Horror Vacui, or fear of the Vacuum, is a remarkable Idea by Aristotle…
intent of Aristotle is to demonstrate that the movement destroys the possibility of existence of the Vacuum, and for this purpose he states that
”the velocities of the same mobile in different media maintain, between them, the inverse proportion of the one between the densities of the media“
where the mobile is a celestial body, the movement is the circular one, the medium is his Aether generating his invisible concentric Spheres, the density of the medium is the intensity of the flow of Aether.
Aristotle is against the introduction of the Vacuum as a necessary condition for the circular motion of celestial bodies… as the density of the Vacuum is infinite times lower than the density (however small) of any medium, then every mobile that moved in a medium at any speed, into the Vacuum should move in an instant… but moving in an instant is not possible… therefore it’s not possible to hypothesize the Vacuum as a necessary condition for the movement of celestial bodies.
So, in accordance with common experience (the observation of the revolutions of the Planets), only a Plenum allows circular motion… indeed Nature abhors a Vacuum and fills it constantly.
Galileo* tries to disprove the reasoning of Aristotle… in his opinion it was specifically conceived against those Men who had introduced the Vacuum as necessary condition for the movement of celestial bodies… introduction derived from the observation that their movement doesn’t slow down with time.
According to Aristotle, instead, cause of all movements must always be sought in a force… the perpetual motion of celestial bodies is then solved with the introduction of a System of concentric Spheres, both impediment and cause for their movement… concentric Spheres representing an elegant model with rotor for the space around Eutòpia*.
Aristotle, reasoning by contradiction, could possibly be criticized… a mobile, in the Vacuum, wouldn’t have any impediment in its movement but, at the same time, wouldn’t be subject to any force… then its movement should remain rectilinear and at a certain constant speed (not infinite)… i.e. it should move in uniform rectilinear motion…
anyway the circular motion of the Planets excludes the possibility of existence of the Vacuum.
Eutòpia ⇒ Planet Earth and Aristotle’s Center of the Universe Galileo ⇒ Galileo Galilei