ntre of the universe.

SIMPLICIUS: Whence, then, do you argue that it is the sun and not the
earth that is the centre of the planetary revolutions?

SALVIATUS: I infer that the earth is not the centre of the planetary
revolutions because the planets are at different times at very different
distances from the earth. For instance, Venus, when it is farthest off,
is six times more remote from us than when it is nearest, and Mars rises
almost eight times as high at one time as at another.

SIMPLICIUS: And what are the signs that the planets revolve round the
sun as centre?

SALVIATUS: We find that the three superior planets--Mars, Jupiter, and
Saturn--are always nearest to the earth when they are in opposition to
the sun, and always farthest off when they are in conjunction; and so
great is this approximation and recession that Mars, when near, appears
very nearly sixty times greater than when remote. Venus and Mercury also
certainly revolve round the sun, since they never move far from it, and
appear now above and now below it.

SAGREDUS: I expect that more wonderful things depend on the annual
revolution than upon the diurnal rotation of the earth.

SALVIATUS: YOU do not err therein. The effect of the diurnal rotation of
the earth is to make the universe seem to rotate in the opposite
direction; but the annual motion complicates the particular motions of
all the planets. But to return to my proposition. I affirm that the
centre of the celestial convolutions of the five planets--Saturn,
Jupiter, Mars, Venus, and Mercury, and likewise of the earth--is the
sun.

As for the moon, it goes round the earth, and yet does not cease to go
round the sun with the earth. It being true, then, that the five planets
do move about the sun as a centre, rest seems with so much more reason
to belong to the said sun than to the earth, inasmuch as in a movable
sphere it is more reasonable that the centre stand still than any place
remote from the centre.

To the earth, therefore, may a yearly revolution be assigned, leaving
the sun at rest. And if that be so, it follows that the diurnal motion
likewise belongs to the earth; for if the sun stood still and the earth
did not rotate, the year would consist of six months of day and six
months of night. You may consider, likewise, how, in conformity with
this scheme, the precipitate motion of twenty-four hours is taken away
from the universe; and how the fixed stars, which are so many suns,