The Ten Books on Architecture, 9.2.3

Vitruvius  Parallel editions

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Gwilt translation

3I shall now subjoin what Aristarchus, the Samian mathematician, learnedly wrote on this subject, though of a different nature. He asserted, that the moon possesses no light of its own, but is similar to a speculum, which receives its splendour from the sun’s rays. Of the planets, the moon makes the smallest circuit, and is nearest to the earth; whence, on the first day of its monthly course, hiding itself under the sun, it is invisible; and when thus in conjunction with the sun, it is called the new moon. The following day, which is called the second, removing a little from the sun, it receives a small portion of light on its disc. When it is three days distant from him, it has increased, and become more illuminated; thus daily elongating from him, on the seventh day, being half the heavens distant from the western sun, one half of it shines, namely, that half which is lighted by the sun.

Morgan translation

3But Aristarchus of Samos, a mathematician of great powers, has left a different explanation in his teaching on this subject, as I shall now set forth. It is no secret that the moon has no light of her own, but is, as it were, a mirror, receiving brightness from the influence of the sun. Of all the seven stars, the moon traverses the shortest orbit, and her course is nearest to the earth. Hence in every month, on the day before she gets past the sun, she is under his disc and rays, and is consequently hidden and invisible. When she is thus in conjunction with the sun, she is called the new moon. On the next day, reckoned as her second, she gets past the sun and shows the thin edge of her sphere. Three days away from the sun, she waxes and grows brighter. Removing further every day till she reaches the seventh, when her distance from the sun at his setting is about one half the extent of the firmament, one half of her is luminous: that is, the half which faces toward the sun is lighted up by him.