Image of the Day: The Transit of Mercury

A photo mosaic taken at 8:15 local time shows Mercury, the sun’s limb, and the sunspots AR2542 (top) and AR2543. (Credit: Ernie Mastroianni) As astronomical events go, Mercury’s solar transit was not as spectacular as say, a solar eclipse, but many observers on this sunny Boston day enjoyed the view of a relatively rare event last seen nearly 10 years ago. Some were getting their first-ever glimpse of the innermost planet. As seen through my telescope with a solar-safe filter, Mercury appeared to be little more than a diminutive dot as it drifted across the solar face, which is nearly 160 times its apparent diameter. The innermost planet is only 3,002 miles across, while the sun spans nearly 865,000 miles. The math doesn't add up because Mercury is closer to the Earth than it is to the Sun. The transit, viewable from most places in the world, began as a ...