As a scientist, Carl Sagan conducted important research in areas such as planetary atmospheres, astrobiology, and the origin of life on Earth. But he was primarily known as a spokesperson for science, with few others having done more to promote and popularize astronomy in the past 100 years. Sagan had a way of instilling curiosity and wonder in even the least scientifically minded listener. And when it came to humankind, he was ever the optimist. He recognized how unique and precious each person is, even on the unimaginably vast cosmic scale. “If a human disagrees with you,” he said, “let him live. In a hundred billion galaxies, you will not find another.”