Mercury’s Slow Glide Across the Sun

May 9th's transit of Mercury is now history. Countless amateurs — and professionals — watched as the innermost planet had its day in the Sun.

It's sometimes the curse of being involved in astronomical outreach that I miss all the action during an important celestial event. For example, I never got to see Comet Shoemaker-Levy 9's crash with Jupiter in 1994 firsthand, because I was constantly flying between observatories or sitting under rain clouds. Likewise, I spent all of totality during November 2012's solar eclipse on the bridge of a cruise ship that had no outside access.

My circumstances weren't any better on Monday, when tiny Mercury crossed the Sun's disk for the first time since 2006. For the entire 7½-hour-long transit, I was sequestered in the Solar Operations Center at the Harvard-Smithsonian Center for Astrophysics (CFA), just down the street from S&T's offices. It was a day full of "Lights! Camera! Action!" as I anchored a live webcast of the entire event.

But don't feel too badly for me. I got to check out video feeds of the transit from all over, starting with Gianluca Masi's Virtual Telescope Project in Italy and ending with hydrogen-alpha images from Big Bear Solar Observatory in California. Our main feed came from the Clay Center Observatory at Dexter Southfield School. But we occasionally switched to the extreme-ultraviolet views from NASA's Solar Dynamics Observatory (SDO), which were nimbly displayed by my host, solar researcher Henry "Trae" Winter. A team at CFA built the main battery of cameras aboard SDO, collectively called the Atmospheric Imaging Assembly.

Mercury's circular silhouette looms large in this image from the 1.6-m New Solar Telescope at Big Bear Solar Observatory in California and its adaptive-optics imaging system.
Jay M. Pasachoff

Mercury's tiny disk didn't put much of a "dent," so to speak, in the Sun. Yet teams of professional astronomers still monitored and recorded the transit for its scientific value. For example, Mercury has virtually no atmosphere, so its silhouette should be razor sharp. But the satellite's cameras record just a hint of light that leaks into the black disk due to internal scattering and diffraction along the optical path. So following Mercury during its transit helps calibrate these effects.

Out at Big Bear, Jay Pasachoff (Williams College) led a multiple-site investigation to further characterize the "black-drop effect." It's a vague, dusky link between the black disk and black space seen just after Mercury fully slides onto the solar disk and just as it's about to leave. Although this was once thought to be an illusion, modern studies show it's a real effect caused by darkening of the solar limb right at its outer edge. Pasachoff has great data from BBSO's 1.6-m New Solar Telescope, which is equipped with adaptive optics for super-high-resolution views of the Sun.

NASA had planned to make space-based views of the event widely available. But its transit-specific websites proved too popular and frequently bogged down from the crush of visitors. This short video provides a nice recap of what SDO saw:

Mercury's Transit Viewed Globally

Monday's slow-motion "dash" lasted from 11:12 to 18:42 Universal Time. That timing favored eastern North America (7:12 a.m. to 2:42 p.m. Eastern Daylight Time). Skywatchers in the western U.S. and Canada got in plenty of viewing too, once the Sun rose. And much (if not all) of the event could be seen from Europe, Africa, and western Asia. Really, only those in Australia and eastern Asia got left out completely.

Reports received at Sky & Telescope suggest that many amateur astronomers took advantage of Mercury's transit to show what was going on to a largely unaware public. Typical is this report from Chris Malicki, who observed with his wife from a public park in the Toronto suburb of Mississauga, Ontario:

"About 40 members of the Royal Astronomical Society of Canada, Mississauga Centre, came out with about 15 telescopes. We were able to show the Mercury transit to a number of members of the public — some of whom had no idea initially why we were there, but were thrilled to see such a rare event. At egress, Liz and I observed in white light with a Celestron 8 and Tele Vue under high magnification, and we were both able to see the impressive black drop effect at around 2:44 p.m."

Members of the New Hampshire Astronmical Society set up their scopes on the sidewalk in downtown Portsmouth, New Hampshire, where the skies stayed clear long enough to capture this crisp view of the transit of Mercury in hydrogen-alpha light.
Herb Bubert

Terry Moseley also had success on the other side of the Atlantic Ocean: "Good views with the Irish Astronomical Association group, together with the Astrophysics Research Centre at Queen's University Belfast. Totally clear skies for about the first 90 minutes, then broken and passing clouds for an hour or so, then totally clear until 4th contact. Excellent outreach, with interest from passing public, and students and staff from QUB."

Out in Flagstaff, Arizona, Lowell Observatory opened for viewing at 8 a.m. (but staffers snuck in peeks earlier, since sunrise came at 5:27 a.m.). According to Lowell historian Kevin Schindler, several hundred people queued up to view the transit's progress through a battery of white-light and hydrogen-alpha-filtered telescopes.

And Ed Faits sent this message about viewing efforts in western Massachusetts by the Amherst Area Amateur Astronomy Association: "Long-time 5A member John Davis is now living in the Arbors assisted-living facility and is not too mobile these days. So club president Crystal Mengele arranged to bring the Mercury transit viewing party to him. The event proved wildly successful; many of the residents came by to enjoy the show. Apparently John is doing a good job educating the senior community about the science of astronomy."

Minimum Magnification for Mercury

During Monday's transit of Mercury, the planet's black silhouette was just 12 arcseconds across — about 1⁄160 of the Sun's diameter. Unlike the transits of Venus in 2004 and 2012, during which keen-eyed observers could spot the planet's silhouette without any optical aid, you needed a small telescope to see such a tiny black dot — or did you? Roger Sinnott, one of S&T's senior contributing editors, decided to see how little it took, optically, to pick out Mercury. He writes:

"I used three high-quality, prismatic optical systems: Nikon 7×21 binoculars, Bushnell 4× Xtra-Wide binoculars, and a 3× tank scope from World War II. For eye protection I held a Thousand Oaks solar filter in front of the objectives. At 7× Mercury's tiny speck was easy to see. At 4× it proved quite difficult, though once spotted I could hold it steadily in view. But at 3× the planet was never seen at all."

So, Sinnott concludes, whereas ancient skywatchers might conceivably have glimpsed a transit of Venus (though no such reports are known), "Apparently, seeing Mercury in transit had to await the invention of the telescope."

When's the Next Transit of Mercury?

At least some of Mercury's transit on May 9, 2016, was visible across the globe — except for Australia, New Zealand, and easternmost Asia. The entire event lasted about 7½ hours.
Sky & Telescope diagram / source: Fred Espenak

Because of the inclination of Mercury's orbit, at present the planet's transits can only occur in May and November — in fact, a transit also occurred on May 9th in 1970. We'll get our next chance to "spot the spot" in just three years, on November 11, 2019, and once again the Americas will be favored. Then comes a transit of Mercury in 2032 and again in 2039, but those won't be visible from North America. So if U.S. observers get clouded out in 2019, they'll have to wait until 2049 for another chance.

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Mercury is far more interesting than just as a little black dot. Check out Sky & Telescope's exquisite 12-inch Mercury globe, which incorporates thousands of images of the planet's surface taken by NASA's Messenger spacecraft.

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