Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/76

 being supposed, and not granted, that some one being placed in the Moon to observe the Earth, he would every day see the whole Terrestrial superficies, by means of the Moons going about the Earth in twenty four or twenty five hours; but we never see but half of the Moon, since it revolves not in it self, as it must do to be seen in every part of it.

So that this, befals not contrarily, namely, that her revolving in her self, is the cause that we see not the other half of her, for so it would be necessary it should be, if she had the Epicycle. But what other difference have you behind, to exchange for this which you have named?

Let me see; Well for the present I cannot think of any other.

And what if the Earth (as you have well noted) seeth no more than half the Moon, whereas from the Moon one may see all the Earth; and on the contrary, all the Earth seeth the Moon, and but onely half of it seeth the Earth? For the inhabitants, to so speak, of the superior Hemisphere of the Moon, which is to us invisible, are deprived of the sight of the Earth: and these haply are the Anticthones. But here I remember a particular accident, newly observed by our Academian, in the Moon, from whchwhich [sic] are gathered two necessary consequences; one is, that we see somewhat more than half of the Moon; and the other is, that the motion of the Moon hath exact concentricity with the Earth: and thus he finds the Phœnomenonand observation. When the Moon hath a correspondence and natural sympathy with the Earth, towards which it hath its aspect in such a determinate part, it is necessary that the right line which conjoyns their centers, do passe ever by the same point of the Moons superficies; so that, who so shall from the center of the Earth behold the same, shall alwayes see the same Discus or Face of the Moon punctually determined by one and the same circumference; But if a man be placed upon the Terrestrial surface, the ray which from his eye passeth to the centre of the Lunar Globe, will not pass by the same point of its superficies, by which the line passeth that is drawn from the centre of the Earth to that of the Moon, save onely when it is vertical to him: but the Moon being placed in the East, or in the West, the point of incidence of the visual ray, is higher than that of the line which conjoyns the centres; and therefore the observer may discern some part of the Lunar Hemisphere towards the upper circumference, and alike part of the other is invisible: they are discernable and undiscernable, in respect of the Hemisphere beheld from the true centre of the Earth: and because the part of the Moons circumference, which is superiour in its rising, is nethermost in its setting; therefore the difference of the said superiour and inferi-