The New International Encyclopædia/Seasons
SEASONS (OF. seson, seison, saison, Fr. saison, from Lat. satio, a sowing, from serere, to sow; connected with OChurch Slav, sēti, Lith. segir, Goth, saian, OHG. sāen, säen, AS. sāwan, Eng. sow) . Divisions of the year based upon climatic conditions. The changes of the seasons are due to two causes: (1) the inclination of the earth's rotative axis to the plane of the ecliptic (q.v.); (2) the varying length of the day as compared with the night, resulting from the inclination of the axis. As a result of the first of these causes, the sun's rays fall more obliquely on the earth in the winter than in the summer. The number of rays striking a surface varies as the sine of the angle of inclination. Thus the greater the obliquity the less the number of rays. In the summer the sun rises to a greater elevation each day than at other seasons, and therefore the number of rays falling on the earth's surface in that season will be greater than in the winter. The second cause is obvious. Since the heat of the earth is due primarily to solarization, it follows that the hot season should occur when the days are longest. Within the tropics the difference in the obliquity of the sun's rays is never so great as to make one part of the year very sensibly colder than another. There are, therefore, either no marked seasons, or they have other causes altogether, and are distinguished as the wet and dry seasons. (See Rain.) But in the temperate zones the year is naturally divided into four seasons: spring, summer, autumn, and winter. In the Arctic and Antarctic regions spring and autumn are very brief, and the natural division of the year is simply into summer and winter, and this is very much the case also in regions of the temperate zones lying near the Arctic and Antarctic circles. In subtropical regions the distinction of four seasons is in like manner very imperfectly marked. This distinction is everywhere somewhat arbitrary as to the periods of the year included in each season, which really vary according to latitude, and partly according to the other causes which influence climate (q.v.), the seasons passing one into another more or less gradually, and their commencement and close not being determined by precise astronomical or other phenomena. The greatest heat of summer is never reached till a considerable time after the summer solstice (q.v.), when the sun's rays are most nearly vertical, and the day is longest; the greatest cold of winter is in like manner after the winter solstice, when the day is shortest, and the sun's rays are most oblique. The reason in the former case is that as summer advances the earth itself becomes more heated by the continued action of the sun's rays, and in the latter, that it retains a portion of the heat which it has imbibed during summer, just as the warmest part of the day is somewhat after midday, and the coldest part of the night is toward morning.