How do slow wobbles in Earth's orbit set the rhythm of the ice ages?

Ice ages are paced by slow, predictable changes in Earth's orbit and spin, which shift how much summer sunlight reaches the far north. There are three main cycles. The shape of the orbit, its eccentricity, swings between nearly circular and mildly elliptical, mostly because of the pull of Jupiter and Saturn, in rhythms of about 100,000 and 405,000 years. The tilt of Earth's axis rocks between 22.1° and 24.5° every 41,000 years, and more tilt means more extreme seasons. And the axis itself wobbles like a top, which, combined with the orbit's slow turning, changes the season in which Earth passes closest to the Sun, on a cycle of about 21,000 years today. The Serbian scientist Milutin Milanković worked out how these combine, focusing on 65° north, where there is a lot of land.

He started under unlikely conditions. In 1914, as a reserve officer of the Serbian army, he was arrested by Austria-Hungary and sent to a prisoner-of-war camp. In his cell he pulled out his notes on 'the cosmic problem' and wrote until past midnight; it felt, he said, like a roadhouse on his trip through the Universe.

The theory has held up: ice and ocean-floor cores show the climate responding to northern summer sunlight. But it hides a puzzle. For the last million years, ice ages have come roughly every 100,000 years, matching eccentricity, the cycle that changes sunlight the least. Before that, they followed the 41,000-year tilt cycle. Explaining that switch is known as the 100,000-year problem.