Today, a look back at the moon landing. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

More than twenty years ago -- on July 20th, 1969 -- astronauts rabbit-hopped across the lunar surface and hit golf balls for the TV cameras. But even then, the bloom was somehow off the rose. The bread and circuses atmosphere was wearing thin. The American public was losing interest.

Before they started home, the astronauts quietly installed an 18-inch, low-tech reflector. It got little if any press. It was set there to help measure the quarter-million-mile distance from earth to the moon. In 1969 we knew the distance within a mile. That was close enough for space navigatation, but NASA had other things in mind. For example, a better knowledge of the distance would let us check Einstein's general relativity theory.

The reflector made laser ranging possible. By bouncing a laser pulse off that little target, we could measure the distance within a few feet. Of course, astronomers had to hit the reflector from a quarter million miles away. It was some help that even a laser beam spreads out in such a long journey. It's a mile wide when it reaches the reflector. But that's still like using a rifle to hit a dime from a distance of two miles. Then they had to catch the reflected beam when it came back.

By now, more reflectors have been set, and the measurement has been refined. Today we measure the distance to the moon within only one inch! That's astonishing accuracy -- 7 billionths of a percent. It's like measuring the distance from New York to Los Angeles within a 50th of an inch.

We've learned a lot from all this. For one thing, Einstein's general relativity theory is on better footing. For another, we've learned about changes in the diameter of the not-quite-solid moon and earth. The most startling result is that the moon is drifting away from us. It moves 2½ inches farther from earth each year.

Meanwhile, these passive reflectors go on serving us. They don't wear out. The worst that can happen is that moon dust might cloud their surfaces, but the lunar surface is so silent that there's been no sign of that in 20 years of use.

So the astronauts went home, and the circus folded up. Public interest faded, and funding faded with it. Yet this modest little reflector reminds us that the real payoff of all that whoop-de-do was more interesting than bread and circuses -- more exciting. Human knowledge has expanded out of sight of the television cameras, and we've grown as an exploratory species.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Morrison, D.C., An Unsung Legacy of the First Lunar Landing. Science, Vol. 246, Oct. 27, 1989, pp. 447-8.