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

Whereat Lucius said: ‘Nay, lest we give the impression of flatly insulting Pharnaces by thus passing over the Stoic opinion unnoticed, do now by all means address some remark to the gentleman who, supposing the moon to be a mixture of air and gentle fire, then says that what appears to be a figure is the result of the blackening of the air as when in a calm water there runs a ripple under the surface.’ ¹ ‘You are <very> nice, Lucius,’ I said, ‘to dress up the absurdity in respectable language. Not so our comrade²; but he said what is true, that they blacken the Moon's eye defiling her with blemishes and bruises, at one and the same time addressing her as Artemis³ and Athena⁴ and making her a mass compounded of murky air and smouldering fire neither kindling nor shining of herself, an indiscriminate kind of body, forever charred and smoking like the thunderbolts that are darkling and by the poets called lurid.⁵ Yet a smouldering fire, such as they suppose that of the moon to be, cannot persist or subsist at all unless it get solid fuel that shelters and at the same time nourishes it⁶; this some philosophers, I believe, see less clearly than do those who say in jest that Hephaestus is said to be lame because fire without wood, like the lame without a stick, makes no progress.⁷ If the moon really is fire, whence came so much air in it? For the region that we see revolving above us is the place not of air but of a superior substance, the nature of which is to rarefy all things and set them afire; and, if air did come to be there, why has it not been etherealized by the fire⁸ and in this transformation disappeared but instead has been preserved as a housemate of fire this long time, as if nails had fixed it forever to the same spots and riveted it together? Air is tenuous and without configuration, and so it naturally slips and does not stay in place; and it cannot have become solidified if it is commingled with fire and partakes neither of moisture nor of earth by which alone air can be solidified.⁹ Moreover, velocity ignites the air in stones and in cold lead, not to speak of the air enclosed in fire that is whirling about with such great speed.¹⁰ Why, they are vexed by Empedocles because he represents the moon to be a hail-like congelation of air encompassed by the sphere of fire¹¹; but they themselves say that the moon is a sphere of fire containing air dispersed about it here and there, and a sphere moreover that has neither clefts nor depths and hollows, such as are allowed by those who make it an earthy body, but has the air evidently resting upon its convex surface. That it should so remain is both contrary to reason and impossible to square with what is observed when the moon is full. On that assumption there should have been no distinction of dark and shadowy air; but all the air should become dark when occulted, or when the moon is caught by the sun it should all shine out with an even light. For with us too, while the air in the depths and hollows of the earth, wherever the suns rays do not penetrate, remains shadowy and unlit, that which suffuses the earth outside takes on brilliance and a luminous colour. The reason is that air, because of its subtility, is delicately attuned to every quality and influence; and, especially if it touches light or, to use your phrase, merely is tangent to it, it is altered through and through and entirely illuminated.¹² So this same point seems right handsomely to re-enforce those who pack the air on the moon into depths of some kind and chasms, even as it utterly refutes you who make her globe an unintelligible mixture or compound of air and fire for it is not possible¹³ that a shadow remain upon the surface when the sun casts his light upon all of the moon that is within the compass of our vision.’