Obliquity

Obliquity is the angle between a planet's spin axis and the normal to its orbital plane. Earth's is currently 23.44° and oscillates between about 22.1° and 24.5° over a 41,000-year cycle, slowly damped by tidal friction. Mars has an obliquity of 25.2° that varies much more wildly because it lacks a large moon to stabilise it; Venus is nearly upside-down at 177°; Uranus lies on its side at 98°.

Obliquity controls seasons: large obliquity means extreme seasonal temperature differences between summer and winter hemispheres. Over geologic time, the slow modulation of Earth's obliquity combines with eccentricity and precession to drive the Milankovitch cycles — the astronomical pacing of the ice ages.