IN THE NAME OF THE LORD.

COMMENTARIES

ON THE MOTIONS OF THE PLANET

M A R S

PART ONE

ON THE

COMPARISON

OF HYPOTHESES.

CHAPTER I.

On the difference between the first motion and the second or "proper" motions, and within the proper motions, the first and second inequality.

Eternity testifies that the motions of the planets are circular. Reason, having borrowed from experience, immediately presumes that their revolutions are perfect circles. For among shapes, the circle—and among bodies, the heavens—are considered the most perfect. But when, paying close attention, experience seems to teach otherwise; namely, that the Planets stray from the simple path of a circle; great wonderment arises, which finally impelled men to inquire into the causes.

From this very beginning, Astronomy was born among men. Its goal is understood to be to teach the causes why the motions of the stars In the 17th century, planets were often called "wandering stars." appear irregular from Earth when they are most orderly in the heavens, and to investigate by which circles the stars are moved, so that by their benefit, the locations and appearances of those stars can be predicted for any given time.

Definitions: 1\. First motion Also known as diurnal motion. is that of the whole sky and of all the stars in it, from the east through the south to the west, and from the west through the bottom of the sky to the east, in a period of 24 hours; in the present diagram, ABCD. 2\. Second motion The orbital motion of the planet itself. is that of the individual Planets from the west toward the east, from A into E, from F into G, in longer periods of time. 4\. A Great circle of a sphere is one which is equally distant from both of its poles. 3\. Lesser circles are those which are closer to one of the poles; just as HLK is closer to pole Q than to pole R.

When the distinction between the 1 first motion and the 2 second motions was not yet established, men looking at the Sun, Moon, and stars noted their daily journeys, which were perceived by the senses to be nearly equal to circles. However, they were joined one to another in the manner of a ball of yarn, and these circles were for the most part 3 lesser circles in the sphere, and very rarely 4 great circles (as now ABCE, FMNG, cutting the equator AB at CN). Part of them were in the South, part in the Nor- The text cuts off here, continuing on the next page with "North."