What is yet more wonderful in these stars which we are speaking of is that sometimes they appear, and sometimes they disappear; sometimes they advance towards the sun, and sometimes they retreat; sometimes they precede him, and sometimes follow him; sometimes they move faster, sometimes slower, and sometimes they do not stir in the least, but for a while stand still.

From these unequal motions of the planets, mathematicians have called that the "great year"[131] in which the sun, moon, and five wandering stars, having finished their revolutions, are found in their original situation.

In how long a time this is effected is much disputed, but it must be a certain and definite period.

For the planet Saturn (called by the Greeks [Greek: Phainon]), which is farthest from the earth, finishes his course in about thirty years; and in his course there is something very singular, for sometimes he moves before the sun, sometimes he keeps behind it; at one time lying hidden in the night, at another again appearing in the morning; and ever performing the same motions in the same space of time without any alteration, so as to be for infinite ages regular in these courses.

Beneath this planet, and nearer the earth, is Jupiter, called [Greek: Phaethon], which passes the same orbit of the twelve signs[132] in twelve years, and goes through exactly the same variety in its course that the star of Saturn does.
Next to Jupiter is the planet Mars (in Greek, [Greek: Pyroeis]), which finishes its revolution through the same orbit as the two previously mentioned,[133] in twenty-four months, wanting six days, as I imagine.
Below this is Mercury (called by the Greeks [Greek: Stilbon]), which performs the same course in little less than a year, and is never farther distant from the sun than the space of one sign, whether it precedes or follows it.