It remains to speak of the earth, of its position, of the question whether it is at rest or in motion, and of its shape.
I.As to its position there is some difference of opinion.Most people-all, in fact, who regard the whole heaven as finite-say it lies at the centre.But the Italian philosophers known as Pythagoreans take the contrary view.At the centre, they say, is fire, and the earth is one of the stars, creating night and day by its circular motion about the centre.They further construct another earth in opposition to ours to which they give the name counterearth.In all this they are not seeking for theories and causes to account for observed facts, but rather forcing their observations and trying to accommodate them to certain theories and opinions of their own.But there are many others who would agree that it is wrong to give the earth the central position, looking for confirmation rather to theory than to the facts of observation.Their view is that the most precious place befits the most precious thing: but fire, they say, is more precious than earth, and the limit than the intermediate, and the circumference and the centre are limits.Reasoning on this basis they take the view that it is not earth that lies at the centre of the sphere, but rather fire.The Pythagoreans have a further reason.They hold that the most important part of the world, which is the centre, should be most strictly guarded, and name it, or rather the fire which occupies that place, the 'Guardhouse of Zeus', as if the word 'centre'
were quite unequivocal, and the centre of the mathematical figure were always the same with that of the thing or the natural centre.But it is better to conceive of the case of the whole heaven as analogous to that of animals, in which the centre of the animal and that of the body are different.For this reason they have no need to be so disturbed about the world, or to call in a guard for its centre:
rather let them look for the centre in the other sense and tell us what it is like and where nature has set it.That centre will be something primary and precious; but to the mere position we should give the last place rather than the first.For the middle is what is defined, and what defines it is the limit, and that which contains or limits is more precious than that which is limited, see ing that the latter is the matter and the former the essence of the system.
II.As to the position of the earth, then, this is the view which some advance, and the views advanced concerning its rest or motion are similar.For here too there is no general agreement.All who deny that the earth lies at the centre think that it revolves about the centre, and not the earth only but, as we said before, the counter-earth as well.Some of them even consider it possible that there are several bodies so moving, which are invisible to us owing to the interposition of the earth.This, they say, accounts for the fact that eclipses of the moon are more frequent than eclipses of the sun: for in addition to the earth each of these moving bodies can obstruct it.Indeed, as in any case the surface of the earth is not actually a centre but distant from it a full hemisphere, there is no more difficulty, they think, in accounting for the observed facts on their view that we do not dwell at the centre, than on the common view that the earth is in the middle.Even as it is, there is nothing in the observations to suggest that we are removed from the centre by half the diameter of the earth.Others, again, say that the earth, which lies at the centre, is 'rolled', and thus in motion, about the axis of the whole heaven, So it stands written in the Timaeus.
III.There are similar disputes about the shape of the earth.Some think it is spherical, others that it is flat and drum-shaped.For evidence they bring the fact that, as the sun rises and sets, the part concealed by the earth shows a straight and not a curved edge, whereas if the earth were spherical the line of section would have to be circular.In this they leave out of account the great distance of the sun from the earth and the great size of the circumference, which, seen from a distance on these apparently small circles appears straight.Such an appearance ought not to make them doubt the circular shape of the earth.But they have another argument.They say that because it is at rest, the earth must necessarily have this shape.For there are many different ways in which the movement or rest of the earth has been conceived.
The difficulty must have occurred to every one.It would indeed be a complacent mind that felt no surprise that, while a little bit of earth, let loose in mid-air moves and will not stay still, and more there is of it the faster it moves, the whole earth, free in midair, should show no movement at all.Yet here is this great weight of earth, and it is at rest.And again, from beneath one of these moving fragments of earth, before it falls, take away the earth, and it will continue its downward movement with nothing to stop it.The difficulty then, has naturally passed into a common place of philosophy; and one may well wonder that the solutions offered are not seen to involve greater absurdities than the problem itself.
By these considerations some have been led to assert that the earth below us is infinite, saying, with Xenophanes of Colophon, that it has 'pushed its roots to infinity',-in order to save the trouble of seeking for the cause.Hence the sharp rebuke of Empedocles, in the words 'if the deeps of the earth are endless and endless the ample ether-such is the vain tale told by many a tongue, poured from the mouths of those who have seen but little of the whole.