May 1, 2008
Mercury — the closest planet to the Sun — makes its best evening appearance of the year this month. Mercury looks about as bright as it ever does, with the view made picturesque May 2 by its location just 2° south of the Pleiades star cluster (M45), which lies in the constellation Taurus the Bull.
At its brightest in May, the innermost planet shines at magnitude –0.9. Brighter than any star through May 8, Mercury makes a stunning sight above the west-northwestern horizon. A 2-day-old crescent Moon joins the scene May 6, when it lies less than 3° to Mercury's upper right. You'll need clear skies and sharp vision to detect the Moon. Its slender crescent will be only 4 percent illuminated.
For the best views, use binoculars just as twilight falls. Camera owners might want to try shooting some images. The most pleasing contain foreground objects — trees, a water tower, or a windmill, for instance — which enhance the scene. They form great silhouettes and add to an image's impact.
A telescope easily reveals Mercury's phase, which shrinks to half-lit by May 8. It then stands 8° north of Aldebaran, Taurus' brightest star. Although Aldebaran is a 1st-magnitude star, it nevertheless shines a full magnitude fainter than Mercury.
Astronomy Senior Editor Michael E. Bakich explains what's happening: "When Mercury is as far east of the Sun as it can get (called greatest eastern elongation), we see it as an evening star low in the west," he said. "When it's west of the Sun, we view it as a morning star in the east before sunrise."
Some elongations are better than others because of Earth's tilt and the stretched-out nature of Mercury's orbit. Even at its farthest from the Sun, Mercury appears no more than 28º away.
Mercury reaches its greatest elongation May 13/14, when it lies 22° east of the Sun and sets 2 hours after our star. Still located in Taurus, Mercury then reveals a 37-percent-illuminated disk through a telescope. If you've followed it from the start of May, you'll notice the planet now shines more than a magnitude fainter. Still, it outshines all but two current nighttime stars.
Mercury continues to dim all month. By the 18th, it equals Aldebaran in brightness, and it appears only half that bright by the 22nd. Its angular distance from the Sun declines as well, making the planet harder to see after this date.