On the Revolutions of the Heavenly Spheres

That the motion of the heavenly bodies is uniform and circular, perpetual, or composed of circular motions. Chapter IV.

Having proved the principle that the world is spherical, he now proves that its motion is spherical.

There are many motions, but all are spherical.

Proof that the motion is circular.

A decorative woodcut initial 'P' depicting a scholar or astronomer sitting at a desk with an open book, positioned in front of a window showing a landscape with buildings and a tower.After this, we shall call to mind that the motion of the heavenly bodies is circular. For the mobility of a sphere is to revolve in a circle, expressing its very form through this action, in the simplest of bodies, where no beginning or end can be found, nor can one part be distinguished from another, as it moves through the same path into itself. However, there are multiple motions because of the multitude of spheres. The most obvious of all is the daily revolution, which the Greeks call nycthemeron original: "νυχθήμερον" — a 24-hour period of day and night., that is, the span of a day and a night. By this motion, the entire universe is thought to glide from east to west, with the Earth alone excepted Copernicus refers here to the perceived motion of the heavens from the perspective of an observer on Earth, though his revolutionary thesis will eventually argue the Earth itself is moving.. This is understood as the common measure of all motions, since we measure time itself primarily by the count of days. Next, we see other revolutions that are, as it were, striving against this one—that is, moving from west to east—namely, those of the Sun, the Moon, and the five wandering stars The "wandering stars" or planets known in the 16th century: Mercury, Venus, Mars, Jupiter, and Saturn.. In this way, the Sun regulates the year for us, and the Moon the months, which are the most common divisions of time; likewise, each of the other five planets makes its own circuit. Yet, they differ in many ways. First, they do not revolve around the same poles as that primary motion, but travel through the obliquity of the zodiac original: "signiferi" — literally the "sign-bearer," the path of the sun and planets across the sky.. Second, in their own circuits, they do not appear to be carried uniformly. For the Sun and Moon are found to be sometimes slow and sometimes faster in their course. As for the other five wandering stars, we perceive them sometimes even moving backward and making stationary points here and there. And while the Sun always proceeds on its own direct path, they wander in various ways, straying sometimes toward the South and sometimes toward the North, which is why they are called "planets" From the Greek word planētēs, meaning "wanderer.". Add also that sometimes they become closer to the earth—and are called "perigee"—and at other times further away—and are called "apogee." Nevertheless, it must be admitted that their motions are circular, or composed of several circles, because these irregularities follow a certain law and return to their original states at fixed intervals, which could not happen if they were not circular. For only a circle can bring back what has already passed; for example, by a motion composed of circles, the Sun brings back to us the inequality of days and nights and the four seasons of the year.