Astronomical Images : Comparison between circular and rectilinear motions

Aristotle

Astronomical Images

<p style='text-align: justify;'>This work comprises Aristotle's <i>Physics</i> and Thomas Aquinas's commentary edited by the Augustinian, Timoteo Maffei of Verona (d. 1470). Aristotle discussed the nature of motion in the latter books of <i>Physics</i>, often referring to figures to elucidate his points. These figures were not always illustrated, and there was no traditional stock of figures like the wind diagrams or concentric circles associated with the Aristotelian analysis of motion. This edition makes an effort to supply such figures. In <i>Physics</i>, book 7, chapter 4, Aristotle discusses various types of motion. Circular and linear motions appear to be comparable in that if circular distance were greater or less than the rectilinear, whenever that relation exists it would imply the possibility of the equality between the two, but Aristotle denies that this is possible. In this figure, A represents time as a bar with two markers, circle B indicates the swifter mobile moving through an arc, and circle C the slower mobile moving in a straight line. The arc would appear to be greater than the line because a higher velocity implies travelling a greater distance in the same time. Therefore the swifter mobile would cover an equal distance in less time, so that there must be some portion of the time A in which B traverses a part of the arc equal to the whole line, which C takes the whole of time A to traverse. But if the two can be so equated, it means that an arc may be equal to a straight line; but this cannot be the case. (Aristotle, <i>Physics</i> VII.4.248a10-248b7, trans. Wicksteed and Cornford.)</p>