One of the rarest of predictable astronomical events will happen May 9 when we on Earth will be able to see Mercury move across the face of the Sun. Mercury transits happen about 13 times per century. The last one was in 2006 and the next is relatively soon, in November of 2019. After that there won’t be another until 2032.
By contrast, transits of Venus occur in pairs eight years apart separated by more than a century. There were Venus tranists in 2004 and 2012, and there won’t be another until December of 2117.
NASA illustration of the 2006 Mercury transit.
Julie Lutz, an emeritus research professor of astronomy at the University of Washington in Seattle, said the Mercury transit will be mostly spectacle, as there isn’t much science to be done.
“We’ve learned about all we can from previous transits of Mercury,” Lutz said. “Early on it was a matter of using the transit timings and things like that to confirm things about the orbit of Mercury, get it more and more accurately, try and deduce the size of the planet, things like that.”
During the transit Mercury will appear as a small, black dot crossing the face of the Sun. Lutz pointed out that watching for such occurrences at other stars is precisely how astronomers are detecting exoplanets.
“We can detect Earths now,” Lutz said. “The Kepler mission has changed the whole picture of planet distributions.” She added that if there is a planet the size of Mercury, which has a diameter of just over 3,000 miles, in orbit around a distant star, we couldn’t spot it in transit. We detect exoplanets by watching for slight reductions in brightness as the planets block some of the light as they transit. Our current instruments just aren’t sensitive enough to see the small change that would be caused by an exo-Mercury.
Mercury will even be a little tough to spot from Earth. While we could see the Venus transit just by looking at the Sun through eclipse glasses, that won’t work with Mercury.
“Mercury is smaller than Venus and further from the Earth, so the apparent size of the planet from our point of view is going to be smaller, so it will be harder to see,” Lutz said.
You’re going to need some magnification, according to Stephanie Anderson, co-owner of Cloud Break Optics in Ballard.
“In order to see Mercury well, you’re going to have to get to 20 to 30 power,” Anderson said. Binoculars with solar filters won’t be quite enough, as these typically have somewhere between eight and 10 power. Binoculars could be used to set up a projection of the Sun’s image on a screen or sheet of paper. Otherwise, Cloud Break’s Matt Dahl said almost any telescope would do the trick.
“You need a front aperture mask of some sort to minimize the light through it. Don’t ever use a telescope to look at the Sun without proper filtration,” Dahl warned. “If you want to see all of the other effects that are going on the Sun, the prominences and granularity all that kind of stuff, a specific solar telescope would be ideal.”
If you don’t have a telescope or a friend who has one, Anderson said they could set you up with a simple refracting telescope or a tabletop reflector for around $100. The aperture mask and solar filter will run $50 to $75, so for less than $200 you’d be ready to roll.
“Then you can use that cool thing to see the (solar) eclipse when it comes up next year in August, and then you can also use it to view the night sky,” Anderson said. “It’s good for more than just viewing the Mercury transit.”
Both Dahl and Anderson are accomplished astrophotographers, and Dahl said shooting the Mercury transit will be relatively easy. He said the Sun is a bright enough object that you wouldn’t really even need a tracking mount. Just hook up your DSLR or a webcam to your telescope. Anderson added that it might even be simpler than that.
“Just use your smartphone to snap a picture right through the eyepiece,” she said. “You might have the technology in your pocket already.”
It can be tough to get a good frame with a smartphone and a telescope, but Anderson said you’ll have time to experiment.
“The event will last for quite a while, so you’ll have some time to mess around with it,” she said.
The full Mercury transit will be visible east of the Dakotas, Kansas, and Nebraska. It will already be in progress when the Sun rises at 5:40 a.m. on May 9 in Seattle, and it will end at about 11:40 a.m. Pacific time. Dahl and Anderson are considering setting up viewing at Magnuson Park, and Seattle Astronomy may be down at Anchor Park at the far north end of Alki Beach Park if the weather is good. If it looks like it will be a cloudy day, we all might hit the road in search of clearer weather, because transits are cool.
“The thing that excites me the most about them is that you get to see the pieces of the solar system moving together,” Anderson said. “You get to see that the Sun has planets going around it. It’s really an amazing thing to see with your own eyes.”
“It gives you a really humbling size reference when you contrast the size of the Sun to the size of the tiny little dot,” of a transiting planet, Dahl said. “The Venus transit four years ago was awe-inspiring for me that way.”