Definitions

That is heavy which is moved towards the middle.

That is light which is moved from the middle.

That is said to be moved in a circle which is continually borne from the same to the same.

Contrary motions are from contraries to contraries.

One motion is contrary to one.

Time is the number of the motion of the celestial bodies.

The motion is one which is without difference according to species, and belongs to one subject, and is produced in a continued time.