A large, white spot has been detected on the surface of Saturn, according to the International Astronomical Union of Cambridge, Mass. The spot appears to be a violent storm that erupted last week on the planet's equator.

"{Aside from the rings} it's the single most prominent feature I have ever seen on Saturn," says Geoff Chester of the Air & Space Museum. Chester and a group of amateur astronomers saw the spot Monday night from the grounds of the Arlington Planetarium. It is only visible through telescopes. The last time a large spot appeared on Saturn was in 1960. Find Saturn preparing to set in the southwest early in the evening. The ringed planet continues to loiter near Sagittarius and lately has only associated with planetary chums Neptune and Uranus. Saturn is to the left of the Sagittarian teapot.

Mars enters the night stage a few hours after sunset in the east with the constellation Taurus becoming brighter as autumn turns into winter.

Jupiter, visiting the constellation Cancer, rises in the east around midnight but probably won't be visible until about 1:30 a.m., when it's high enough above the horizon to see.

That big, bright ball, grinning at you as it glides across the southern sky tomorrow night, is the full Harvest Moon. It's always the closest moon to the beginning of fall and bright enough for farmers to see as they gather their harvest.

The Great Square of Pegasus -- another time indicator that leaves will fall -- is riding high in the eastern sky, by the time the sun sets. The Great Square of Pegasus looks like a baseball diamond or it looks like a kite, depending on your perspective. As a kite, it sails across the southern evening sky, with a tail trailing behind. The W shape of Cassiopeia sits above the wisp of the kite's tail.

Look for another autumnal symbol, this time in the north: Two similar constellations, Ursa Minor and Ursa Major dominate that part of heaven. You know them as the Big and Little Dippers. At the end of the Little Dipper handle, Polaris beams brightly. Quite literally, it's all dressed up with nowhere to go. Since our own North Pole is pointed directly at that star, from our perspective it looks as if it just stays there. During this season, the Little Dipper pours a few stars into the Big Dipper. That will change in a few months.

Bringing a preview of Winter, Orion begins to rise around midnight. By the end of the month, it will mark time throughout the southern sky before the first snowflakes. Orion is the constellation shaped like an H and when it rises the H will be on its side. Accompanying Orion on its eternal journey, the twins Castor and Pollux, of the Gemini constellation, follow hot on Orion's shoulder.

Down-to-Earth Events Astrophysical Lecture Series -- The Smithsonian and the Air & Space Museum present a series of free lectures on astrophysics. The lectures, which run on consecutive Wednesday nights in October, begin at 8 o'clock at the Einstein Planetarium.

"From Solar Constant to Stellar Cycles: A Century of Sunwatching" will be presented tonight by astrophysicist Sallie Baliunas.

Karl Henzie, a NASA senior scientist, explains "Smithsonian Meets Sputnik: When Astronomy Entered the Space Age" on Oct. 10.

Astronomer Brian Marsden discusses "Comets, Meteors, Asteroids and Other Small Stuff in Space" on Oct. 17.

Steve Murray, astrophysicist, talks about "New Windows on the Universe: Observations Above the Atmosphere" on Oct. 24.

David Latham, an astronomer, focuses on "Innovative Instruments for a Golden Age of Research" on Oct. 31.

Oct. 6 -- If you can't figure out why it's so hard to find constellations this close to the city, the Air & Space Museum's Geoff Chester presents an enlightening talk on light pollution, "The Vanishing Night." 10 a.m. Einstein Planetarium at the Air & Space Museum. Free.

Oct. 12 -- Visit a space colony and discover the cosmos in "Destination Universe: Our Future in Space," scheduled to run on the weekends through Nov. 18 at the Arlington Planetarium, adjacent to Washington-Lee High School. Friday and Saturday evenings at 7:30 p.m.; Sunday matinees at 1:30 p.m. and 3 p.m. $2 for adults; $1 for children. 703-358-6070 for reservations.

Oct. 13 -- Actor Kevin Reese portrays a young boy growing up during the early days of the American space program in the one-man play "Apollo to the Moon." 9:30 a.m. Einstein Planetarium. Free.

Oct. 15 -- Sit under the stars for Jane Ira Bloom's suite "Most Distant Galaxy," performed at the Eistein Planetarium. Bloom will be on the saxophone, Rufus Reid on bass and Jerry Granelli on drums. 8 p.m. Free.

Oct. 24 -- Sten Odenwald, an astronomer with the Naval Research Lab, explores the future of searching the cosmos in his lecture "Astronomy in the 21st and 22nd Centuries." Arlington Planetarium. 7:30 p.m. $2 for adults, $1 for kids. Reservations at 703-358-6070.

Naval Observatory Tours -- Due to budgetary constraints, weekly tours of the heavens through the oldest and biggest telescopes in town will end on Oct. 29 at the Naval Observatory, just when autumn skies and crisp winter heavens provide choice viewing. According to Susan Glutting, public liaison at the observatory, biweekly tours are scheduled to begin the first Monday in May.

In the meantime, free tours will take place each Monday to the end of October, beginning at 7:30 p.m.