Let us first decide the question whether the earth moves or is at rest.For, as we said, there are some who make it one of the stars, and others who, setting it at the centre, suppose it to be 'rolled'
and in motion about the pole as axis.That both views are untenable will be clear if we take as our starting-point the fact that the earth's motion, whether the earth be at the centre or away from it, must needs be a constrained motion.It cannot be the movement of the earth itself.If it were, any portion of it would have this movement; but in fact every part moves in a straight line to the centre.Being, then, constrained and unnatural, the movement could not be eternal.But the order of the universe is eternal.Again, everything that moves with the circular movement, except the first sphere, is observed to be passed, and to move with more than one motion.The earth, then, also, whether it move about the centre or as stationary at it, must necessarily move with two motions.But if this were so, there would have to be passings and turnings of the fixed stars.Yet no such thing is observed.The same stars always rise and set in the same parts of the earth.
Further, the natural movement of the earth, part and whole alike, is the centre of the whole-whence the fact that it is now actually situated at the centre-but it might be questioned since both centres are the same, which centre it is that portions of earth and other heavy things move to.Is this their goal because it is the centre of the earth or because it is the centre of the whole? The goal, surely, must be the centre of the whole.For fire and other light things move to the extremity of the area which contains the centre.It happens, however, that the centre of the earth and of the whole is the same.Thus they do move to the centre of the earth, but accidentally, in virtue of the fact that the earth's centre lies at the centre of the whole.That the centre of the earth is the goal of their movement is indicated by the fact that heavy bodies moving towards the earth do not parallel but so as to make equal angles, and thus to a single centre, that of the earth.It is clear, then, that the earth must be at the centre and immovable, not only for the reasons already given, but also because heavy bodies forcibly thrown quite straight upward return to the point from which they started, even if they are thrown to an infinite distance.From these considerations then it is clear that the earth does not move and does not lie elsewhere than at the centre.
From what we have said the explanation of the earth's immobility is also apparent.If it is the nature of earth, as observation shows, to move from any point to the centre, as of fire contrariwise to move from the centre to the extremity, it is impossible that any portion of earth should move away from the centre except by constraint.For a single thing has a single movement, and a simple thing a simple: contrary movements cannot belong to the same thing, and movement away from the centre is the contrary of movement to it.
If then no portion of earth can move away from the centre, obviously still less can the earth as a whole so move.For it is the nature of the whole to move to the point to which the part naturally moves.
Since, then, it would require a force greater than itself to move it, it must needs stay at the centre.This view is further supported by the contributions of mathematicians to astronomy, since the observations made as the shapes change by which the order of the stars is determined, are fully accounted for on the hypothesis that the earth lies at the centre.Of the position of the earth and of the manner of its rest or movement, our discussion may here end.
