tend to be increased reports of UFO sightings around the world.

Due to its interior orbit, from our point of view Venus appears to go slowly back and forth on either side of the sun, like a pendulum. At the moment, it’s the ‘evening star’, the first star that pops out at night, chasing the sun into the west. But after Venus gets onto the other side of the sun, it will become the ‘morning star’, appearing just before the sun rises. Every once in a while, Venus crosses in front of the sun. This is called a transit of Venus.

Why don’t we get a transit of Venus every year?

This is similar to another question I get a lot: if the moon goes around the earth, shouldn’t we get a lunar or a solar eclipse every single month? The reason we don’t is because the two orbits are on different planes.

Imagine the planet Earth orbiting the sun at the edge of a flat plane, like a DVD. A lot of people imagine that the orbit of Venus would be interior but in the same plane, that is, flat with the DVD. In fact, it’s tilted by about 3.4 degrees up from the plane of our orbit around the sun. So there are only a couple of times that we all sync up.

How often does this happen?

Our year is about 365 days; on Venus it would be about 224 days. If you do the math, you find out that 243 of our orbits is roughly equal to 395 orbits of Venus. So if you were to line the Sun, Venus and Earth up, 243 years later we’d all be back in the same spot. That means we are guaranteed to get a transit at least once every 243 years. But because of the relative motions of Earth and Venus, there are other spots where they line up. If you put their paths into an equation and look for the times when everything is positioned just right, you find a periodic pattern. Transits come in pairs, 8 years apart. The pairs are separated from each other by either 105.5 years or 121.5 years, depending on which pair you start counting from. If you add 105.5 + 8 + 121.5 + 8 you get 243, and we’re back where we started.

The transit happens when Venus is crossing from above our plane to below our plane, or vice versa. So it can be what we call a descending node, or an ascending node. The pairs are always of the same type: two ascending nodes, then two descending nodes and so on. The 2004 and 2012 transits are both descending nodes, so from our point of view Venus is going from top left of the sun down to the bottom right as it crosses from above our orbital plane to below.

When did transits of Venus start to attract scientific attention?

Johannes Kepler, the grandfather of modern orbital mechanics, predicted that there would be a transit of Venus in 1631. Unfortunately, he didn’t live long enough to observe it. Jeremiah Horrocks, an English scientist, re-did Kepler’s calculations . He found some small errors. Kepler hadn’t realized that the transits would happen in pairs every eight years, but Horrocks did. He was lucky enough to observe this in December 1639.

Horrocks made his discovery only about a month before the transit, so he wasn’t able to spread the word. But after his successful observation in 1639, the scientific community got interested, and they were able to prepare for 1761 transit and the expected 1769 one as well.

Is it true that observing transits can allow you to measure the distance from the earth to the sun?

Absolutely. It’s all about something called parallax, an apparent shift of an object based on where you’re looking at it from. We all know this effect: if you put your finger out at arm’s length and look at it first with your left eye and then your right, your finger appears to shift from side to side. Although it’s not actually changing its position, there’s a very specific mathematical relationship between its apparent shift and its actual distance; the further away it is, the less it will appear to shift. So as long as you know that shift angle, and how far apart your eyes are, you can measure how far away it is from you.

So if you were to observe the transit of Venus from two locations on earth, and you know how far apart those locations are, you would be able to figure out how far away the sun was from the Earth?

That’s right, and that’s exactly what people did. Teams from many countries spanned the globe, from India to Tahiti, trying to get as many accurate measurements as they could. After the 1769 transits, a French scientist named Jérôme Lalande analysed the data and calculated a distance of 153 million kilometres with an uncertainty of 1 million kilometres. That’s less than one per cent uncertainty, although they were hoping for better.

What do you mean?

Part of the problem is what’s called the black drop effect, which you can still see today. If you take your thumb and pointer finger and bring them really close together, you see a little bridge of skin. They seem to connect just before they actually do. The same thing happens when the dark smudge of Venus gets projected onto the brightness of the sun. That means that it’s really hard to tell exactly when the transit started and when it stopped, which you need to know if you want to get perfectly accurate measurements.

What causes this?

For a long time scientists thought it was a result of Venus’s atmosphere, but that’s not very likely to be the real cause. It may be distortion due to Earth’s atmosphere error or a telescope error; we don’t know for sure. As telescopes have gotten better over the years the effect has been reduced, but it’s not gone. At any rate, today our ability to measure distances using radar is much more effective than using transits, so we know that earth is actually 149.598 million kilometres from the sun.

What’s the best way to view the transit?

Safety first: do not look at directly the sun! You will damage your eyes, and you can’t get new ones! There are safe ways to observe the transit though.

One thing you can do is to create a pinhole camera. Take a piece of cardboard and cut a small window in it, maybe 4 or 5 inches square. Cover it in tinfoil and poke a pinhole in it. Point that hole at the sun, then put a piece of paper behind it. It will project an image of the sun’s disc on the paper, and you’ll be able to see the little black dot of the planet Venus travelling across it.

Another way is to get yourself a pair of solar viewing glasses. These are essentially just filters that let in only .001 per cent of the light that hits them; in normal light, you can’t see through them at all. But when you put them on and look at the sun you’ll see it, along with the black dot passing in front of it.

If you have access to a telescope you can get yourself a solar filter that fits on the front of it. Again, don’t ever point your telescope at the sun without a filter, and don’t look through your telescope with normal solar viewing glasses. A telescope magnifies the sun, and the glasses were not built to take that kind of magnification. But a proper solar filter designed for a telescope will block enough light to allow you to look at it safely.

Any other advice?

Get in contact with your local astronomy club. Here in Toronto it’s the Royal Astronomical Society of Canada. They are also partnering with us at the Ontario Science Centre, and we’re putting on a massive transit of Venus celebration. There will be many qualified astronomers who will encourage you to look through their telescopes. We’re even going to put a telescope on our roof and project it onto a big screen so that there will be an unobstructed view. We’ll also have solar glasses and other safety information. It’s completely free and everybody is encouraged to show up.

If you’re not in Toronto, if you don’t have a local astronomy club or if it’s cloudy or nasty, you can always hit the internet. Places like the Keck observatory and tons of other outlets will be broadcasting it for sure.

When does it all happen?

In Toronto, it’s happening on the 5th of June, which is a Tuesday. The transit itself starts at about 6:04 PM Eastern Daylight Saving Time; that’s when the planet just touches the edge of the sun. It will be completely on the sun at 6:21 PM. It will then slowly move across, and exit about 6 hours later. Unfortunately, for us the sun sets at 8:56PM, so we won’t see the end of the transit.

Go to www.transitofvenus.org to find out exactly when it will happen based on where you are in the world. Have fun!