In 13,000 years Polaris will no longer mark true north, because Earth is not a perfectly spinning top. This episode unpacks the three orbital rhythms that Serbian geophysicist Milutin Milankovitch calculated by hand in the 1920s: eccentricity, the stretching of Earth's orbit on roughly 100,000 and 400,000 year timescales that can change incoming solar radiation by up to 23 percent; obliquity, the axial tilt rocking between 22.1 and 24.5 degrees every 41,000 years; and precession, the 25,700 year wobble that decides which hemisphere faces the Sun at closest approach. Along the way the hosts explain why Northern Hemisphere winter is about four days shorter than summer thanks to Kepler's second law.

The heart of the episode is Milankovitch's counterintuitive trigger: not brutal winters but cool summers at 65 degrees north, where the continents ...