Mercury's March madness
The closest planet to the Sun shines brightly in the twilight sky.
March 10, 2005Mercury orbits the Sun every 87 days, 23 hours, and 18 minutes. For every 1 Earth "year," Mercury experiences 4.Mercury's diameter is 3,032 miles, only 38-percent that of Earth's.The largest crater on Mercury, named Beethoven, is the largest known crater in the solar system. Beethoven is nearly 400 miles in diameter.From Mercury, the Sun is 6.3 times brighter than from Earth.Mercury has a total of 297 named features, of which 239 are craters.Mercury next will pass in front of (or, transit) the Sun November 8, 2006. This event will be visible from the United States.
Mercury — the closest planet to the Sun — makes its best evening appearance of the year this month. Half an hour to an hour after sunset, look low in the west for a bright dot piercing the deep orange to blue twilight sky. The planet was brightest March 1 when it stood 6° above the western horizon 30 minutes after sunset. (For comparison, your closed fist held at arm's length spans 10°.) Each day after March 1, Mercury fades a bit, but even 2 weeks later it will be brighter than any star in its area.
Mercury will be easiest to find March 11, when a 2-day-old crescent Moon lies nearby. You can spy Mercury 4° to the Moon's lower right, about 11° (a bit more than a fist's width) up from the horizon in the west. A view of Mercury through a telescope that evening will reveal the planet's small disk, which will appear 48-percent lit, similar to a quarter Moon.
The next evening, Mercury reaches a point called greatest elongation — its maximum distance from the Sun as seen from Earth. In the days following, Mercury remains fairly high but fades rapidly. On March 17 it lies 10° above the western horizon 30 minutes after sunset and sets 1 hour later. Mercury appears in front of the faint stars of Pisces the Fishes during this entire period of visibility.