Season, one of several divisions of the year, characterized by cyclical changes in the predominant weather pattern. In the temperate zones there are four seasons; spring, summer, autumn (fall), and winter. These result from the constant inclination of the earth's polar axis as the earth orbits the sun: during summer in the Northern Hemisphere the North Pole is tilted toward the sun; in winter, when the solar radiation strikes the hemisphere more obliquely, it is tilted away from the sun. The summer and winter solstices (about June 21 and Dec. 22), popularly known as midsummer and midwinter, mark the beginnings of summer and winter, respectively. Spring begins on the day of the vernal equinox (about Mar. 21) and autumn at the autumnal equinox (about Sept. 23).