Axial Tilt, Seasons & Precession: Structures & Vocabulary

Earth's seasons are caused primarily by the 23.5-degree tilt of its rotation axis, not by large changes in Earth-Sun distance. The tilted axis changes both the angle at which sunlight strikes a hemisphere and the number of daylight hours during the year. Over much longer timescales, the direction of the axis itself slowly changes through precession.

A hemisphere tilted toward the Sun receives sunlight at a steeper angle and for more hours each day, concentrating more solar energy per unit area. Six months later the geometry reverses. Precession arises mainly because the gravitational pulls of the Sun and Moon act on Earth's equatorial bulge, producing a torque that slowly changes the axis direction.

Seasonal changes in noon solar altitude and day length match the geometry of axial tilt. The two hemispheres experience opposite seasons at the same time, which rules out Earth-Sun distance as the main cause. Historical star positions and modern astrometry show the slow shift of the celestial poles expected from precession.

The important geometry includes the 23.4-degree obliquity, solstices, equinoxes, celestial poles, ecliptic, and precessional cone. At a solstice one hemisphere is tilted most toward the Sun; at an equinox the Sun crosses the celestial equator. Precession slowly shifts equinox directions over roughly 26,000 years.