Born on 9 November 1934 in Brooklyn, New York, Carl Sagan was an influential astronomer and science popularizer. He studied physics and astronomy at the University of Chicago. Sagan and his collaborators discovered that the greenhouse effect warms the surface of Venus, that wind-blown dust causes certain features on the Martian surface, and that sooty materials found in the outer solar system arise from the action of sunlight on methane. He helped select landing sites for the Viking probes on Mars. Sagan cofounded the Planetary Society in 1980 to promote space exploration, and he became a vocal proponent of the search for extraterrestrial intelligence. Despite his many scientific accomplishments, Sagan is best known for sharing science with the masses. He wrote the science-fiction novel Contact, which became a popular movie. And he starred in the TV series Cosmos, which he wrote with his wife, Ann Druyan. (A reboot of the series, narrated by Neil deGrasse Tyson and cowritten by Druyan, aired in 2014.) Sagan died in 1996 at age 62. Sagan’s most famous words came when describing the 1990 photo of Earth that, at Sagan’s suggestion, was taken by NASA’s Voyager 1 spacecraft from 6 billion kilometers away: “Look again at that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. … It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we’ve ever known.” (Photo credit: NASA/JPL)