Seasons on Earth are found only in the temperate zones. These zones extend from 23.5° north (and south) latitude to 66.5° north (and south) latitude. In these regions of Earth nature exhibits four seasons; spring, summer, autumn (or fall) and winter. Each season is characterized by differences in temperature, amounts of precipitation, and the length of daylight. Spring comes from an Old English word meaning to rise. Summer originated as a Sanskrit word meaning half year or season. Autumn comes originally from a Etruscan word for maturing. Winter comes from an Old English word meaning wet or water. The equatorial regions or torrid zones have no appreciable seasonal changes and here one generally finds only a wet season and a dry season. In the polar regions we have only a light season and a dark season.
In the Northern Hemisphere, astronomers assign an arbitrary starting date for each season. Spring begins around March 21 and summer begins around June 21. Autumn begins around September 23 and winter around December 21. Because every fourth year is a leap year and February then has 29 days, the dates of these seasonal starting points change slightly. In the Southern Hemisphere the seasons are reversed with spring beginning in September, summer in December, fall in March, and winter in June. Seasons in the Southern Hemisphere are generally milder due to the moderating presence of larger amounts of ocean surface as compared to the Northern Hemisphere.
Changes in the seasons are caused by Earth's movement around the Sun. Because Earth orbits the Sun at varying distances, many people think that the seasons result from the changes in the Earth-Sun distance. This belief is incorrect. In fact, Earth is actually closer to the Sun in January compared to June by approximately three million miles.
Earth makes one complete revolution about the Sun each year. The reason for the seasons is that the axis of Earth's rotation is tilted with respect to the plane of its orbit. This tilt, called the obliquity of Earth's axis, is 23.5degrees from a line drawn perpendicular to the plane of Earth's orbit. As Earth orbits the Sun, there are times of the year when the North Pole is alternately tilted toward the Sun (during northern hemispheric summer) or tilted away from the Sun (during northern hemispheric winter). At other times the axis is generally parallel to the incoming Sun's rays. During summer, two effects contribute to produce warmer weather. First, the Sun's rays fall more directly on Earth's surface and this results in a stronger heating effect. The second reason for the seasonal temperature differences results from the differences in the amount of daylight hours versus nighttime hours. The Sun's rays warm Earth during daylight hours and Earth cools at night by re-radiating heat back into space. This is the major reason for the warmer days of summer and cooler days of winter. The orientation of Earth's axis during summer results in longer periods of daylight and shorter periods of darkness at this time of year. At the mid-northerly latitudes summer days have about 16 hours of warming daylight and only eight hours of cooling nights. During mid-winter the pattern is reversed and we have longer nights and shorter days. To demonstrate that it is the daylight versus darkness ratio that produces climates that make growing seasons possible, one should note that even in regions only 30° from the poles one finds plants such as wheat, corn, and potatoes growing. In these regions the Sun is never very high in the sky but because of the orientation of Earth's axis, the Sun remains above the horizon for periods for over 20 hours a day from late spring to late summer.
Astronomers have assigned names to the dates at which the official seasons begin. When the axis of Earth is perfectly parallel to the incoming Sun's rays in spring the Sun stands directly over the equator at noon. As a result, daylight hours equal night time hours everywhere on Earth. This gives rise to the name given to this date, the vernal equinox. Vernal refers to spring and the word equinox means equal night. On the first day of fall, the autumnal equinox also produces 12 hours of daylight and 12 hours of darkness everywhere on Earth.
The name given for the first day of summer results from the observation that as the days get longer during the spring, the Sun's height over its noon horizon increases until it reaches June 21. Then on successive days it dips lower in the sky as Earth moves toward the autumn and winter seasons. This gives rise to the name for that date, the Summer Solstice, because it is as though the Sun "stands still" in its noon height above the horizon. The Winter Solstice is likewise named because on December 21 the sun reaches the lowest noon time height and appears to "stand still" on that date as well.
In the past, early humans celebrated the changes in the seasons on some of these cardinal dates. The vernal equinox was a day of celebration for the early Celtic tribes in ancient Britain, France, and Ireland. Other northern European tribes also marked the return of warmer weather on this date. Even the winter solstice was a time to celebrate, as it marked the lengthening days that would lead to spring. The ancient Romans celebrated the Feast of Saturnalia on the winter solstice. And even though there are no historical records to support the choice of a late December date for the birth of Christ, Christians in the fourth century A.D. chose to celebrate his birth on the winter solstice. In the Julian calendar system in use at that time this date fell on December 25.
See also Global climate.
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Darrel B. Hoff