Carl Edward Sagan (November 9, 1934 - December 20, 1996), born in Brooklyn, NY, was an American astronomer and astrophysicist. His best-known scientific contribution was his research on the possibility of extraterrestrial life, including experimental demonstration of the production of amino acids from basic chemicals by exposure to light. He assembled the first physical messages sent into space, the Pioneer plaque and the Voyager Golden Record, which were universal messages that could potentially be understood by any extraterrestrial intelligence that might find them. The following is a lecture Sagan gave in 1977 during which he discussed the place, scale and geometry of earth--the third planet from the Sun, formed 4.5 billion years ago--in our solar system.