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.
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.