Galileo's Invisible Mountains

I have recently been reading (White, 2007) about what happened when Galileo turned his telescope towards the Moon and reported mountains there. This upset people at the time since Aristotle had stated that the Moon was a perfect sphere. Christopher Clavius defended Aristotle saying that the Moon was surrounded by a "crystalline invisible layer" so it was still a perfect sphere.

Galileo's brilliant response was: "If anyone is allowed to imagine whatever he pleases, then someone could say that the Moon is surrounded by a substance that is invisible, provided that I can say that the crystal has on its outer surface some mountains that are 30 times higher than the terrestrial ones, and also invisible."

Surely no-one today would invent an invisible substance to defend a historical figure ;)

Reference:

White, M., 2007. Galileo Antichrist: a biography. Phoenix Press.