Season

Season, one of four divisions of the year, defined by the position of Earth in its orbit around the Sun (see Ecliptic). The seasons—winter, spring, summer, and autumn or fall—are characterized by differences in average temperature and in the amount of time that the Sun is in the sky each day.

The seasons occur because the axis on which Earth turns is tilted with respect to the plane of Earth’s orbit around the Sun. Earth’s tilt causes the North Pole to be tilted toward the Sun for half of the year, and the South Pole to be tilted toward the Sun for the other half of the year. The hemisphere that is tilted toward the Sun has a longer day, receives more of the Sun’s rays, and receives the Sun’s rays more directly than the hemisphere tilted away from the Sun. When it is summer in the Northern Hemisphere, this hemisphere is tilted toward the Sun; this corresponds to winter in the Southern Hemisphere, when the Southern Hemisphere is tilted away from the Sun. If Earth’s axis was not tilted, each night and day everywhere on Earth would always be 12 hours long and there would be no seasons. The axis of the planet Mars is also tilted with respect to the plane of Mars’s orbit around the Sun, so Mars experiences seasons much like those of Earth.

II

EQUINOX AND SOLSTICE

In late March and late September both hemispheres are the same distance from the Sun and the Sun is said to be at an equinox. The Northern Hemisphere’s vernal equinox usually occurs on March 20 or 21 and marks the beginning of spring. The Northern Hemisphere’s autumnal equinox usually occurs on September 22 or 23 and marks the beginning of autumn. In the Southern Hemisphere, the vernal equinox occurs in September and the autumnal equinox occurs in March. At the equinoxes, the Sun appears to be directly over Earth’s equator. The lengths of day and night are then equal over almost all Earth, except at the poles. At the North Pole and South Pole, Earth’s atmosphere bends the Sun’s rays enough to make the Sun visible throughout the day and night, even during the 12 hours the Sun is below the horizon.

The Sun is said to be at a solstice when the difference between the distances from each pole to the Sun is at its greatest. The solstices usually occur on December 21 or 22 and June 21 or 22. In December, the South Pole is tilted farther toward the Sun than it is at any other time of the year and the North Pole is tilted farther away from the Sun; the Southern Hemisphere has its summer solstice and the Northern Hemisphere has its winter solstice. In June, during the Southern Hemisphere’s winter solstice and the Northern Hemisphere’s summer solstice, the North Pole is at its most direct tilt toward the Sun, and the South Pole tilts away from the Sun. The hemisphere most tilted toward the Sun on the solstice experiences its longest day of sunlight and its shortest night. The other hemisphere experiences its shortest day of sunlight and its longest night.

The seasons have an unequal number of days because Earth’s orbit is slightly elliptical, or oval shaped, and the Sun is not exactly at the center of the orbit. Earth moves slightly faster when it is close to the Sun than when it is farther away, so the seasons that occur when Earth is close to the Sun pass more quickly. Earth is closest to the Sun in January and farthest away in July, so the summer is longer than the winter in the Northern Hemisphere. In the Southern Hemisphere, the winter is longer than the summer.

III

LENGTH OF DAY AND TEMPERATURE

The changes in temperature and in the length of daylight that accompany the seasons differ greatly at different latitudes. At the poles, summer is three months of daylight and winter is three months of darkness. Near the equator, however, days and nights remain about 12 hours long throughout the year. The Arctic and Antarctic circles, at latitude 66°30’ north and 66°30’ south respectively, mark the farthest points from the poles at which there can be 24 hours of daylight or 24 hours of darkness. Midway between the poles and the equator, the length of daylight varies from about 8 hours in winter to about 16 hours in summer.

The days when the Sun is near the summer solstice are not the hottest days of the year, even though the hemisphere experiencing summer receives the greatest amount of radiation from the Sun then. Temperature depends not only on the amount of heat that the atmosphere receives, but also on the amount of heat that the atmosphere loses through absorption by water and ground or through reflection. The oceans’ temperatures change much more slowly than the atmosphere’s temperature because water is much denser than air, and therefore takes longer to heat up or cool down. Although the atmosphere warms up quickly in the spring and summer, the atmosphere still loses much of its heat to the cooler water of the oceans; eventually the oceans absorb enough heat to reach equilibrium with the temperature of the atmosphere. The solid ground has a lesser but similar effect. For several weeks after the summer solstice, the atmosphere of the hemisphere experiencing summer receives less heat than it did at the solstice, but it also loses less heat to absorption and reflection. As a result, the average temperature continues to increase. When the loss of heat to the oceans and ground equals the gain from the Sun’s radiation, the temperature reaches a maximum. In the Northern Hemisphere this effect occurs in August, in the Southern Hemisphere in February. The oceans and the ground stay warm and release heat into the atmosphere as the atmosphere begins to cool off during the fall or winter, so the coldest days of winter do not occur until well after the winter solstice.