One of the four periods into which the year is divided based upon the position of the Sun in the sky. The four seasons are spring, summer, autumn (also called fall), and winter. Some elementary textbooks claim that seasons are caused by the Earth getting slightly closer to the Sun at perihelion, resulting in higher temperatures. THIS IS COMPLETELY WRONG. First of all, seasons are shifted by a half year in the northern and southern hemispheres (so that it is summer in the southern hemisphere when it is winter in the northern hemisphere, etc.), which contradicts the perihelion explanation. Second of all, the Earth turns out to be closest to the Sun in January (January 4 at 21:00 UT in 1998), which is winter in the northern hemisphere.

The real cause of seasons is the 23.5° obliquity of the Earth's rotational axis to its orbital plane. The hemisphere of the Earth which is tilted toward the Sun receives a greater flux of solar energy ("flux" is just a fancy word for energy per unit area per unit time) than the hemisphere tilted away, resulting in higher temperatures. The effect of incidence angle on solar flux is well-known to everyone, since the day is warmest when the Sun is overhead (at which point the Earth's surface is nearly perpendicular to it) and then cools as the Sun nears the horizon (at which point the sunlight grazes the ground at an angle, resulting in a smaller amount of heating per unit area of the ground). Here is a QuickTime movie illustrating the tilt of the Earth's equatorial plane relative to the Sun which is responsible for the seasons. The dates of maximum tilt of the Earth's equator correspond to the summer solstice and winter solstice, and the dates of zero tilt to the vernal equinox and autumnal equinox.

Autumn, Autumnal Equinox, Equinox, First Point in Aries, Obliquity, Solstice, Spring, Summer, Summer Solstice, Vernal Equinox, Winter, Winter Solstice