The Art of Navigation

Alhazen in the 39th proposition of his Optics.

Aristotle in his book on the quality of sight.

Note.

composed of the elements shall be dissolved and corrupted. The heaven is imperishable original: "ingenerable"; meaning it was not created by natural processes and cannot be destroyed by them., and cannot be increased or receive any other impression: it is neither light nor heavy, hot nor cold, dry nor wet, either in form or in reality. It is instead full of virtue In this context, "virtue" refers to a celestial power or influence that acts upon the earthly world.: for its virtue and influence provide heat, as the Philosopher Aristotle was often referred to simply as "The Philosopher" in Renaissance texts. says in the first book of On the Heavens and the World. Therefore, one might improperly say that the heaven is unbreakable and impenetrable, and likewise improperly say it is thick or thin. It has color only in an improper sense, and yet it shines. And as for the color that it seems to us to have, it must be known that our senses are often deceived: for sight can be cheated more readily than any other natural sense, because our eyes are of such tender composition that we feel something like a passion from every species The term "species" here refers to the visible "images" or "forms" that objects were thought to transmit through the air to the eye. sent back to us by every visible thing. This is according to what Alhazen original: "Alacen"; referring to the 11th-century Arab scholar Ibn al-Haytham, whose work on optics was foundational in Europe. says in his Optics original: "perspectiue.", in the thirty-ninth proposition, and Aristotle in his book on the quality of sight. Therefore, I say that no one can see anything above the Moon except for the Sun or the stars, which we see by means of the light they receive from the Sun. But, regarding the color of the heaven, it should be noted that color is understood in one of two ways: the first is "proper" color, which is a secondary quality caused by the primary qualities—such as white, black, and other colors. In this manner, the celestial bodies have no color at all. Color is also understood in another way, namely, as a common term for everything that limits our sight and extends to things that are luminous and transparent original: "diafane.". In this sense, the heaven does have color: namely, it is shining.

C H A P T E R V.

An ornamental woodcut initial 'P' featuring a seated figure in a courtyard with architectural arches and a landscape visible in the background, surrounded by decorative floral patterns.Since I have declared the number of the heavens, their shape, and their quality, I shall speak of each of them in particular according to our purpose. To begin, it is quite obvious that the heaven turns before our eyes. Regarding this, it must be noted that motion is a passage from one point original: "terme"; a boundary or limit. to another, such that everything that moves passes from one place to another place. Such motion can occur in one of three ways: circularly, directly, or from top to bottom (and the reverse). The first of these three, which is circular motion, does not change its place according to its entire volume, but rather according to its parts—as can be seen in a wheel, which, without moving at all from one place to another, moves in all its parts when it is turned. Such is the motion of the heaven: so that what is at this hour in the East shall afterwards be in the West. Thus, although the heaven does not change...