On the Revolutions of the Heavenly Spheres

NICOLAUS COPERNICUS

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

After this, we shall recall that the motion of the heavenly bodies is circular. For the natural property of a sphere is to turn in a circle; by this very act, it expresses its own form as the simplest of bodies, wherein one can find neither a beginning nor an end, nor distinguish one part from another, as it moves through the same path back into itself. However, there are multiple motions because of the vast number of spheres. The most obvious of all is the daily revolution, which the Greeks call nychthēmeron original: "νυχθήμερον"; a period of twenty-four hours, encompassing both a full day and a full night.. By this motion, the entire universe—excepting the Earth—is thought to be carried from east to west. This is understood as the common measure of all motions, since we measure time itself primarily by the number of days.

Then, we see other revolutions that seem to move in the opposite direction—that is, from west to east; I am speaking of the Sun, the Moon, and the five wandering stars planets From the Greek "planētēs," meaning "wanderer." In the 16th century, the Sun and Moon were often grouped with the five visible planets as "seven wanderers.". In this way, the Sun regulates our year and the Moon our months, which are the most common divisions of time. Likewise, each of the other five planets completes its own circuit.

Yet, these motions differ in many ways. First, they do not revolve around the same poles as the primary daily motion, but instead travel through the obliquity of the zodiac signifer original: "signiferi"; literally the "sign-bearer," the Latin name for the belt of the heavens containing the twelve constellations of the zodiac.. Secondly, in their own circuits, they do not appear to move uniformly. We find that the Sun and Moon are sometimes slow and at other times faster in their courses. As for the other five wandering stars, we perceive them sometimes even moving backward original: "repedare"; this refers to "retrograde motion," where a planet appears to move in reverse against the background of stars. and coming to a standstill in various places. And while the Sun always proceeds along its own direct path, the planets wander in various ways, straying sometimes toward the South and sometimes toward the North; it is for this reason they are called "planets." Add to this that they are sometimes nearer to the Earth, called perigee From the Greek, meaning "near the Earth.", and at other times further away, called apogee From the Greek, meaning "away from the Earth.".

Nevertheless, we must admit that these motions are circular, or composed of several circles. This is because these irregularities observe a fixed law and occur at regular intervals, which could not happen if they were not circular. For only a circle can bring back what has already passed; for example, the Sun, through a motion composed of circles, restores to us the inequality of days and nights and the four seasons of the year.
restores...