Etymologiae, Book III, chapters 52-58: De itinere solis to De eclipsi solis
Chapter LII. On the journey of the sun The sun, rising, takes its way through the south. After it has come to the west and dipped itself in the Ocean, it goes by unknown paths beneath the earth and runs back again to the east.
Chapter LIII. On the light of the moon Some philosophers say that the moon has a light of its own, and that one part of its globe is luminous and the other dark, and that by turning gradually it produces different shapes. Others on the contrary say that the moon has no light of its own but is illumined by the rays of the sun. Hence it also suffers eclipse if the shadow of the earth comes between it and the sun. For the sun is higher than it. Hence it happens that, when it is beneath the sun, it shines on its upper part, while the lower part, which it has toward the earth, is dark.
Chapter LIV. On the phases of the moon The first figure of the moon is two-horned, thus: The second has the sickle shape, thus. The third has the half, thus.
The fourth is full, thus. The fifth is again a half, from the greater, thus. The sixth is again a sickle, thus. The seventh is two-horned, thus. The seventh and a half, and the twenty-second and a half, are the middle points in its orbit. The rest are in proportion.
Chapter LV. On the interlunium of the moon The interlunium of the moon is that time between the waning and the new moon. It is the thirtieth day, on which the moon does not shine. It cannot then be seen because, joined to the sun, it is darkened; but being born again at the same moment, it is seen as it gradually draws away from it.
Chapter LVI. On the course of the moon The moon governs the monthly spaces by the alternations of light lost and regained. It advances in an oblique course and not a straight one, as the sun does, namely lest it fall into the center of the earth and often suffer eclipse. For its circle is close to the earth. Waxing, it looks to the east with its horns; waning, to the west: fittingly, because it is about to set and to lose its light.
Chapter LVII. On the nearness of the moon to the earth The moon is nearer to the earth than the sun. Hence, in a shorter orbit, it accomplishes its course more swiftly. For the journey that the sun makes in three hundred and sixty-five days, the moon runs through in thirty days. Hence the ancients also reckoned months by the moon, but years by the course of the sun.
Chapter LVIII. On the eclipse of the sun An eclipse of the sun occurs whenever the moon, on the thirtieth day, comes to the same line by which the sun is carried, and, placing itself in front of it, darkens the sun. For the sun seems to us to fail when the orb of the moon is set against it.