Mercury Gets Its Bright, Shining Moment on the Western Horizon

May is a long month with a solar system full of action. The evenings open with Mercury in the western sky stealing attention from larger planets, including Saturn and Mars, which entertain sky gazers high above the horizon.

Usually, Mercury is hard to see, but this fleet planet is quite visible now in the western sky after dusk. Just after sunset, the planet is about 17 degrees above the west-northwestern horizon, and on Tuesday, Mercury is joined by a new crescent Moon. (Thin crescent moons in the western sky are always new moons, and they set shortly after the sun. Conversely, full moons always rise in the east, opposite sundown in the west.)

Mercury will be at zero magnitude, bright enough to see from the city, but make sure you have a good view of the western horizon. At mid-month, Mercury puts a little more cosmic distance between itself and the setting sun, which means we get to see Mercury's beauty for a little longer after sunset.

However, after the middle of May, Mercury nudges closer to the sun. By the end of the month, this planet will be hiding, once again, in the sun's glare.

Of the two planets -- Saturn and Mars -- on center stage at sundown, Saturn is the brightest and easiest to find. The ringed planet is at zero magnitude and high in the southern sky after dusk, loitering as usual with the constellation Leo. You'll see it due south about 60 degrees from the horizon. The dim Mars, visible after dusk, high in the south-southwestern sky, hangs out between the constellations Gemini and Cancer.

And speaking of Mars, on May 25, NASA's Phoenix Mars Lander will land on the planet's arctic surface to examine the polar region in great detail. It was launched in August from Cape Kennedy, Fla., and its instruments will look for evidence of water and carbon-containing compounds and report atmospheric processes. Follow the mission's progress at http://phoenix.lpl.arizona.edu.

For night owls, Jupiter ascends the east-southeast about 2 a.m. now, and by sunrise, the large gaseous planet is due south. Look for this negative second magnitude (very bright) object to the east of the constellation Sagittarius. By month's end, Jupiter climbs from the eastern sky about two hours earlier, giving gazers grand opportunities to enjoy this planet's splendor on the morning side of night.

If you are looking for Venus, don't bother. This effervescent planet will return in late summer.

Down-to-Earth Events

· Tomorrow -- "Stars Tonight," a planetarium program describing May's night sky, 7:30 p.m., at Arlington County's David M. Brown Planetarium, 1426 N. Quincy St., adjacent to Washington-Lee High School. Information: 703-228-6070.

· Tomorrow -- Astronomer Mia Bovill discusses "Near Field Cosmology (or The Little Galaxies That Could)," at an open house, University of Maryland Observatory, College Park. Afterward, enjoy the stars and planets through a telescope, weather permitting. 9 p.m. Information: 301-405-6555; http://www.astro.umd.edu/openhouse