In a town of tourists and politicians, where onetime White House aides -- and their lawyers -- are this summer's celebrities, it's still possible to spend an evening with some real stars. And a few planets, too.

Every Monday at sunset, while the doors of bureaucratic Washington are closing, the doors at the U.S. Naval Observatory on Massachusetts Avenue are opened to the more than 100 astronomy buffs who have lined up at the main gate for a closer look at the night sky.

The Naval Observatory, the nation's oldest government scientific agency (established in 1844), employs about 70 civilian astronomers at its Northwest Washington site, which it shares with the official home of the vice president.

The astronomers, part of an observatory work force of about 200, have the use of seven telescopes for their work, which involves tracking the movement of celestial objects, writing nautical almanacs and determining the precise time for everywhere in the country.

The observatory does not chart as precise a course for tourists, however.

There are no advertisements of its weekly walk-throughs, but enough stargazers find their way to make the program a popular attraction in the Washington area. Almost every Monday night, all 140 places on the tour are filled.

"I saw it on an old Exxon map," Dana Timaeus, visiting from Beaumont, Tex., said of the observatory, which has occupied its current location since the 1890s, when it moved from Foggy Bottom.

Some visitors hear of the observatory while visiting the offices of their senator or representative on Capitol Hill.

Others discover it while flipping through the phone book.

And some come specifically for the celestial tour.

On a recent Monday, students from the West Virginia Governor's Honors Academy, a summer program for gifted high school students, drove the 70 miles to Washington.

"I brought 'em down to see the telescopes," said Tim McCarthy, the group's leader.

He said the students, who are taking several science classes this summer at Shepherd College in Shepherdstown, are interested in lenses of all types.

Two telescopes of particular interest on the tour are a 26-inch refracting telescope built more than 100 years ago, and a 12-inch reflecting telescope built in 1892.

"It's interesting that scopes are this old and they're still using them," Timaeus said.

In fact, the astronomers at the observatory saved one of the scopes from the junk heap a few years ago.

The Navy had "retired" and dismantled the 12-inch reflecting telescope in 1971, but observatory volunteers restored it and it is now a popular stop on the tour.

For those not of a scientific mind, in a refracting telescope, light passes through a series of lenses, magnifying the image.

In a reflecting telescope, light is bounced between a series of curved mirrors.

Reflecting telescopes have greater light-gathering power, and thus can see fainter objects.

The observatory's 26-inch telescope made astronomical history in August 1877 when Asaph Hall, an astronomer at the Foggy Bottom site, used it in discovering two moons of Mars: Deimos and Phobos.

On a recent tour, astronomer Sean Urban aimed the telescope toward the double star Alpha Hercules in the eastern sky.

The faint celestial object became strikingly clear.

Richard Schmidt, another astronomer, aimed the 12-inch reflecting telescope toward Saturn, revealing a rare view of the planet's rings to each person on the tour.

Urban said he enjoys leading tours at the observatory because "everyone who comes here wants to be here."

After Nicholas Thomas, a high school student from Augusta, Ga., saw Saturn and its rings, he said: "It makes me want to study astronomy. We don't have astronomy classes like this. I wish we did."

Although the observatory's telescopes are kept in mint condition, the five buildings at the site are showing some signs of aging, and occasionally the astronomers are called on to supply a little brawn as well as brains.

When Urban tried to turn the observatory's revolving dome on the recent tour, a mechanism stuck and the dome would not budge.

Recruiting a man from the tour group to hold the control button, Urban climbed the low rafter of the dome and nudged it.

The dome finally moved, its open slit facing the east.

The appreciative crowd gave him a round of applause. For further information about Naval Observatory tours, call 653-1543.