Earth Axial Tilt (Obliquity of the Ecliptic)

Seasons come from this angle, not from the varying distance to the Sun: the tilt points the northern hemisphere sunward in June and away in December, changing both the height of the Sun and the length of the day. The tilt fixes the tropics at 23.44° latitude, where the Sun can stand overhead, and the polar circles at 66.56°, where it can stay up all day. Eratosthenes measured the obliquity around 230 BC to within a fraction of a degree by comparing midsummer and midwinter shadows.

It is slowly changing. Planetary perturbations rock the obliquity between about 22.1° and 24.5° over a 41 000-year cycle — currently decreasing by roughly 47 arcseconds per century — and that cycle is one of the Milanković pacemakers of glaciation. The Moon's stabilising torque keeps the swing small; without it, models suggest Earth's tilt would wander chaotically like Mars's has.