Astronomical Images : Zones of the Earth: habitable and uninhabitable zones

Johannes Sacrobosco

Astronomical Images

Very little is known about Johannes Sacrobosco except that he was probably British, taught astronomy at Paris University, and died there in the second quarter of the thirteenth century. Sphaera mundi, his major work, was an extraordinarily popular astronomical textbook for several generations. Manuscripts of it circulated through all the main European centres of learning. It was first published in 1472 in Ferrara, and went through dozens of editions up to the mid-seventeenth century. This edition of Sacrobosco's Sphaera mundi was printed with Georg Peuerbach's Theoricae novae planetarum and Johannes Regiomontanus's Disputationes contra Cremonensia deliramenta. It is illustrated throughout with woodcuts, some of which were coloured. This figure shows the five zones of the Earth, divided according to the limits of certain heavenly circles, and whether or not they are habitable. Proceeding from the North Pole to the South Pole along a given meridian, the circles that divide the zones are the Arctic circle, the tropic of Cancer, the celestial equator, the tropic of Capricorn and the Antarctic circle. The regions of the Earth that correspond to each of these regions of the celestial sphere share their characteristic temperatures. Thus, the zone that lies between the tropics is said to be uninhabitable on account of the heat of the Sun, which is always confined to the region between these two latitudes. Conversely, the zone to the north of the Arctic circle and the zone to the south of the Antarctic circle are deemed uninhabitable because the Sun is never overhead in these regions and so does not provide enough heat to sustain life. However, the zones in between the Arctic circle and the tropic of Cancer and the Antarctic circle and the tropic of Capricorn respectively are habitable, with just the right temperature for life, being shielded from the heat of the Sun, but enjoying its warmth. In this figure, it seems that the southern habitable zone is depicted featuring irrigation and signs of civilization, while the northern and southern zones of extreme cold seem to have the appearance of cracked ice.