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Kepler, writing back to Galileo after having read his newly published work on the discoveries made using his new telescopes [after noting the problems of spherical aberration facing Galileo’s new telescopes, suggesting a hyperbolic curvature to prevent it]:
“I cannot help wondering about the meaning of that large circular cavity in what I usually call the left corner of the mouth [of the face in the Moon]. Is it a work of nature, or of a trained hand? Suppose there are living beings on the moon . . [?] It surely stands to reason that the inhabitants express the character of their dwelling place, which as much bigger mountains and valleys than our earth has. consequently, being endowed with very massive bodies, they also construct gigantic projects. Their day is as long as 15 of our days, and the feel insufferable heat. Perhaps they lack stone for erecting shelters against the sun. On the other hand, maybe they have a soil as sticky as clay. their usual building plan, accordingly, is as follows. digging up huge fields, they carry out the earth and heap it in a circle, perhaps for the purpose of drawing out the moisture down below. In this way they may hide in the deep shade behind their excavated mounds and, in keeping with the sun’s motion, shift about inside, clinging to the shadow. They have, as it were, a sort of underground city. They make their homes in numerous caves hewn out of that circular embankment. They place their fields and pastures in the middle, to avoid being forced to go too far away from their farms in their flight from the sun.“
Later in the same letter:
“What other conclusion shall we draw from this difference [in appearance between fixed stars and planets] than that the fixed stars generate their light from within, whereas the planets, being opaque, are illuminated from without; that is, to use [Giordano] Bruno‘s terms, the former are suns, the latter, moons or earths?“