Five hundred million people in sixty countries watched him explain the universe. Almost none of them could name a single thing he discovered.

Carl Sagan established that the surface of Venus is hot enough to melt lead, that the seasonal color changes on Mars are blowing dust rather than creeping vegetation, and that the orange smog of Titan is made of complex organic molecules drifting down onto an alien shore. He briefed the Apollo astronauts. He designed the first physical message our species ever sent beyond the Solar System. He also spent a portion of his youth calculating, for the United States Air Force, what a nuclear detonation on the Moon would look like from Earth.

This book restores the scientist without diminishing the showman, and then asks the question that neither his admirers nor his critics have asked properly: what actually happens to a piece of science when it is translated for people who are not scientists? Sagan is the most thoroughly documented case in history. He built instruments of translation — a calendar that compressed fourteen billion years into twelve months, a gold-plated record carrying Bach and Chuck Berry into interstellar space, a photograph of Earth as a single dim pixel — and every instrument, like every telescope, has a range of validity and a characteristic error.

Sometimes the translation produced more truth than the original: a novel that needed a shortcut to a distant star sent a physicist into genuine research on the structure of spacetime. Sometimes it produced distortion, and Edward Teller wrote to tell him that a propagandist is most effective precisely when he does not appear to be one. The National Academy of Sciences refused him membership and then, two years later, handed him its highest medal for service to the public. He died at sixty-two, having spent his final strength writing not about galaxies but about a country he feared was quietly losing the ability to tell what feels good from what is true.