The northern hemisphere welcomes an evening treat, the kind of gift not seen in several years: a naked-eye comet. Comet Levy is visible right after sunset in the southern sky, where it is moving with the constellation Sagittarius westward.

"This comet is behaving as most comets are supposed to behave," says Geoff Chester of the Air and Space Museum. Last spring, he notes, Comet Austin "fizzled miserably."

But Comet Levy sits in the sky at +3.0 magnitude, which is well within unaided visibility. There's no denying it will be dim, since it looks like a tiny, firm cotton ball hanging above the spout of teapot-shaped Sagittarius. In urban and suburban areas, where light pollution poses the biggest problem, a small telescope or binoculars will enhance chances of catching Levy. The best idea is to journey beyond the Beltway to find a dark sky.

Levy will fade rapidly in a few weeks, heading for the southern horizon by the end of the month. Meanwhile, Mars waits patiently in the wings, beginning its best showing of the last two years. It rises in the northeast, about an hour before midnight in the Taurus constellation. The red planet loiters with the star Aldebaran, in Taurus, through the end of the month.

Mars will reach -0.8 magnitude, bright enough to cut through the light pollution of any major city. It's taking a southern tour, coasting along with Aldebaran in the early morning hours. It will be very high in the southern sky just prior to breakfast.

The best evening planet to watch now is Saturn. Its marginally bright +0.4 magnitude makes it a decent target from the city, while a small telescope might discern the rings. Catch it coasting along the southern sky, close to Sagittarius and Levy as dusk falls. Saturn will be to the upper left of the Sagittarian teapot.

Venus will be leaving by the end of September. Still, it's departing at a -3.9 magnitude; the only brighter objects are the moon and the sun. Just before the morning alarm clangs, the planet will be very, very low on the eastern horizon. In fact, a chance to watch Venus's last days is as good an excuse as any for a final weekend at the beach.

In a little over two weeks, the real summer ends. The Autumnal Equinox ushers the southbound sun past the Equator on Sept. 23 at 2:55 a.m. EST, according to the Naval Observatory. At that precise moment, fall begins.

In case you're looking for the Harvest Moon, forget it. There won't be one until Oct. 4. The Harvest Moon is always the fullest lunar disc closest to the equinox. October wins this year. Down-to-Earth Events

Sept. 8 -- Astrophysicist Jeff Goldstein conducts the first in a series of kid-oriented astronomy programs at the Einstein Planetarium in the Air & Space Museum. 9:30 a.m. Free.

Sept. 8 -- Astronomer Maria Zuber of the Goddard Space Flight Center discusses the vast Martian vistas in a program sponsored by the National Capital Astronomers. 7:30 p.m. Einstein Planetarium. Free.

Sept. 10 and 15 -- Bill Burton of the Northern Virginia Astronomy Club explores the Milky Way in his "Astronomy Overnight" class. Sponsored by the Northern Virginia Community College's Continuing Education program at its Loudoun campus, the class meets for an introductory lecture, then hunts for sky objects a few days later. $35. For further information, (703) 450-2551.

Sept. 10 -- Track your favorite constellation at the Arlington Planetarium with "The Stars Tonight." Adults $2, kids $1. 7:30 p.m. For further information 358-6070.

Sept. 17 -- Cosmic phenomena such as neutron stars were predicted before they were discovered. University of Maryland astrophysicist Virginia Trimble explains how to determine if the universe is made of "dark matter." 8 p.m. Langley Theater at the Air and Space Museum. Free.

Sept. 22 -- The National Capital Astronomers and the Park Service jointly sponsor a view of the sky at Rock Creek Park. 7:30 p.m. Glover and Military roads NW. Free.

Sept. 22 -- Say goodbye to Comet Levy when Geoff Chester of the Air and Space Museum points out that and other heavenly objects at Sky Meadows State Park, located between Paris and Delaplane, Va., on Route 17. Free. 8 p.m.

For those into advance planning, on the nights of Oct. 12 and Oct. 20 National Capital Astronomers Walter Nissen and Dan Costanzo will each take an overnight tour groups for a closer look at the heavens. This program is sponsored by the Smithsonian Resident Associates. For further information, call 357-3030.