If the Moon is similar to the earth, then there could be life on the Moon. Galileo understood this perfectly, but some theologians threw up their hands in dismay. If human beings exist on the Moon, how can they be descended from Adam? And if they did not descend from Adam, how can they have the "original sin" that the Church believed everyone had inherited from Adam and Eve, who had disobeyed God in the Garden of Eden? Matters were not made easier by the enthusiasm with which the Protestant, Johann Kepler, and the Catholic radical, Tommaso Campanella, greeted the possibility of rational life on the Moon. A high-level official of the Vatican, Giovanni Ciampoli, wrote from Rome on 28 February 1615 to warn Galileo of the dangerous speculation to which his astronomical discoveries were giving rise:
Your interpretation of the play of light and shade on the clear and spotted surfaces of the Moon assumes some similarity between the Earth and the Moon. Someone adds to this and says that you assume that the Moon is inhabited. Then another asks how they could be descended from Adam or how they have gotten out of Noah's Ark, and many other extravagant ideas that you never dreamt of.
A year later Galileo was in Rome on an extended visit and the issue was raised at a meeting at the residence of Cardinal Tiberio Muti. Galileo recognized the gravity of the situation, and in order to dispel any ambiguity about his position, he wrote to the Cardinal's brother, who had been present and could be relied upon to make the letter known. Galileo insisted that he had never said that there was life on the Moon: there is no water there, and no water no life! His reason for affirming that the Moon was arid is that whereas daylight on the Earth lasts about 12 hours, daylight on the Moon lasts some 15 days because it rotates so much slower on its axis than does the Earth. The consequence is that the surface of the Moon is subjected alternately to scorching heat and devastating cold. In the First Day of his Dialogue Concerning the Two Chief World Systems, Galileo expanded his argument for saying that there was no water on the Moon by pointing out that we do not see clouds on its surface. He did not deny the possibility of change on the Moon, but he offered a powerful anology to explain why we cannot even imagine what those changes might be:
I am certain that anyone, who was born and raised in a vast forest among wild beasts and birds, and who knew nothing about water, would never be able to imagine that there exists another world unlike the Earth filled with animals that travel very fast without legs or wings not only upon the surface like beasts upon the earth but deep below, and that they not only walk but stop motionless wherever they please, something that birds in the air cannot do. Furthermore it could not be imagined that people live there, build palaces and cities, and travel with such ease that without tiring themselves they can move to remote countris with their families, their household and even the entire population. Now as I say, I'm sure that no one could, even with the liveliest imagination, picture to himself fishes, the ocean, ships, fleets and armadas. This and much more might happen on the Moon, which is separated from us by so much greater an interval and made, perhaps, of very different material.
SHEA, W. R. & BASCELLI, T. "The Invention of the Telescope". In: GALILEI, Galileo. Sidereus Nuncius. Galileo's Sidereus Nuncius, or, A sideral message. Translated from the Latin by William R. Shea; introduction and notes William R. Shea and Tiziana Bascelli. Watson Publishing International LLC, 2009, pp.24-26.