13 Nov 2006
(Source: Johns Hopkins University Applied Physics Laboratory)
A report on Mercury transiting the Sun, by Clark R. Chapman, of the Southwest Research Institute, Boulder, Colo.
Mercury is a very difficult planet to see in the twilight or dawn skies, because it stays so close to the Sun. This innermost planet, and the smallest one (if you accept the recent demotion of Pluto), is occasionally easy to see, by special means, when it passes directly between the Earth and the Sun. These events, called "transits of Mercury," occur about a dozen times a century. They are visible from somewhere on the Earth, but require a small telescope, equipped with special filters, so that the observer isn't blinded by the Sun. They can also be viewed by projecting an image of the Sun onto a white card.
I had the good fortune to be in Tucson, Ariz., on November 8, when the most recent transit occurred with the Sun high in the cloudless Arizona skies. This was the best Mercury transit opportunity in North America since I saw my first transit of Mercury in 1960. My wife and I visited famed comet discoverer David Levy at his home in the desert near Tucson. Also attending were Eli Maor, author of the book "Venus in Transit," which is about the even rarer transits of Venus (these occur only twice a century; the last one was in 2004, the next one in 2012, and then not again until the 22nd century). Maor and his had wife traveled from Chicago to be assured of sunny skies. Several other amateur astronomers also converged on Levy's "telescope farm" for the event.
Levy is one of the co-discoverers of Comet Shoemaker-Levy 9, which famously split into about 20 fragments - each of which crashed into Jupiter - during a one-week period in 1994. Although he has discovered many comets in earlier years (21 to be exact) professionally operated search programs are claiming most of the comet discoveries these days. But on this day, Levy was still smiling about find number 22, his first comet discovery in a dozen years. Just a few weeks before the Mercury transit, he had found a fuzzy blob next to Saturn in his telescope's field-of-view. He feared it was simply a strange optical reflection of Saturn, but it turned out to be a real comet!
On Wednesday, Mercury started to cross the face of the Sun around mid-day, Tucson time. We arrived mid-afternoon to find the planet already two-thirds of its way across the Sun. Looking through a telescope equipped with a hydrogen-alpha filter, I was startled by the deep red color of the Sun's chromo sphere, mottled by the gas jets called spicules. Then I noticed the solid-black disk of Mercury, perfectly round in shape, silhouetted in front of the Sun. From minute-to-minute, it slowly crawled from spicule to spicule.
Levy's back-yard hospitality extended beyond his dozen fellow astronomers and friends onsite; as a SkyGuide for Slooh.com, he hosted thousands of virtual transit watchers. He was leading a live broadcast featured on AOL, in association with Slooh.com, an online portal for live astronomy that offers its members an opportunity to explore space live in real time, seeing many objects difficult to view with typical backyard telescope equipment through its easy to use web interface.
Levy provided commentaries during the five-hour event while live images of the transit were being net cast on AOL. As Mercury approached the end of its transit, he interviewed me about MESSENGER. I told the audience about MESSENGER's recent pass by Venus, and I described the spacecraft's future path and plans for exploring Mercury.
One of the most beautiful features of the Sun, when viewed through a hydrogen-alpha filter, are the enormous, wispy "prominences" projecting beyond its edge. The looping structures visible on November 8, confined by the Sun's magnetic field, were larger than Earth. As Mercury slowly approached the edge of the Sun, it became apparent that it might pass in front of a particularly large prominence just after leaving the edge of the Sun.
Levy described to Slooh.com listeners the optical illusion called the "black-drop effect," just before the leading side of little Mercury's disk reached the edge of the Sun. And then, suddenly, Mercury became a notch in the solar profile, rather than a complete disk. The notch dwindled in size and, at about 5:10 p.m. MST, the transit was over.
Or was it? Maybe we could watch the planet's disk faintly framed by the prominence. But, unfortunately, the Sun was lowering toward the horizon, and the telescope was now peering through branches of a desert Palo Verde tree, so the viewing was obscured.
A few minutes later, we watched for the "green flash" as the top edge of the Sun finally blinked out behind a distant mountain range, but I saw nothing special as the light began to fade and the air quickly cooled. Later, as the group enjoyed supper at the Levy's home, we talked about the miracles of shadows. In a sense, we had just been in Mercury's shadow, as cast on Earth. But, like the "shadow" of a stratospheric jetliner, Mercury's shadow only infinitesimally dimmed the sunlight on Earth. But by using the proper telescopic equipment, we were rewarded with a good view of the entire circumference of the planet, which will soon to be orbited (in 2011) by the MESSENGER spacecraft.
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as principal investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.