Mercury is tiny. But it's a big deal for thousands of amateur astronomers in the Western hemisphere who will be watching the planet on Monday as it makes a transit across the disk of the sun.
In fact, on a list of prime celestial events this year, Sky & Telescope's observing editor, Diana Hannikainen, says "it's definitely very high up there."
If you have a telescope, it must be equipped with an appropriate sun filter to keep from damaging your eyes. Looking through it, this is what — weather permitting — you might see:
Beginning at about 7:35 a.m. ET, Mercury, looking "like a little freckle," according to Hannikainen, will begin a slow trek across the sun — a tiny black dot silhouetted against the sun's immense brightness. About three hours later, it will appear midway across the sun, and by shortly after 1 p.m. ET, Mercury will exit the stage as a black drop — an optical illusion that will make it momentarily appear as a teardrop.
Martin Russell Wise, 37, is a pretty serious amateur astronomer. He even moved to Trenton, Fla., west of Gainesville, because of the area's dark skies.
"But I find myself more and more often during the day just observing and monitoring our local star — the sun," he says. "I've kind of specialized in that the past three years or so now."
Wise plans to use a standard astro-imaging technique — taking high-speed video and then stacking the best images in the computer to get crisp, clear shots of the transit.
You won't necessarily need your own telescope to view the transit. Dozens of local astronomy clubs will be hosting events and if you're lucky enough to have a public observatory nearby, there's a good chance it will have telescopes set up for viewing.
Lowell Observatory in Flagstaff, Ariz., is one such place. The observatory — made famous by Percival Lowell and his fanciful observations of the red planet and what he believed were canals built by Martians — has a new observing plaza with six telescopes, several of which will be equipped with solar filters for the public to view the transit.
"In Flagstaff, the transit starts at 5:35," says Kevin Schindler, a spokesman for the observatory.
"But the sun doesn't rise until 6:57. And so, you know, the first hour and 25 minutes or so will miss," he says. "So what we're going to do is start at 7 o'clock."
If you can't get to the observatory, Lowell is one of many observatories that will be livestreaming the transit. At least one of these, the space-based Solar Dynamics Observatory, won't have to worry about the possibility of clouds.
Meanwhile, more than a dozen observing sites will be set up by the Citizen Transit of Mercury (ToM) Project, made up mostly of students. They hope to re-create a method Sir Edmund Halley (of comet fame) used to determine the Earth-sun distance. The method uses data gathered by observations of transits of Venus in the 18th century to determine (with the help of some trigonometry) the distance between Earth and sun. The same thing can be worked out from a transit of Mercury.
If you have a chance to access a sun-filtered telescope, Monday could be an opportunity to see one of the innermost planets directly, rather than simply as reflected light, says Hannikainen, who worked as a professional astronomer before coming to Sky & Telescope.
"You can actually see its dimension. You can see that it is spherical. And that is something that is is quite a rare phenomena," she says.
It's a thought echoed by Wise, the amateur astronomer.
"[We] accept that there are most probably planets and we can all suspend our belief and just accept that without much argument," he says, adding that what's magical is "being able to walk outside and physically watch a planet just float by over a course of hours between you and a star."
Transits of Mercury occur 13 or 14 times a century; the next one visible from the U.S. will be in 2049.