The “perfect sciences” they define as those which, depending on no external hypothesis, have their origin, and the evidence of their propositions, in themselves. Here are a few of the facts established by natural philosophers.[705] The earth and heavens are spheroidal. The tendency of all bodies having weight, is to a centre. Further, the earth being spheroidal, and having the same centre as the heavens, is motionless, as well as the axis which passes through both it and the heavens. The heavens turn round both the earth and its axis, from east to west. The fixed stars turn round with it, at the same rate as the whole.[706] These fixed stars follow in their course parallel circles; the principal of which are, the equator, the two tropics, and the arctic circles. While the planets, the sun, and the moon, describe certain oblique circles comprehended within the zodiac. Admitting these points in whole or in part, astronomers proceed to treat of other matters, [such as] the motions [of the stars], their revolutions, eclipses, size, relative distance, and a thousand similar particulars. On their side, geometers, when measuring the size of the entire earth, avail themselves of the data furnished by the natural philosopher and astronomer; and the geographer on his part makes use of those of the geometer. 3. The heavens and the earth must be supposed to be divided each into five zones, and the celestial zones to possess the same names as those below. The motives for such a division into zones we have already detailed. These zones may be distinguished by circles drawn parallel to the equator, on either side of it. Two of these will separate the torrid from the temperate zones, and the remaining two, the temperate from the frigid. To each celestial circle there shall be one corresponding on earth, and bearing the same name, and likewise zone for zone. The [two] zones capable of being inhabited, are styled temperate. The remaining [three] are uninhabitable, one on account of the heat, the others because of the extreme cold. The same is the case with regard to the tropical, and also to the arctic circles, in respect of those countries for which arctic circles can be said to exist. Circles on the earth are supposed, corresponding to those in the heavens, and bearing the same name, one for one.