For three generations, astronomers were certain that Mercury kept one face permanently turned toward the sun, leaving the other side in eternal, frozen darkness. It was a tidy and elegant conclusion drawn from trembling glimpses of the planet at dawn and dusk, when viewing conditions were at their worst. This episode explores how that seemingly settled astronomical fact was overturned.

We look at the difficult history of observing Mercury through Earth's turbulent atmosphere, where early observers like Giovanni Schiaparelli and Percival Lowell saw faint, persistent markings that seemed to confirm a tidally locked world. The breakthrough did not come from a larger optical telescope, but from a completely different kind of instrument. In the 1960s, radio telescopes and radar pulses revealed that the dark side was too warm for permanent shadow ...