Seasons
the annual recurrence of spring, summer, autumn, and winter. The change of seasons is due to three facts:
1. The earth swings around the sun once each year.
2. The various positions of the axis on which the earth turns daily are parallel.
3. The earth's axis inclines to the earth's annual path at an angle of 23 1/2 deg.
During one-half of the earth's annual trip the northern hemisphere receives more than its share of light and heat from the sun. During the other half of the year the southern hemisphere receives the greater share. If the earth remained in one place before the sun, we still might have night and day but no changes of seasons. If the earth's axis changed direction, or wobbled, we should have irregular changes of weather. If the earth's axis were perpendicular to its path around the sun, we should have night and day only, without change of season. If the angle between the axis and a perpendicular position were greater we should have a longer, hotter summer and a longer, colder winter, with a shorter fall and spring.
Astronomically spring begins March 21st with the vernal equinox and ends at the summer solstice, June 21st. Summer extends from the summer solstice to the autumnal equinox, September 22d; autumn, from the autumnal equinox to the winter solstice, December 22d; winter, from the winter solstice to the vernal equinox. According to the calendar, March, April, and May are the spring months. If we reckon from the same astronomical date, the seasons in the southern hemisphere come in the reverse order,--fall, winter, spring, and summer. This brings the southern summer at the time of the northern winter, etc.
Theoretically spring and autumn should have the same temperature. A given locality is exposed to as much sunshine in the spring as in the fall, but a large amount of heat is carried over from the summer into the autumn. Early autumn is warmer than early summer, and early winter is warmer than early spring.
The earth travels in an ellipse, and is nearer the sun in one part of its yearly path than in the other. The northern hemisphere is nearer the sun in winter than in summer. The earth passes through the nearer portions of its orbit, that is, from the autumnal to the vernal equinox, in 179 days, but spends 186 days in passing through the other part of its course from the vernal to the autumnal equinox. Our northern summer, therefore, is a week longer than our winter. Our nearness to the sun makes our winter a trifle milder, and it is cut short by a week which has a still greater effect. Our summer is a week long, but the extra length is about offset by our greater distance from the sun. In the southern hemisphere the winter comes in the long part of the orbit. The effect of an extra week in length is increased by the greater distance from the sun. The southern summer is a week short, but the nearness to the sun ensures a fierce midsummer heat. As compared with northern seasons, the seasons of the southern hemisphere show greater extremes. The summer is shorter and hotter. The winter is longer and colder. Southern autumn comes at the time of northern spring, southern summer at the time of northern winter, etc.
The farther a region is from the equator, the greater the difference between summer and winter temperatures, and the greater the difference between the length of the summer and the winter day. In tropical countries there is little variation in the length of day or in temperature. The different seasons are marked chiefly by a difference in rainfall.
See Wheat.