Just over 45 years ago, midway on their journey to the Moon, the astronauts of Apollo 8 looked back at the Earth and became the first humans in history to see our world wholly in the round, whirling through the void.

The Earth was so shrunken by distance, the astronaut Jim Lovell noted, that he could cover it with an outstretched thumb. Looking at the small disk of white, brown and royal blue far below, he grew pensive. “What I keep imagining is, if I am some lonely traveler from another planet, what I would think about Earth at this altitude, whether I’d think it would be inhabited or not,” Mr. Lovell mused. He saw no obvious signs of life or civilization, and wondered whether an outsider would even recognize the blue as ocean and the brown as land.

In lunar orbit on Christmas Eve, the Apollo 8 crew read the creation story from Genesis, and snapped pictures of the Earth rising like a fragile jewel above the Moon’s monochrome craters. The photographs awakened something long slumbering within all who saw them, a greater sense of worth for our world and all its wonders. Among those who sent Apollo on its lunar voyage, it had become fashionable to believe our species and our planet were unremarkable footnotes in an indifferent and pointless universe. Yet glimpsed from a spacecraft orbiting the Moon, alive and alone in the endless dark, the Earth was revealed without question as the most precious and significant object in the solar system, having birthed beings to behold such an expansive view.

The crew of Apollo 8 had set out to visit the Moon but rediscovered the Earth and some fraction of our faded glory. Today, we are poised for another epochal discovery, one that could once again transform our perceptions of our place in creation. We are on the verge of taking world-changing pictures — not of our Earth, but of other planets that might be like it, far beyond our solar system.