Jupiter knows summer. The large gaseous planet straddling the western evening horizon will take a hiatus beginning June 21, only to return as a morning beacon just before school starts in September.

Summer officially arrives in the northern hemisphere when the sun hangs directly above the Tropic of Cancer. The sun hits the Tropic of Cancer at 11:33 a.m. EDT June 21, according to the Naval Observatory.

Meanwhile the whole earth-side surface of Jupiter currently is illuminated at -1.9 magnitude (bright). Up until its departure, Jupiter hangs out with Castor and Pollux, two bright stars in Gemini. A brand new moon joins Jupiter's bon-voyage party after sunset on June 23.

Venus will illuminate the eastern morning horizon at a magnitude of -4.0 (very bright).

Aside from Venus and Jupiter, the summer constellations stagger onto the starry stage: Regally crossing the summer evening, Leo heads west. Leo's front is shaped like a backward question mark, while its mane and lion's body face east. Leo's tail points to Arcturus, the prominent beacon in the constellation Boo tes. Shaped like a warped kite, it will be almost overhead in the late evenings.

Just to the east of Boo tes, the crown of Corona Borealis and the strong silhouette of Hercules roll across the heavens.

Later in the evening, the "summer triangle" -- the stars Deneb, Vega and Altair -- rises from the east and introduces the hot, sultry, seasonal nights. Deneb and Altair lie in the Milky Way stream while Vega sits on its banks.

The Milky Way, however, will be downright impossible to see anywhere close to the city because of excessive light pollution. Dark-sky locations on the perimeter of Washington's influence offer the best opportunity to view the Milky Way.

A new comet was discovered by David Levy of Tucson on May 20. Not much is known about Comet Levy but astronomers currently predict that the comet might be a "force magnitude" object, decent enough to see in binoculars. The comet is about 280 million miles from here and is expected to arrive in our part of the cosmic neighborhood late this summer.

A rock fragment that geologists believe might be a piece of Mars will be on exhibit at the Air and Space Museum beginning June 16. It's a gray, two-inch piece of meteorite that was discovered on Antarctica in 1979. Scientists have determined that the rock's chemical composition is nearly identical to the atmosphere found on Mars as it was measured by the Viking landers in 1976, according to the Air and Space Museum.

Down-To-Earth Events Tonight -- Voyager Project scientist Ed Stone of the Jet Propulsion Laboratory recaps the missions of Voyagers I and II. He will describe how the Voyager data will be used and he will unveil a "family portrait" of the solar system. Langely Theater at the Air and Space Museum. 7:30 p.m. Free.

June 13 -- Catching stars hiding behind the moon and other galactic objects, David Dunham will describe the results of watching occultations -- eclipses. He is the founder of the International Occultations Timing Association. Arlington Planetarium. 7:30 p.m. $2 for adults; $1 for children.

June 13 -- Two area poets (both recent winners of the Washington Writers Publishing House poetry competition) read their own astronomical creations. Patricia Garfinkel, who wrote "From the Red Eye of Jupiter," and Elisabeth Murawski, who wrote "Moon in Mercury," bring their starry poems to the Einstein Planetarium. Air and Space Museum. 8 p.m. Free.

June 18 -- Under The Stars -- spend a summer night under the stars, the galaxies, stray meteors and deep-sky objects. Bill Burton provides an introductory sky lecture before the overnight trip at the Shenandoah National Park June 23. This outdoor adventure is sponsored by the Loudoun campus of Northern Virginia Community College and there is a $40 fee. Information, (703) 450-2551.

June 21 -- Astronomers are exploring the center of our Milky Way galaxy with infrared, X-ray and radio telescopes to confirm the existence of a massive black hole. Join radio astronomer Kwok-Yung Lo of the University of Illinois as he describes this telescopic search. Einstein Planetarium. 7:30 p.m. Free.

June 30 -- Catch the moon, Saturn, Uranus and a few friendly star clusters at Sky Meadow State Park near Paris, Va. On Route 17, seven miles north of I-66 and two miles south of Route 50. Viewing begins at 8:30 p.m. Free.