We have next to show that the movement of the heaven is regular
and not irregular. This applies only to the first heaven and the
first movement; for the lower spheres exhibit a composition of
several movements into one. If the movement is uneven, clearly
there will be acceleration, maximum speed, and retardation,
since these appear in all irregular motions. The maximum may
occur either at the starting-point or at the goal or between the
two; and we expect natural motion to reach its maximum at the
goal, unnatural motion at the starting-point, and missiles
midway between the two. But circular movement, having no
beginning or limit or middle in the direct sense of the words,
has neither whence nor whither nor middle: for in time it is
eternal, and in length it returns upon itself without a break.
If then its movement has no maximum, it can have no
irregularity, since irregularity is produced by retardation and
acceleration. Further, since everything that is moved is moved
by something, the cause of the irregula