The Art of Navigation

FIRST BOOK

...assembled: And it is to be understood that the first astronomers In the 16th century, the term 'astrologer' often referred to what we now call an astronomer; Medina uses it here to describe those who study the physical movements of the heavens. held that there were only eight heavens, and they found it so by the movement of the stars. For the stars do not make any movement on their own, but rather by their heavens, as the Philosopher Aristotle (384–322 BCE), whose work 'De Caelo' (On the Heavens) is referenced here. says in the second book of On the Heavens original: "du ciel & du monde", that the stars are fixed in their heaven like a knot in a board; so that the first difference between erratic and fixed stars was recognized by their movements.

Erratic stars are those we call planets, and they are known to be different from one another by their movements in speed, slowness, and position. But the fixed stars are in such great number that they cannot be fully grasped. It was through long experience and the observations of astronomers that they came to understand that they all move together at an equal distance and proximity, which they always maintain with one another; so that their movement is but one, as the Philosopher alleges in his first book of On the Heavens.

And as for what I have said, that the astronomers estimated that there is only one diurnal diurnal From the Latin 'diurnalis' (daily); the apparent daily motion of the stars around the Earth caused by the Earth's rotation. movement in the eighth sphere—namely, from East to West—and that this was the Primum Mobile Primum Mobile Latin for 'the first moved'; the outermost moving sphere in the Ptolemaic system which drives all other spheres., it seems that the Philosopher agrees in the aforementioned passage, where he says that all the fixed stars are in the Primum Mobile, and that for this reason there are so many of them, whereas in each of the inferior spheres, there is only one. Other astronomers were not satisfied with eight spheres, but added a ninth, because they saw that the eighth heaven has two contrary movements: one is from East to West, and the other is opposite—namely, from West to East—and it occurs so slowly that if someone wished to take notice of it, they would hardly perceive it in a hundred years.

(Ptolemy Claudius Ptolemy (c. 100–170 CE), the Greco-Egyptian mathematician whose 'Almagest' served as the authoritative text on astronomy for over a millennium. declares this movement with good and lively reasons in the seventh book of his Almagest) thus, considering these two contrary or different movements, they came to know that the eighth heaven was not the Primum Mobile; for the Primum Mobile has only one single and simple movement.

The most recent astronomers have found through long speculation certain fixed stars that sometimes move from West to North, to the South, and to the East, more hastily than to the West; and at other times sooner toward the North than toward the South. And because they could not explain this appearance by the two aforementioned movements, they added a third movement which is of itself, which occurs in two small circles at the heads of Aries and Libra This describes the theory of 'trepidation,' a hypothesized oscillation of the equinoxes used by medieval astronomers to explain variations in the rate of precession.; so that since the eighth sphere has three movements, it is not only necessary to...