Sunwatchers prepare for Mercury

Observers across the globe — and even above it — are planning for Mercury's November 8 transit.
By | Published: November 2, 2006
Mercury transits the Sun November 8. Portions of the event are visible throughout North America to observers with clear skies, a telescope, and safe solar-viewing techniques.
November 2, 2006
Skywatchers from Canada to China are making preparations for next week’s rare planetary shadow play. On Wednesday, November 8, the planet Mercury will be visible in silhouette as it slides across the Sun’s face. The event, called a transit, lasts about 5 hours as Mercury, which orbits the Sun in just 88 days, laps Earth.

But amateur astronomers won’t be the only ones watching. Sun-observing satellites like NASA’s Transition Region And Coronal Explorer (TRACE), Japan’s newly orbited Hinode, and the joint NASA/ESA Solar and Heliospheric Observatory (SOHO) also will see the event.

For Bernhard Fleck, SOHO project scientist at NASA’s Goddard Space Flight Center in Maryland, the transit offers an opportunity to learn more about the instruments on the 11-year-old orbiting spacecraft. “SOHO’s instruments should show Mercury as a completely black dot. It doesn’t take a rocket scientist to see that’s not the case,” he says. Light scattered inside SOHO’s instruments contaminates the planet’s shadow. Transit observations offer a way for scientists to measure this effect and correct for it in images.

Watch Mercury transit the Sun from your desk
The Exploratorium in San Francisco
NASA’s Sun-Earth Day web site
Hawaii’s AstroDay

Jay Pasachoff at Williams College in Massachusetts hopes to use observations of Mercury from TRACE and Hinode to confirm his ideas about the so-called black-drop effect. This optical illusion, in which a dark thread appears attached to a transiting planet long after its disk has moved onto the Sun, stymied 18th- and 19th-century astronomers who tried to use Venus transits to measure the Sun-Earth distance. Its visibility depends in part on how sharply a telescope brings light to a point, but Pasachoff has identified other effects as well.

Mercury Transit with Airplane, 7 May 03, Germany
Hans Schremmer captured this wonderful image of Mercury’s transit from Moers, Germany (North 51° 26.557′ East 6° 36.927′) at 8:15 UT. He used an 8-inch Schmitt-Cassegrain (C-8), Thousand Oaks Optical Film Solar Filter, and a Canon AE-1 Camera with Fuji 200ISO Color Slide Film. The nose of the plane is pointing directly to Mercury’s tiny, black dot.
Hans Schremmer
Transit time
Observers under clear skies across North America will be able to see portions of the event before sunset, but the entire transit will be visible in its entirety for those near the West Coast. Mercury will look like a small, dark sunspot as it moves across the solar surface. A properly filtered telescope will provide the best view, because Mercury will appear only 1/194 the Sun’s size. A brief glimpse of the Sun through a telescope can permanently damage vision, so observers must use safe Sun-viewing techniques, such as using proper filters or projecting the image onto paper.

Mercury begins its star trek at 2:12 p.m. EST, when its disk first appears to touch the Sun’s limb as seen from Earth. Two minutes later, and for the next 4 hours and 54 minutes, Mercury’s entire disk appears against the bright solar surface. At 4:41 p.m. EST, Mercury makes its deepest solar incursion, passing just 7’03” away from center of the Sun’s disk. The transit ends at 7:10 p.m. EST as the planet slips off the Sun’s edge.

Mercury last transited the Sun in May 2003, and next week’s event marks the planet’s second of 14 transits this century. From Earth, only the planets Mercury and Venus can transit the Sun, and both events are relatively rare. After November 8, Venus will be the next planet to cross the Sun, in June 2012. Mercury follows suit 4 years later, in 2016.

In the zones
Why are Mercury transits so rare? Mercury’s orbit is tipped with respect to Earth’s and crosses the plane of our orbit at only two points. For a transit to occur, Mercury must lie near one of those points when it passes us. To meet these conditions, Mercury must slip between Earth and the Sun within 3 days of May 8 or within 5 days of November 10.

The unequal size of these “transit zones” reflects the eccentricity of Mercury’s orbit, which gives us twice as many transits in November as in May. Mercury lies farther from the Sun and moves much slower in May, which reduces our chance of catching it in the right place. But Mercury’s disk appears 20-percent larger during May transits because it’s closer to us then.