On Veterans Day, Monday, Nov. 11, you can see the littlest planet pass directly in front of the sun. The event is called a transit, and they’re fairly rare. Mercury transits the sun about 13 times a century. What makes this event unique, at least for North America, is that we won’t see it happen again for almost 30 years — the next transit happens on May 7, 2049.
Additional transits occur in 2032 and 2039, but they’re visible from other parts of the planet. The sun won’t be up during those times for the U.S. You know what that means — don’t miss this one! Given my current age, Monday’s event will likely be the last of my lifetime. Astronomical prediction can sting like that, but it comes with the territory.
During a transit, Mercury appears as a perfectly circular, inky-black dot a little smaller than a BB. As it marches along its orbit, the planet slowly moves across the face of the sun. To see it, you’ll need at minimum a pair of binoculars magnifying 10x capped by a safe solar filter. Unlike Venus, which transited the sun in 2004 and 2012, Mercury is too small to see without optical aid.
I’ve seen the solar system’s fastest planet parade in front of the sun at least five times, and wouldn’t miss one. Each has been an adventure. My first was May 9, 1970, at age 16. I remember toting my telescope through the forest to an open field to catch a minutes-long glimpse of Mercury shortly after sunrise. Monday’s transit will seem like easy-street in comparison with hours and hours of viewing time. That depends, of course, on where you live. If that’s South America, the east coast of the U.S., eastern Canada and western Africa, you’ll see the entire 5½-hour-long event. The farther west, the less time you’ll have.
For example, in the Midwest, the sun will rise with Mercury already in view and 5 hours of transit time in store. That dwindles to around 4 hours in the mountain states and 3 hours along the West Coast. From my observing perch in Duluth, observers can start watching from the moment the sun clears the trees until the transit ends at 12:04 p.m.
Each provides an opportunity to appreciate the true giganticness of the sun. In our daily lives, there’s little with which to compare it. All we see is a bright, shiny disk the same apparent size as the moon, a rocky orb a mere 2,160 miles across. Pfft!
The sun is some 864,000 miles across, or 194 times the diameter of Mercury. During a transit, it looks enormous compared to the tiny planet and offers a truer glimpse of its real size. All transits occur in four acts, called contacts. At first contact at 6:35 a.m., the leading edge of Mercury’s disk kisses the sun’s outer edge (called the limb). One minute and 41 seconds later, the planet’s trailing edge fully enters the sun’s disk at second contact, touching the inner edge.
If you have a telescope, this is the time to look for the black drop effect, when the planet appears momentarily “stuck” to the sun’s limb by a taffy-like, dark filament. What you’re seeing is a combination of effects that include diffraction (the bending and interference of light waves as they squeeze through a narrow opening) and limb darkening. Even in a small, filtered telescope the center of the sun’s disk appears brighter than its edge. Square-on, we face directly into its brilliant, incandescent gases and it looks brightest. Along the edge we can’t peer as deeply so the material looks darker in comparison.
After second contact, Mercury spends hours crossing the solar disk until reaching third contact at the opposite inner edge and providing a final opportunity to witness the black drop. One minute, 41 seconds later, it departs the disk at fourth contact, and the show is officially over. Or is it? Observers with hydrogen alpha (H-a) solar filters might see the planet both before and after the official transit times. The specialized filter reveals pink prominences sticking out from the edge of the sun normally only visible during a total solar eclipse. A well-positioned prominence would serve as a spectacular backdrop for Mercury’s black silhouette.
If you don’t have a safe solar filter, you can safely project the sun’s image onto a white sheet of paper or a wall using either a pair of binoculars or a telescope. Take care to never look directly at the sun. In Duluth, the Arrowhead Astronomical Society and staff at the University of Minnesota-Duluth planetarium will combine forces and set up telescopes outside the campus planetarium from 9 a.m. till noon on Monday. Come on by! Clouds or not, you can watch the transit via live-streaming.
You can also catch the transit live on your phone or computer at Gianluca Masi’s Virtual Telescope Project site starting at 6:30 a.m., as well as at SLOOH’s Livestream, also beginning at 6:30 a.m. We’ve got you covered!