The film is only 28 minutes long. It is shown in only one theater here, with 485 seats. It isn't advertised except for brochures and weekly newspaper notices. It costs 50 cents, and you have to wait in line for tickets, and usually you wind up seeing a later show than you had planned.

The film is "To Fly," and in its four years at the Smithsonian's Air and Space Museum, 4.5 million people have gasped at it.

It is not just the longest running commercial film in Washington history, it is a national Sight. Vistors come here to see the stone presidents, the FBI Building -- and "To Fly."

It is so big that even the music played while people are filing into the spectacular, steeply canted autorium has become famous. Johann Pachelbel's luminous and simple Canon was not exactly something that the average American hummed before 1976. The composer, one of the great pre-Bach organ masters, died in 1706, his contrapuntally uncomplicated work known mostly to organists and music students. The curiously modern Canon has had a revival of sorts anyway, appearing for instance in the Werner Herzog film about Kaspar Hauser.

But surely that wasn't what caused the tidal wave of interest that has put the short piece on 18 separate stereo records.

For the past year "To Fly" has been shown back to back with "Living Planet," a companion travalogue made by the same people, but the original, which opened on the same day as the museum itself, July 1, 1976, on schedule and under budget as the Smithonian loves to remind us, is the one that audiences seem to love best.

Where do they learn about it? Why do Washington visitors -- up to 10 million a year at the Air and Space alone -- seem to know all about "To Fly" before they even get here?

It's partly word of mouth, of course, because you just don't forget that experience, that vast screen. But also, officials at sponsoring Conoco Oil Co. pointed out, it has been seen on TV at least twice, hundreds of 16mm prints are circulated to schools all over the world, and there are several other IMAX screens in this country and in Germany, Japan and Mexico. Theme parks go for them, in particular.

The picture begins on a standard screen, with a costume piece involving some nonsense about an 1831 balloonist who is evidently, like the other period figures scattered through the film, a bow to the Bicentennial for which the movie was made. The balloon takes off. The balloonist tips his beaver hat.

And suddenly the conventional screen blows up to an enormous white wall five stories high and seven stories wide, and we are in the balloon, sailing over the clean forests of Vermont.

It catches the throat. You hear the audience go "ahhh." A few seconds later, to a thunderous crescendo, we waft over Niagara Falls, and then mount a hillside to burst like the sun over a yawning valley.

Now, having tasted the exhilaration of flight, we do some filmic acrobatics:

the vast screen breaks into three parts, nine parts, 36 parts. The Blue Angels team of Air Force jets spirals joyously into endless blue. A field of sun-glided wheat rushes beneath us. Like wheeling hawks, we shear past red buttes in the sunset. Finally, as the (dispensable but not obnoxious) narrator muses that "We first flew in dreams . . ." we follow a hang-glider swooping, diving, schooning above the sudden mountains of Kauai.

If ever there was a moment that said, This is what it is like to fly, that was it.

The picture, produced by Francis Thompson (who got an Oscar for his triple-screen documentary "To Be Alive") and photographed on 70mm film by Greg MacGillivray and James Freeman, draws on sophisticated techniques for its aerial effects. The camera was rigged under the nose of their helicopter for a more vertiginous impact. In what has to be the most brethtaking single sequence, we fly upside down over the landscape in a stunt plane.

Even the pilot, when he saw this part of the film, couldn't bear to look.

The director-photographers -- already known in the business for their aerial work on "The Towering Inferno," "Jonathan Livington Seagull" and Sky Riders" -- had to develop a whole new set of techniques to handle their oversize IMAX camera. They designed special camera mounts so they could shoot from beneath a Boeing 747, their helicopter and an F4 jetfighter. cThey built a remote control aiming gun which operates any of nine lenses. The illusions are astonishing: at one point we seem to be flying through St. Louis' Gateway Arch in a 747 jet liner. Actually, we are in a copter equipped with a specially titled lens to give the appearance of great speed.

Freeman, tragically, died just three weeks after "To Fly" opened in 1976. His copter went down in the Sierra Nevada foothills during a filming job.

The picture has six-track stereo sound on 11 speakers. The score was composed by Bernardo Segall, a Brazilian concert pianist, and recorded by a 50-piece symphony.

The last part of "To Fly" seems to be a return to the Bicentennial idea: the inevitable gaze into the future with the 1975 Apollo liftoff and imagined space vehicles of an era to come. By this time we have seen more than enough movies that end with an Apollo liftoff; it's the successor to the atomic cloud ending cliche.

Besides, you could argue with the notion that floating in space is flying at all.

But forget these quibbles. You come away from the film remembering the flying, the freedom of it, the glee, the exaltation. No wonder "To Fly" is a national monument.