Jupiter( 4) completes its smaller course in twelve years, Mars in two; the moon performs its much smaller revolution within a month.

Just as clearly in the Medicean stars, we see that the one nearest Jupiter completes its revolution in a very short time--about forty-two hours; the next in about three and one-half days, the third in seven, and the most distant one in sixteen days.

This rule, which is followed throughout, will still remain if we ascribe the twenty-four-hourly motion to a rotation of the earth.

If, however, the earth is left motionless, we must go first from the very short rule of the moon to ever greater ones--to the two-yearly rule of Mars, from that to the twelve-yearly one of Jupiter, from here to the thirty-yearly one of Saturn, and then suddenly to an incomparably greater sphere, to which also we must ascribe a complete rotation in twenty-four hours.

If, however, we assume a motion of the earth, the rapidity of the periods is very well preserved; from the slowest sphere of Saturn we come to the wholly motionless fixed stars.