Etymologiae, Book III, chapters 43-48: De hemisphaeriis to De magnitudine lunae
Chapter XLIII. On the hemispheres A hemisphere is half a sphere. The hemisphere above the earth is the part of heaven that is seen by us in its entirety; the hemisphere beneath the earth is that which cannot be seen, as long as it is beneath the earth.
Chapter XLIV. On the five circles of heaven There are five zones of heaven, by whose divisions some regions are inhabited because of their temperate climate, while others are uninhabitable through the extremity of cold or heat. These are called zones or circles because they exist in the revolution of the sphere. The first circle is called the arctic, because within it the signs of the Bears are seen enclosed. The second circle, the summer one, which is called the tropic, is so named because in that circle the sun, making summer in the northern regions, does not pass beyond that circle but at once turns back; and hence it is called the tropic. The third circle is the equinoctial, which is called by the Latins aequinoctialis because when the sun has come to that orbit it makes the equinox. For the day-and-night is called in Latin aequinoctium, since in that circle half of the sphere is seen laid out. The fourth circle is called the antarctic, because it is opposite to the circle that we name the arctic. The fifth circle is the winter tropic, which is called by the Latins the hibernal or brumal, because when the sun has come to that circle it makes winter for those who are toward the North, and summer for those who dwell in the southern parts.
Chapter XLV. On the zodiac circle The zodiac is a circle that consists of five angles of lines, and of one line
Chapter XLVI. On the white circle The milky circle is a track that is seen in the sphere, so called from its whiteness, because it is white. Some say that it is the way by which the sun travels around, and that it shines thus from the passage of its splendor.
Chapter XLVII. On the size of the sun The size of the sun is greater than that of the earth, and hence at the same moment when it rises it appears equally to the east and to the west at once. But as to its seeming to us only a cubit across, we must consider how far the sun is from the earth, a distance which makes it appear small to us.
Chapter XLVIII. On the size of the moon The size of the moon too is said to be less than that of the sun: for since the sun is higher than the moon and yet appears larger to us than the moon, if it had come near us it would be seen to be much larger than the moon. And as the sun is greater than the earth, so the earth is greater than the moon by some amount.