Galilei, Galileo, The systems of the world, 1661

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ſame, not to ſerve for a neceſſary demonſtration, but to adorn a
Platonick Conceit; to which I will add another particular obſer­
vation of our Academick, which hath in it ſomething of admira­
ble. Let us ſuppoſe amongſt the decrees of the divine Architect,
a purpoſe of creating in the World theſe Globes, which we be­
hold continually moving round, and of aſſigning the centre of
their converſions; and that in it he had placed the Sun immoveable,
and had afterwards made all the ſaid Globes in the ſame place,
and with the intended inclinations of moving towards the Centre,
till they had acquired thoſe degrees of velocity, which at firſt ſee­
med good to the ſame Divine Minde; the which being acquired,
we laſtly ſuppoſe that they were turned round, each in his Sphere
retaining the ſaid acquired velocity: it is now demanded, in
what altitude and diſtance from the Sun the place was where the
ſaid Orbs were primarily created; and whether it be poſſible that
they might all be created in the ſame place? To make this inve­
ſtigation, we muſt take from the moſt skilfull Aſtronomers the
magnitude of the Spheres in which the Planets revolve, and like­
wiſe the time of their revolutions: from which two cognitions is
gathered how much (for example) Jupiter is ſwifter than Sa­
turne; and being found (as indeed it is) that Jupiter moves more
ſwiftly, it is requiſite, that departing from the ſame altitude, Ju­
piter be deſcended more than Saturne, as we really know it is, its
Orbe being inferiour to that of Saturne. But by proceeding for­
wards, from the proportions of the two velocities of Jupiter and
Saturne, and from the diſtance between their Orbs, and from the
proportion of acceleration of natural motion, one may finde in
what altitude and diſtance from the centre of their revolutions,

was the place from whence they firſt departed. This found out,
and agreed upon, it is to be ſought, whether Mars deſcending
from thence to his Orb, the magnitude of the Orb, and the ve­
locity of the motion, agree with that which is found by calcula­
tion; and let the like be done of the Eartb, of Venus, and of
Mercury; the greatneſs of which Spheres, and the velocity of
their motions, agree ſo nearly to what computation gives, that it
is very admirable.
The circular mo­
tion is never ac­
quired naturally,
without right mo­
tion precede it.
Circular motion
perpetually uni­
form.
The magnitude of
the Orbs, and the
velocity of the mo­
tion of the Planets,
anſwer proportion­
ably, as if deſcend­
ed from the ſame
place.

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