ZONES (lat. zone, a girdle), portions of the earth's or of a planet's surface, chiefly in form of belts or girdles, divided by parallels of latitude which are defined by the inclination of the plane of the planet's eqnator to that of its orbit, or ecliptic. The plane of the earth's equator is inclined to that of its orbit, 23i°, and when the planet is at the summer solstice (21st of June), the sun's rays fall perpendicularly upon that parallel of latitude (the tropic of Cancer), which is 231° n. of the equator. Conversely, at the winter solstice (22d of Dec.), the sun's rays fall perpendicularly upon that parallel called the tropic of Capricorn, which is 23• s. of the equator. The belt thus defined, lying between the tropics, and having a breadth of 47° (23ix2), is called the torrid zone. Now, when the sun's rays are vertical to the tropics, and the ELM always pours his ver tical rays over some portion of it, they will be tangent to certain parallels (polar cir cles) lying 23e from the poles, throwing that portion of the earth's surface bounded by one of these circles into shadow, and continually shining upon that portion bounded by the other. These circles define the boundaries of the frigid or polar zones. Lying be tween these and the torrid zone there are, therefore, two belts (one in each hemisphere) bounded by the tropics and the polar circles, each 43° in breadth, called respectively the n. temperate and s. temperate zone. Over some portion of their surfaces the sun always shines, but never vertically. The zones, therefore, are not bounded by arbitrary
but by natural fixed lines, although in the ordinary, non-geometrical sense, they are imaginary. Connected with these natural divisions of the earth's surface are the phenomena of the seasons and climate, and of length of day and night. Scu SOL STICE, SEASONS, and CLIMATE. Let us consider here the zones of a planet whose equa torial inclination is much less than the earth's. The inclination of the plane of Jupiter's equator to that of his orbit is only 3°; consequently, the tropics defining the torrid zone, that belt is only 6° wide, and the frigid zones each the same, while the temperate zones are each 84° in breadth. This condition is theoretical. Practically, if Jupiter received most of its present heat from the sun, as the earth does, and had real seasons, and sup ported life, there would be a series of sub-torrid and also of sub-frigid zones lying on either side of the really temperate zones. The length of day and night varies but little on Jupiter except near the poles, at which points the days and nights are alternately six of our years long, the planet making its revolution round the sun in twelve of our years, but the nights cannot be dark. The seasons, if there were any practically, would vary but little in one latitude. There would he continual summer within, and to a considerable distance beyond the tropics; continual spring further beyond, and still further continual winter.