Before Luna 3 photographed it in October 1959, no human had ever seen the far side of the Moon. This episode explains the physics behind that frozen face: tidal locking. The hosts describe how Earth's gravitational gradient stretches the Moon into a prolate spheroid with tidal bulges, how the Moon's rotation drags those bulges out of alignment, and how the resulting torque and internal rock friction slowed its spin until one rotation matched one 27-day orbit.

The conversation follows conservation of angular momentum to the Moon drifting away at 3.8 centimeters per year, Earth's day lengthening from about six hours to 24, the prospect of mutual tidal locking, and why the Sun will become a red giant first. It also covers libration and parallax that let us see 59 percent of the lunar surface, Mercury's 3:2 spin-orbit resonance, the chaotic tum ...