illuminated parts of the Terrestrial superficies appear beheld from the Moon, depends not on this alone, but on the divers aspects which the Moon is still changing with the Sun; so that, if for instance, the Moon punctually followed the motion of the Sun, and stood, for example, always in a direct line between it and the Earth, in that aspect which we call Conjunction, it looking always to the same Hemisphere of the Earth which the Sun looks unto, she would behold the same all light: as on the contrary, if it should always stay in Opposition to the Sun, it would never behold the Earth, of which the dark part would be continually turn'd towards the Moon, and therefore invisible. But when the Moon is in Quadrature of the Sun, that half of the Terrestrial Hemisphere exposed to the sight of the Moon which is towards the Sun, is luminous; and the other towards the contrary is obscure: and therefore the illuminated part of the Earth would represent it self to the Moon in a semi-circular figure.
Sagr.I clearly perceive all this, and understand very well, that the Moon departing from its Opposition to the Sun, where it saw no part of the illumination of the Terrestrial superficies, and approaching day by day nearer the Sun, she begins by little and little to discover some part of the face of the illuminated Earth; and that which appeareth of it shall resemble a thin sickle, in regard the figure of the Earth is round: and the Moon thus acquiring by its motion day by day greater proximity to the Sun, successively discovers more and more of the Terrestrial Hemisphere enlightned, so that at the Quadrature there is just half of it visible, insomuch that we may see the other part of her: continuing next to proceed towards the Conjunction, it successively discovers more and more of its surface to be illuminated, and in fine, at the time of Conjunction seeth the whole Hemisphere enlightned. And in short, I very well conceive, that what befalls the Inhabitants of the Earth, in beholding the changes of the Moon, would happen to him that from the Moon should observe the Earth; but in a contrary order, namely, that when the Moon is to us at her full, and in Opposition to the Sun, then the Earth would be in Conjunction with the Sun, and wholly obscure and invisible; on the contrary, that position which is to us a Conjunction of the Moon with the Sun, and for that cause a Moon silent and unseen, would be there an Opposition of the Earth to the Sun, and, to so speak, Full Earth, to wit, all enlightned. And lastly, look what part of the Lunar surface appears to us from time to time illuminated, so much of the Earth in the same time shall you behold from the Moon to be obscured: and look how much of the Moon is to us deprived of light, so much of the Earth is to the Moon illuminated. In one thing yet these mutual operations in my judgment seem to differ, and it is, that itbeing
