Carl Sagan, the 43-year-old glamor-boy of astronomy, is hunched down in the fifth row of the Ziegfeld Theater on West 54th Street, waiting for the five o'clock matinee to roll.Whoooooosh! Suddenly viewers are bathed in yellow haze as a sandstorm rages across the screen.

It's Sagan's first encounter with Steven Spielberg's $20 million cosmic gamble, "Close Encounters of the Third Kind." He watches UFO sleuth Claude Lacombe (Francois Truffaut) emerge from the swirling Mexican desert. Lacombe is about to fall upon a squadron of Navy fighters mysteriously lost over Florida in 1945. Where are the pilots, everyone wants to know.

Sagan scoffs. "There's not a smidgen of evidence to suggest that lights in the sky or the disappearance of ships or planes are due to extraterrestrial intervention. The return of those planes is a favorite incident of the most uncritical panderers of the Bermuda Triangle mysteries. Extraordinary claims require extraordinary evidence."

So it goes. All around, the audience sits gaping. Kids cease crunching popcorN. You can hear parents' heavy breathing, their children's gasps.

What Spielberg serves up is a tale of UFOs' visiting Muncie, Indiana, and one of the witnesses - power-company line-man Roy Neary (Richard Dreyfuss) - becoming obsessed with finding an explanation. Visiting spacecraft zip and zap about with the help of animation and Doug Trumbull's special-effects wizardry.

Carl Sagan yawns. "I'm not able to say such a thing is impossible. It would certainly be a much more interesting world if such a thing had happened. But in the real world, it hasn't.

"I find science so much more fascinating than science fiction. It also has the advantage of being true. My general feeling is that there is a lot of life throughout the universe. But feelings don't count for all that much in science. The essential point is to find out. But this movie doesn't even represent a plausible scenario."

As you might guess from the title, there's a close encounter with cosmic munchkins. THis was especially displeasing to Sagan, who calls Hollywood's propensity to represent alien beings as humanoids "earth chauvinism." "That's the part of science fiction that's so impoverished." he says. "They take a human being and warp him slightly and you get laughable caricatures of human beings. THe range of possibilities is so much greater."

Carl Sagan is an exobiologist - a scientist involved in the search for extraterrestrial life. Yet, since no evidence exists to prove such a notion, he operates in that halfworld between laboratory and cosmic fancy. To bridge the gap between sci-fi and hard fact, he tries to spark curiousity in the possibility of life Out There. He writes best-sellers such as "The Cosmic Connection"; at Cornell, where he runs the Laboratory for Planetary Studies, his lectures are standing-room-only, Carl Sagan Productions, his media incarnation, is currently preparing a 13-part series. "Man and the Cosmos," for 1980 broadcasting over PBS; he continues to serve as a NASA adviser, as he did on the Mariner and Viking surveys of Mars. And he frequently tromps to Washington to lobby for giant radio telescopes capable of eavesdropping on other galaxies. Sagan feels they might be bombarding us with messages at this very moment.

Although no signal has yet been received, the scientific super-star doesn't rule out the grand possibility of life elsewhere. "I would be astounded if life weren't coursing through the cosmos," he says.

If man ever greets life in the galaxy, our neighbors are more likely to look like the Blob, or a Thing, than Barbarella, Darth Vader or Spielberg's little wobblies. The same ingredients that, eons ago, went into the cosmic blender and produced Earth are believed to be salted throughout the universe. But chances are slim. Sagan says, that the elements would ever commingle again to produce anything resembling man.

Human beings came about as sort of a "cosmic accident," he explains, though that concept might not sit well with large egos. A cosmic ray zapped a gene 4 1/2 million years back; the gene mutated and evolved and, voila, man and woman. "Most biologists would agree that if you started the earth out again and merely let the same random factors operate, you would never wind up with anything looking like a human being. If that's the case, than some very different physical planet would have zero chance of producing the aliens you see in the movie."

It's dark and bitter cold outside as "Close Encounters" blasts off the screen and Carl Sagan, poker-faced, scouts out an escape hatch. His gray-green eyes dart down the alley.No National Enquirer reporters lurking about.

The Enquirer sustains a hefty 4 million weekly circulation by packing its pages full of Hollywood romance, instant happines, psychic phenomena and fantastic tales of UFOs. Especially UFOs. Reporters are paid handsomely to secure quotes from "exprts" that lend credence to the latest speculation, and there's a reported million-dollar bounty for absolute proof of a visitor from space. Sagan has stiff-armed the tabloid for years, but its agents keep landing on his doorstep. A UFO endorsement from Carl Sagan would be tantamount to convertin gLarry Flynt.

So far, there's no evidence to support even an encounter of the first kind - sighting a UFO. Sagan calls contentions of astronauts from other worlds popping down "fundamentally silly." Yet his own speculation on extraterrestiral life draw fire from scientific peers who call him too freewheeling.

Such comments he repeats as a frequent guest on The Johnny Carson Show, where his boyish face is beamed into bedrooms acorss the land. People recognize him - especially people who snap up The National Enquirer in supermarkets.This also makes Carl Sagan "an expert."

Down the street, over an encounter with a dish of manicotti, Sagan struggles to choose between "Close Encounters" and "Star Wars." He confesses affection for Spielberg's notion of communicating with aliens in ways other than words. In "Close Encounters," John Williams' thematic score booms through a kind of cosmic calliope to cement a galactic friendship.

Sagan likes the idea of music as the language of the universe, and for NASA's Voyager flight beyond our solar system, he concocted an LP of "earth sounds" to ride shotgund. Along with "Hi there!" in 60 tongues, there is whalespeak, volcano grumble, wave crashing, animal talk - all in "evolutionary sequence" - and music. Classical, Eastern and, yes, even rock'n'roll. If Voyager finds any space rockers beyond Pluto, they may thrill to Chuck Berry twanging "Johnny Be Good." "There are lots of ways to communicate what we know, but few ways to communicate what we feel," says Sagan. "Music is one way to communicate emotions."

Still, he sniffs, both films remain riddled with humanoid aliens and "scientific inaccuracies." He just doesn't understand why producers don't hire some starving grad student to guard against errors. In "Star Wars," pilot Han Solo shoots into hyperspace in so many "parsecs." A parsec is a measure of distance not speed. "That's like saying. 'I got up at 32 miles this morning,'" says Sagan. And he found the award ceremony a subtle case of discrimination characters in white received medals for valor, while the Wookie, a minority player who braved equal hardship, was ignored.

Asked to choose between the two films, though, Sagan gives the nod to Lucas. "The 11-year-old in me liked 'Star Wars.'" He banishes "Close Encounters" to the Siberia of "pop theology."

But "2001 Space Odyssey" remains his favorite sci-fi movie. It's also, of course, the only one of the three he was asked to consult on. Sagan advised director Stanley Kubrick not to depict aliean beings as some nephew of the Purple People-Eater, and Kubrick's close encounter was left to the imagination.

What especially worries Sagan is the negative impact such films might have on the future of space exploration. What if viewers come away from "Close Encounters" believing there is little need for a space program because UFOs will surely zip down some day, as they do in the film? Some, he fears, might even come away thinking scientists are holding back evidence as good as Spielberg's fantasies.

On the other hand, he says, "These are excellent movies for eight-year-olds. They excite a sense of wonder not too taxing for a child's mind. They may even end up intriguing children with the idea of space, and play the same role the Mars novels of Edgar Rice Burroughs played for me. If that happens and 20 years from now we have a host of young scientists who were turned on by 'Star Wars' and 'Close Encounters,' Hollywood will have performed a service. But we'll have to wait and see."*