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 tumbling of Pluto’s small moons, the distance-to-the-sixth-power locking timescale, the estimated 85 percent of detected exoplanets in their stars’ locking zones, and Tau Bootis, a star apparently locked by its own planet.

• How gravitational gradients create tidal bulges in solid rock and act as a planetary brake
• Where the Moon’s lost spin went and why Earth’s day gains 2.3 milliseconds per century
• Libration and parallax: why we actually see 59 percent of the Moon
• Mercury’s 3:2 resonance and the gravitational washing machine around Pluto and Charon
• Eyeball exoplanets, twilight habitable zones and the star tidally locked by its planet