Zone (Gr. A Belt Or Girdle), in the mathematical sense, the circular belt or portion of the surface of a sphere lying between any two parallel circles of the latter, or the convex surface cut off to one side by a circle of the sphere. Owing to the inclination of the earth's equator to the ecliptic or plane of the sun's path at an angle of 23° 28', the tropics have respectively this distance N. and S. of the equator, and when the sun is over either tropic its rays are withdrawn from the surface within a like distance of the opposite pole. (See Seasons.) There are thus naturally established four parallel circles of the earth, the two tropics and two polar circles, which with the equator divide the entire surface into six belts or portions, corresponding in pairs N. and S. of the equator. As within these several pairs of belts the relative lengths of day and night and the character of the seasons and climate differ, these portions of the earth's surface have been designated as the zones of the earth. The two lying on both sides of the equator, and bounded N. and S. respectively by the tropics, are termed the torrid or burning zones; these have continual alternation of night and day, and over any point within them the sun is vertical twice yearly.

The zones lying between a tropic and polar circle on either side of the equator are respectively the northern and southern temperate zones; these also have continual alternation of night and day, but the sun is never vertical to any part of them. The portions within the polar circles respectively are the northern and southern frigid or frozen zones; throughout these zones there is in each year a period, varying from 24 hours at the polar circles to six months at the poles, during which the sun does not rise, and a corresponding period during which it does not set. The names of the torrid and frigid zones are expressive of the character of their climates; the temperate ones are characterized by alternation of hot and cold seasons, though not reaching generally the respective extremes presented by the others. The transition from any zone to another is gradual both in the relative lengths of day and night and in temperature.