Stephen Hawking breathlessly reports in the new edition of A Brief History of Time that the publisher has sold one copy for every 750 men, women, and children on the planet. I finally joined that constellation of readers and came away awed at the vastness and complexity of our universe.

Science teeters between hubris and humility. Hawking's book shows how much we have learned. Yet just in the last few years astronomers have admitted underestimating the number of galaxies by 50 billion or so (oops!) and missing the age of the universe by around 8 billion years—and, oh yes, there is that embarrassing "dark matter," which no one has found yet but which may constitute 90 percent of the matter in the universe.

I remember the early Apollo missions, when engineers took pride in the spacecraft's 5 million parts, all of which had to work together with precision. This, the greatest technological achievement of our species, gave the first real glimpse of our puny place in the universe. As Stephen Hawking describes it, Earth is "a medium-sized planet orbiting around an average star in the outer suburbs of an ordinary spiral galaxy, which is itself only one of about a million million galaxies in the observable universe."

To the Apollo astronauts, though, that humdrum planet looked just fine. Jim Lovell, reflecting on the scene, said, "It was just another body, really, about four times bigger than the moon. But it held all the hope and all the life and all the things that the crew of Apollo 8 knew and loved. It was the most beautiful thing there was to see in all the heavens."

Scientists have a hard time imagining how it all happened. As astronomer Chet Raymo puts it, "If, one second after the Big Bang, the ratio of the density of the ...