Plutarch, De facie quae in orbe lunae apparet
Harold Cherniss and William C. Helmbold, Ed.

Speaking generally,” he said, ‘I marvel that they adduce against us the moon's shining upon the earth at the half and at the gibbous and the crescent phases too.¹ After all, if the mass of the moon that is illuminated by the sun were ethereal or fiery, the sun would not leave her² a hemisphere that to our perception is ever in shadow and unilluminated; on the contrary, if as he revolves he grazed her ever so slightly, she should be saturated in her entirety and altered through and through by the light proceeding easily in all directions. Since wine that just touches water at its surface³ or a drop of blood fallen into liquid at the moment <of contact> stains all the liquid red,⁴ and since they say that the air itself is filled with sunshine not by having any effluences or rays commingled with it but by an alteration and change that results from impact or contact of the light,⁵ how do they imagine that a star can come in contact with a star or light with light and instead of blending and producing a thorough mixture and change merely illuminate those portions of the surface which it touches?⁶ In fact, the circle which the sun in its revolution describes and causes to turn about the moon now coinciding with the circle that divides her visible and invisible parts and now standing at right angles to it so as to intersect it and be intersected by it, by different inclinations and relations of the bright part to the dark producing in her the gibbous and crescent phases,⁷ conclusively demonstrates that her illumination is the result not of combination but of contact, not of a concentration of light within her but of light shining upon her from without. In that she is not only illuminated herself, however, but also transmits to us the semblance of her illumination, she gives us all the more confidence in our theory of her substance. There are no reflections from anything rarefied or tenuous in texture, and it is not easy even to imagine light rebounding from light or fire from fire; but whatever is to cause a repercussion or a reflection must be compact and solid,⁸ in order that it may stop a blow and repel it.⁹ At any rate, the same sunlight that the air lets pass without impediment or resistance is widely reflected and diffused from wood and stone and clothing exposed to its rays. The earth too we see illuminated by the sun in this fashion. It does not let the light penetrate its depths as water does or pervade it through and through as air does; but such as is the circle of the sun that moves around the moon and so great as is the part of her that it intercepts, just such a circle in turn moves around the earth, always illuminating just so much and leaving another part unilluminated,¹⁰ for the illuminated portion of either body appears to be slightly greater than a hemisphere.¹¹ Give me leave then to put it in geometrical fashion in terms of a proportion. Given three things approached by the light from the sun: earth, moon, air; if we see that the moon is illuminated not as the air is rather than as the earth, the things upon which the same agent produces the same effects must be of a similar nature.’ ¹²