Etymologiae, Book III, chapters 49-51: De natura solis to De effectu solis
Chapter XLIX. On the nature of the sun The sun, since it is fiery, grows hotter the more by the excess of motion in its revolution. The philosophers say that its fire is nourished by water, and receives the power of light and heat from the opposite element. Hence we see it often moist and dripping with dew.
Chapter L. On the course of the sun The sun moves by itself and is not turned with the world. For if it remained fixed in heaven, all days and nights would be equal; but since we see that it will set tomorrow in one place and set yesterday in another, it is clear that it moves by itself and is not turned with the world. For it completes its yearly orbits over unequal spaces on account of the changes of the seasons. Rising, it makes day; setting, it brings on night. For going farther toward the south, it makes winter, so that the earth may grow rich with the moisture and frosts of winter. Drawing nearer to the north, it brings summer, so that the crops may be hardened to ripeness, and what is unripe in damp places may be warmed and grow mild.
Chapter LI. On the effect of the sun The sun, rising, makes day; setting, it brings on night; for day is the sun above the earth, night is the sun beneath the earth. From it come the hours: from it the day, when it has risen; from it also the night, when it has set; by it months and years are counted; from it come the successions of the seasons. When it runs through the south, it is nearer the earth; but when it runs along the north, it is lifted on high. For this reason God appointed it different places and times for its course, lest, if it always stayed in the same places, it should consume them with its daily heat; but, as Clement says: “It takes different courses, by which the temperature of the air is dispensed according to the reckoning of the seasons, and the order of successions and changes is preserved. For when it has mounted to the higher regions, it tempers the spring; when it has come to the summit, it kindles the summer heats; descending again, it restores the mildness of autumn. But when it returns to the lower circle, it leaves us the rigor of winter cold from the icy fabric of heaven.” ‸