Chapter 44: On the five circles of the heavens

1. There are five zones in the heavens, according to the differences of which certain parts of the earth are inhabitable, because of their moderate temperature, and certain parts are uninhabitable because of extremes of heat and cold. And these are called zones or circles for the reason that they exist on the circumference of the sphere.

2. The first of these circles is called the Arctic, because the constellations of the Arcti are visible enclosed within it; the second is called the summer tropic, because in this circle the sun makes summer in northern regions, and does not pass beyond it but immediately returns, and from this it is called tropic.

3. The third circle is called ἰσημερινὸς, which is equivalent to _equinoctialis_ in Latin, for the reason that when the sun comes to this circle it makes equal day and night (for ἰσημερινὸς means in Latin day equal to the night) and by this circle the sphere is seen to be equally divided. The fourth circle is called Antarctic,[271] for the reason that it is opposite to the circle which we call Arctic.

[271] This order is repeated in 13, 6.

4. The fifth circle is called the winter tropic (χειμερινὸς τροπικός), which in the Latin is _hiemalis_ or _brumalis_, because when the sun comes to this circle it makes winter for those who are in the north and summer for those who dwell in the parts of the south.