seasons, divisions of the year characterized by variations in the relative lengths of day and night and in the amount of heat received from the sun. These variations depend on the inclination of the equator to the plane of the ecliptic and on the revolution of the earth around the sun. The amount of heat received at a given point on the earth's surface depends chiefly on the angle at which the sun's rays strike the earth at that point and on the daily duration there of exposure to the sun's rays; the more vertical the rays and the longer the exposure, the more heat will be received. Seasonal change varies greatly with latitude. Near the equator there is little change; in high latitudes spring and autumn are very short. In the temperate zones there are four well-defined seasons; in the north temperate zone, spring begins about Mar. 21, the vernal equinox; summer, about June 22, the summer solstice; autumn, about Sept. 23, the fall equinox; and winter, about Dec. 22, the winter solstice. However, the weather lags somewhat behind the seasons because, at the time of maximum sunlight (summer solstice for the Northern Hemisphere) the ground is still too cold to radiate as much heat as it receives, so average temperatures usually continue to rise for several weeks until a balance is reached between reception and radiation of heat. In low latitudes and in certain other areas (e.g., India) where oceans and winds are the chief factors governing seasonal changes, the terms "wet season" and "dry season" are used. The seasons play an important part in mythology and folklore; many holidays are connected with the changes of season.