Its shape must necessarily be spherical.For every portion of earth has weight until it reaches the centre, and the jostling of parts greater and smaller would bring about not a waved surface, but rather compression and convergence of part and part until the centre is reached.The process should be conceived by supposing the earth to come into being in the way that some of the natural philosophers describe.Only they attribute the downward movement to constraint, and it is better to keep to the truth and say that the reason of this motion is that a thing which possesses weight is naturally endowed with a centripetal movement.When the mixture, then, was merely potential, the things that were separated off moved similarly from every side towards the centre.Whether the parts which came together at the centre were distributed at the extremities evenly, or in some other way, makes no difference.If, on the one hand, there were a similar movement from each quarter of the extremity to the single centre, it is obvious that the resulting mass would be similar on every side.For if an equal amount is added on every side the extremity of the mass will be everywhere equidistant from its centre, i.e.the figure will be spherical.But neither will it in any way affect the argument if there is not a similar accession of concurrent fragments from every side.For the greater quantity, finding a lesser in front of it, must necessarily drive it on, both having an impulse whose goal is the centre, and the greater weight driving the lesser forward till this goal is reached.In this we have also the solution of a possible difficulty.The earth, it might be argued, is at the centre and spherical in shape: if, then, a weight many times that of the earth were added to one hemisphere, the centre of the earth and of the whole will no longer be coincident.
So that either the earth will not stay still at the centre, or if it does, it will be at rest without having its centre at the place to which it is still its nature to move.Such is the difficulty.A
short consideration will give us an easy answer, if we first give precision to our postulate that any body endowed with weight, of whatever size, moves towards the centre.Clearly it will not stop when its edge touches the centre.The greater quantity must prevail until the body's centre occupies the centre.For that is the goal of its impulse.Now it makes no difference whether we apply this to a clod or common fragment of earth or to the earth as a whole.The fact indicated does not depend upon degrees of size but applies universally to everything that has the centripetal impulse.Therefore earth in motion, whether in a mass or in fragments, necessarily continues to move until it occupies the centre equally every way, the less being forced to equalize itself by the greater owing to the forward drive of the impulse.
If the earth was generated, then, it must have been formed in this way, and so clearly its generation was spherical; and if it is ungenerated and has remained so always, its character must be that which the initial generation, if it had occurred, would have given it.
But the spherical shape, necessitated by this argument, follows also from the fact that the motions of heavy bodies always make equal angles, and are not parallel.This would be the natural form of movement towards what is naturally spherical.Either then the earth is spherical or it is at least naturally spherical.And it is right to call anything that which nature intends it to be, and which belongs to it, rather than that which it is by constraint and contrary to nature.
The evidence of the senses further corroborates this.How else would eclipses of the moon show segments shaped as we see them? As it is, the shapes which the moon itself each month shows are of every kind straight, gibbous, and concave-but in eclipses the outline is always curved: and, since it is the interposition of the earth that makes the eclipse, the form of this line will be caused by the form of the earth's surface, which is therefore spherical.Again, our observations of the stars make it evident, not only that the earth is circular, but also that it is a circle of no great size.For quite a small change of position to south or north causes a manifest alteration of the horizon.There is much change, I mean, in the stars which are overhead, and the stars seen are different, as one moves northward or southward.Indeed there are some stars seen in Egypt and in the neighbourhood of Cyprus which are not seen in the northerly regions;
and stars, which in the north are never beyond the range of observation, in those regions rise and set.All of which goes to show not only that the earth is circular in shape, but also that it is a sphere of no great size: for otherwise the effect of so slight a change of place would not be quickly apparent.Hence one should not be too sure of the incredibility of the view of those who conceive that there is continuity between the parts about the pillars of Hercules and the parts about India, and that in this way the ocean is one.As further evidence in favour of this they quote the case of elephants, a species occurring in each of these extreme regions, suggesting that the common characteristic of these extremes is explained by their continuity.Also, those mathematicians who try to calculate the size of the earth's circumference arrive at the figure 400,000 stades.This indicates not only that the earth's mass is spherical in shape, but also that as compared with the stars it is not of great size.