Seasons
the alterations in the relative length of day and night, heat and cold, etc., which take place each year. In the United States there are four seasons, spring, summer, autumn, and winter. The Anglo-Saxons reckoned only three, spring, summer, and winter, the words for which are are all from Anglo-Saxon; AUTUMN (q. v.) was borrowed from the Romans.
In India there are but three well-marked seasons of four months each, the hot (February-May), the rainy (June-September), and the cold (October-January). The essential astronomical fact on which the recurrence of the successive seasons depends is that the axis of the earth always points in the same direction, whatever portion of the orbit the earth may at the time be traversing. The inclination of the equator to the ecliptic is 23° 27'. On June 21, when the sun is at the highest point of the ecliptic, the North Pole necessarily inclines toward the sun, and is as much irradiated as it ever can be by his beams, while the South Pole, on the contrary, is as little. It is therefore midsummer in the Northern and midwinter in the Southern Hemisphere. Six months later, Dec. 21, the South Pole points toward the sun. It is therefore now midwinter in the Northern and midsummer in the Southern Hemisphere. At the intermediate periods (March 21 and Sept. 21), the axis of the earth is at right angles to the direction of the sun; hence, in both hemispheres it is the equinox, the vernal at the former date in the Northern and at the latter in the Southern Hemisphere.