It was hard to grasp. You were actually seeing what amounted to an Instamatic snapshot of Saturn's rings from a mere 125,000 kilometers out. But what you got was a small TV screen full of black-and-white diagonal lines.

People began collecting at the National Air and Space Museum long before the live satellite coverage of the Saturn fly-by yesterday at 5. Almost 500 of them clustered around the batteries of television sets, with some 200 sitting quietly on the floor. There is another session today from 2 to 3 p.m.

The Saturn pictures came about every 2 1/2 minutes. The ones that Voyager took of Jupiter in March 1979 came every 48 seconds, but that was millions of miles ago, and the signal is weaker now. So last night, scientists (in color) talked and talked between transmissions. Mostly they pointed out craters and assorted spots they hadn't seen in earlier shots.

The crowd quickly turned restless, drifting off to view a free film on Voyager, retreating to the rear to chat, while newcomers picked their way through sprawled legs and coats to vacant places. These were babies in strollers, students, young couples, old couples, grave scientific-looking savants who nodded wisely at the pictures, a very wrinkled man with a white beard, and a family that spoke Polish in an undertone the whole time.

"I sure thought there'd be a bigger screen," said Doris Newton of Bowie, who came all the way in by train just to sit on a planter box, watching the historic event."You can't hear very well either. I'll probably watch it on the 11 o'clock news anyway. But I hope they do more things like this in the space program. They shouldn't keep it on ice. We have to know. It's out there, and it's not going anyplace. We have to know."

Children seemed more fascinated with local TV coverage of the scene, goggling at the monitors and the cameras and the announcers. The audience was told that a storm had come up at the Madrid tracking station just at the wrong moment, but the transmission was okay. Whew.

"I've seen better on TV, and better explanations," said James Cooperman of Chevy Chase. "I'll go home and watch it on the news tonight."

Now there was a sharp view of the moon Mimas. Someone would have to count the craters, the experts were saying. Dr. Andrew Ingersoll of Cal Tech, a specialist in planet atmospheres, talked about the mystery of Saturn's liquid hydrogen surface and why its clouds are so different from Earth's brushstroke cloud patterns. A wait for another transmission. More people left.

Just overhead, not 10 feet from the TV sets, hung the Wright Brothers' plane, which first flew in 1903, only a lifetime ago.