The Moon once more...

"In the philosophy of Aristotle (384-322 BCE), the features on the Moon's surface presented somewhat of a problem [since it was believed that the Moon, planets and stars were fixed, perfect spheres]...

"The medieval followers of Aristotle, first in the Islamic world and then in Christian Europe, tried to make sense of the lunar spots in Aristotelian terms. Various possibilities were entertained. It had been suggested already in Antiquity that the Moon was a perfect mirror, and that its markings were reflections of earthly features, but this explanation was easily dismissed because the face of the Moon never changes as it moves about the Earth.

"Perhaps, then, there were vapors between the Sun and the Moon, so that the images were actually contained in the Sun's incident light and thus reflected to the Earth. The explanation that finally became standard was that there were variations of 'density' in the Moon that caused this otherwise perfectly spherical body to appear the way it does. The perfection of the Moon, and therefore the heavens, was thus preserved.

"The telescope delivered the coup de grace to attempts to explain away the Moon's spots and to the perfection of the heavens in general. With his telescope, Galileo saw not only the 'ancient' spots, but many smaller ones never seen before. In these smaller spots, he saw that the width of the dark lines defining them varied with the angle of solar illumination. He watched the dark lines change and he saw light spots in the unilluminated part of the Moon that gradually merged with the illuminated part as this part grew. The conclusion he drew was that the changing dark lines were shadows and that the lunar surface has mountains and valleys. The Moon was thus not spherical and hardly perfect."

via Moon River