, in Geography and Astronomy, a division of the earth's surface, by means of parallel circles, chiefly with respect to the degree of heat in the different parts of that surface.

The ancient astronomers used the term Zone, to explain the different appearances of the sun and other heavenly bodies, with the length of the days and nights; and the geographers, as they used the climates, to mark the situation of places; using the term climate when they were able to be more exact, and the term Zone when less so.

The Zones were commonly accounted five in number; one a broad belt round the middle of the earth, having the equator in the very middle of it, and bounded, towards the north and south by parallel circles passing through the tropics of Cancer and Capricorn. This they called the torrid Zone, which they supposed not habitable, on account of its extreme heat. Though sometimes they divided this into two equal torrid Zones, by the equator, one to the north, and the other south; and then the whole number of Zones was accounted 6.

Next, from the tropics of Cancer and Capricorn, to the two polar circles, were two other spaces called temperate Zones, as being moderately warm; and these they supposed to be the only habitable parts of the carth.

Lastly, the two spaces beyond the temperate Zones, about either pole, bounded within the polar circles, and having the poles in the middle of them, are the two frigid or frozen Zones, and which they supposed not habitable, on account of the extreme cold there.

Hence, the breadth of the torrid Zone, is equal to twice the greatest declination of the sun, or obliquity of the ecliptic, equal to 46° 56′, or twice 23° 28′. Each frigid Zone is also of the same breadth, the distance from the pole to the polar circle being equal to the same obliquity 23° 28′. And the breadth of each temperate Zone is equal to 43° 4′, the complement of twice the same obliquity. See these Zones exhibited in plate 35, fig. 16.

The difference of Zones is attended with a great diversity of phenomena. 1. In the torrid Zone, the sun passes through the zenith of every place in it twice a year; making as it were two summers in the year; and the inhabitants of this Zone are called amphiscians, because they have their noon-day shadows projected different ways in different times of the year, northward at one season, and southward at the other.

2. In the temperate and frigid Zones, the sun rises and sets every natural day of 24 hours. Yet every where, but under the equator, the artificial days are of unequal lengths, and the inequality is the greater, as the place is farther from the equator. The inhabitants of the temperate Zones are called heteroscians, because their noon-day shadow is cast the same way all the year round, viz, those in the north Zone toward the north pole, and those in the south Zone toward the south pole.

3. Within the frigid Zones, the inhabitants have their artificial days and nights extended out to a great length; the sun sometimes skirting round a little above the horizon for many days together: and at another season never rising above the horizon at all, but making continual night for a considerable space of time. The inhabitants of these Zones are called periscians, because sometimes they have their shadows going quite round them in the space of 24 hours.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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