Milankovich cycle
When the obliquity increases, the summers in both hemispheres receive more heat and light from the Sun, and winters less. Conversely, when the obliquity decreases, summers receive less sunshine and winters more. These slow 2.4° obliquity variations are roughly periodic. They take about 41,000 years to shift between a tilt of 22.1° and 24.5° and back again.