CHAPTER ONE

...attained. It is easily evident, however, that if those motions are not uniform original: "aequabiles." This refers to motion at a constant rate., as they are believed to be, then this measurement of time would also be erroneous. And in fact, more recent astronomers The author refers to post-Copernican astronomers who realized that the Earth's orbit and rotation are not perfectly constant. have detected an inequality in these motions and have found that not all days are equal to one another. For this reason, they are also accustomed to applying a correction derived from other, more uniform motions; this they call the equation of time original: "temporis aequationem." This is the mathematical difference between time as measured by a sundial (apparent solar time) and time measured by a reliable clock (mean solar time)., through which the inequality of days is recognized.

DEFINITION 5.

21. Every body that moves is said to have speed or velocity, and this is measured by the distance original: "spatio." While literally "space," in this physical context it refers to the length of the path traveled. that the body, moving uniformly, traverses in a given time. Specifically, when body B completes twice the distance through uniform motion in the same time that body A, also moving uniformly, traverses a single distance, body B is said to have twice the speed of body A.

Corollary 1.

22. Therefore, because in uniform motion a body traverses equal distances in equal times (17.), a body moving uniformly will perpetually have the same speed or velocity. In non-uniform motion original: "motu vero inaequabili." This refers to acceleration or deceleration., however, a body successively takes on different speeds.

Corollary 2.

23. Furthermore, the speed that a body moving non-uniformy has at any point of the distance traversed,