This book tells the story of how we learned what controls the temperature of the planets, including our own. From the first ideas on climate in ancient times, to the magnificent intellectual leaps of the Renaissance, then via the progressive understanding of heat and the daring explorations of the Earth and atmosphere by ship, mountain ascent and balloons in the 19th century, to the modern era of mathematical prediction of weather and climate, the story sweeps in parallel with astronomical observation of our planetary neighbors. This scientific tour sees everything from long and meticulous calculations by lone geniuses, to international diplomacy and globetrotting adventures of discovery. Science is a human endeavor, and its forward march has been sometimes punctuated by self-delusion, dismissal of radical ideas, and untimely death. Critical advances have been sometimes lost for years, but the centuries leading up to the era of planetary exploration progressively built our knowledge of the sun, the greenhouse effect, and the ice ages, setting the stage to understand our neighboring worlds, and our past and future.