Why does it all seem so long ago? Alan Shepard smiling wanly from the steel cocoon of Freedom 7 . . . Yuri Gagarin having his hand raised fighter-style by Khrushchev . . . Jack Kennedy telling Congress, in that clipped twang of his, that we should commit ourselves to landing a man on the moon "before this decade is out."

We made it with five months to spare, on July 20, 1969, and today the Library of Congress opens a 10th anniversary exhibit on those events, the dawning of the space age.

A similar display of Apollo 11 artifacts and videotapes opened over the weekend at the Air and Space Museum, where on Friday the first moon men themselves, Neil Armstrong, Edwin "Buzz" Aldrin and Michael Collins, will be honored in special ceremonies.

Even those names have to be pulled out from the dusty stacks of memory. Is it perhaps some principle that Einstein overlooked, that time is compressed as our understanding of space expands?

Sputnik, I which set off the whole hullaballoo, seems as old as George Washington. But is was only 22 years ago. There is a picture of it in the Library of Congress show, a fine, human-scale display of videotapes, recordings, photos and 16th-century moon maps, and a nice sequence simulating in a series of stills what an astronaut would see on approaching the moon.

One reason those days seem so distant is that they covered an incredibly short span. In 1961, while we were still firing off 70-foot Redstone rockets, the Russians sent Gagarin into orbit on April 12. On April 22 Werner von Braun told Kennedy that "there is a sporting chance" we could still beat the Soviets to the moon. Thirteen days later, Shepard was shot into space.

All these events are dramatized in the display, along with such curiosities as a letter written by rocket pioneer Robert Goddard in 1920 to one "Lieutenant Aldrin," who turns out to be astronaut Edwin Aldrin's father. Goddard was still six years away from his first successful flight.

There is also a good record, "Sounds of Earth," addressed to Anybody Listening Out There and featuring all kinds of sounds, from erupting volcanoes to kisses, from pygmy ritual songs to Chuck Berry's birthday "Johnny B. Goode."

Especially intriguing are the political aspects of this show: reminders of the shock that Sputnik dealt to the somnolent Eisenhower era and the resulting stampede to catch up, and then the urgent need to recoup prestige after the Bay of Pigs. That was in April of '61, too, a year o' action and decision.

It seems like a century ago.