THE SPOTS ON THE MOON.
Some have said that vapours rise from the moon, after the manner of clouds and are interposed between the moon and our eyes. But, if this were the case, these spots would never be permanent, either as to position or form; and, seeing the moon from various aspects, even if these spots did not move they would change in form, as objects do which are seen from different sides.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.