Epitome Astronomiae Copernicanae: Liber I, part 7
Part V. Of the diurnal motion of the Earth
I hold the figures of the heavens, of the Earth and of the air poured round the Earth, and the place of the Earth in the world, so far as suffices for the knowledge of the first motion; it remains for me to ask what makes the stars daily emerge from one part of the horizon and, in its opposite part, after an interval of some hours, hide themselves again: is it the motion of the heavens or of the Earth? Copernican astronomy teaches that sight is deceived concerning the first motion: for the stars do not truly ascend above the mountains or rise toward our zenith, but on the contrary the mountains which lie around us, standing on the surface of the globe of the Earth, being parts indeed with the whole globe, are turned about its axis from the quarter of the setting toward the quarter of the rising, and by this turning the immovable stars of the east are disclosed to us one after another, and the stars of the west are covered; so that the stars do not pass through the zenith, but the vertical point passes through the immovable stars, so far as concerns the first motion.
Do you say that all the appearances of the first motion can be sufficiently accounted for by this marvellous position, and the whole spherical doctrine delivered? Plainly, most exactly, and so this is the one aim of this little book, that what has now been promised in words may be proved in fact.
But since at the beginning of the book, when the question was of hypotheses, you laid down this law for the astronomer, that he should not posit whatever he pleases, but also prove his positions by the counsels of nature, I ask therefore whether you hope to prove this absurd position, and by what arguments? That the first motion takes place by the revolution of the yoke of the Earth about her axis, the heavenly bodies being at rest (so far as the first motion goes), can be proved by seven kinds of argument chiefly, of which the first is from the subject of the motion, the second from the swiftness of the motion, the third from the uniformity of the motion, the fourth from the cause of the motion or moving faculty, the fifth from the organs of motion, that is, from the axis and poles, the sixth from the end of the first motion, the seventh from signs or effects.
First, prove from the subject of the motion. Nature always does by the easier ways what she can, and does not act by difficult circuits. But by the rotation of the Earth, a very small body, about her axis toward the east, nature could plainly effect the same as by the revolution of the vast world about the same axis (extended to both extremities of the world) toward the west. As, therefore, it is more credible that a man’s head should be turned in a lecture-room than that the lecture-room should be rolled about the motionless head of the man, so it is more credible also that the Earth is rotated from the setting to the rising than that the whole remaining machine of the world should be turned from the rising to the setting, since the same things follow on both sides.
If the first motion is in the celestial orbs, then two motions are in the same subject, one common to all the spheres, the other proper to each sphere; but it is much more likely that the first motion and the second motions are distinct in their subjects, so that the second, which are several, belong each to its own sphere, but the first, which is single, belongs also to a single body, the Earth, and indeed alone, in respect of her form, which is defined by an axis and poles as if immovable; notwithstanding that below, in the theoretical doctrine, another motion will be added to it also, in a far different respect, by which together with the body the axis, poles and centre will also be moved.
The argument gathers only likelihood; demonstrate the necessity. That some motion occurs, the eyes testify without error. If it occurs with the Earth at rest, then all the remaining machine of the world would be turned; a third is not given, and this is beyond controversy. But the whole machine of the world cannot be moved by the diurnal motion, the Earth alone being at rest; therefore it is necessary that the Earth be moved by the diurnal motion.
Why cannot the whole machine of the world be moved? The world is either infinite or finite. Let the first be granted, according to the opinion of William Gilbert, who thinks that the omnipotence of God is illustrated in this, if he supposes the world infinite outward, so that from the creature of infinite quantity the infinite power of the Creator may be known. Although it was refuted above by metaphysical arguments, yet from astronomy, in which men, on account of the evidence of the senses, trust more than in reasonings most remote from sense, nothing could be drawn to refute it. But the world being supposed infinite outward, Aristotle demonstrated that it cannot be moved as a whole in a motion of revolution, as a whole.
Let the world now be finite: outside the world, then, there is nothing that gives the world place, but it should be at rest. But where there is nothing that rests, there is no motion. For first, motion is the separation of a movable thing, as movable, from its place and its translation to another place. Second, in particular the motion of the machine about axis and poles at rest cannot be understood where there is nothing in respect of which the poles may be understood to rest. For in the turning of a sphere its poles are fixed in the immovable meridian, the meridian rests upon the horizon and the foot, the foot of the table, the table on the Earth; but outside the machine of the world there is nothing that has the character of a resting meridian. What has no place, therefore, in the intellect cannot even happen in these things which partake of geometry. Third, not unjustly does Maestlin also ask how it can be that, the whole system of the world being driven round, with none of its orbs excepted, not the sphere of fire (if there be one), not the upper region of the air, this one little globe, whose diameter is less than the twenty-thousandth part of the diameter of the world, should not be swept round with it? The force of which objection shines out most when there is also asked about the means or supports by which that little globe should keep its place in the centre of the world, which follows from the rest of the Earth, of which matter we treated in the fourth part.
Aristotle thinks that the rest of a very slight little body in the centre suffices, in respect of which at rest the motion of the surrounding machine is both understood and made, and on which that motion rests. The rest of the Earth in the centre does not suffice. For first, although the separation of the stars from the parts of the Earth lying under them is then perceived, yet by whose motion this is done, the heaven’s or the Earth’s, cannot yet be understood, unless along with the Earth there also rest some body outside the heaven, containing the heaven and giving it place, of which kind there is no body, or certainly it is unknown; and if there were, the heaven would be turned in it just as Copernicus now says the Earth is turned in the heaven, having no need for this of an outer body.
Next, that the motion of the vastest machine of the world be either compared to something at rest or rest upon something at rest, there must be a fitting proportion between the movable and the resting. But the Earth in respect of the heaven is of most contemptible slightness and like a point.
Second, give the argument from the swiftness of the motion. It is more likely that nature expresses in the small Earth a proportionate and small motion than in the vast heaven a motion incredibly swift and one that exceeds the vastness in swiftness in an immense ratio. For since motion is a geometrical thing, no less than magnitude, slowness corresponds to the great, swiftness to the small, not conversely, as we learn in the Harmonics. Now if the machine of the world were turned about the Earth, so great a mass of so many stars, many times greater than the Earth, of so many most vast orbs, would be turned in one minute of an hour through fifty thousand, or in one hour through thirty hundred thousand German miles, all the stars, so vast, being continually carried from place to place. And this is to be understood of the Tychonic constitution of the world, which has much affinity with the Copernican; in the ancient Ptolemaic, as the world is greater, so this swiftness is far more incredible still.
But if the Earth is turned, then she does not desert her own place (that of the first motion, in which we now are engaged), but contains herself within it, sustaining the little parts of her body going round and its parts, which, the nearer they are to the axis, are the nearer to rest; finally, the outermost parts, those namely which are rotated in the greatest circle of all, midway between the poles, move in one minute of an hour through a space not the thirteen- or twenty-thousandth of the former space through which the outermost heaven would have had to pass; for in one minute they fly over three and three quarters miles, 225 in one hour.
Yet this motion also is very rapid. Indeed, first, where it is swiftest, it is twice slower than the motion of the Moon, who traverses an orbit more than sixty-fold greater in a time at least thirty-fold and more. And so in proportion it answers beautifully to the heavenly things. Second, this rapid swiftness does not much exceed the examples of motions near at hand, if you weigh circumstances against circumstances. Maestlin gives the flight of clouds as equal in swiftness. The Landgrave and Brahe measured the time that elapses while a great ball of a bombard, driven out by the force of fire, flies through the air before it strikes the ground; they found it two minutes, the space traversed a great German mile. A place under the equator, by reason of the first motion, is therefore only seven or eight times swifter than a cannon-ball; indeed it is scarcely swifter than the beginning of the violent motion, when the ball has just been cast from the tube; for the nearer it is to its fall, the slower it becomes. But the aethereal breeze, through which the highest ridges of mountains are borne, is not merely eightfold, but almost infinitely many times thinner than the air through which a leaden ball is borne. The flight, therefore, of the ridges through the aether is many times more tranquil than that of the iron ball through the air. Then the ball is transferred wholly from place to place, wearing away the air all round on every side and as it were boring and exhausting it behind it, thrusting it forward; the places of the surface of the Earth cling by their roots, parts of the whole, and bring no force upon the aethereal breeze poured over them, as they do not drive it from its place, especially the deeper places, nor on every side, but only wear it away upward. Third, which is the chief point, the leaden ball by the force of gravity is meanwhile drawn out of the line of its violent motion downward toward the centre of the Earth; the parts of the surface of the Earth, although heavy in potency, are yet not in act drawn out of the circular curve of the diurnal motion, since they are sustained by that which could draw. But the whole Earth herself can no longer be called heavy as a whole, since there is nothing outside her to attract her.
Third, what argument have you from the uniformity of the motion? This can be sought by anticipation from the theoretical doctrine of the motions of the Moon. For by manifest experiments we are convinced that the first motion, although in any one revolution it is found most uniform, yet we must grant it a little and in parts insensibly unequal, slower indeed when the Sun is in apogee, swifter when in perigee. But if, the Earth alone excepted, the whole remaining machine of the world sustained this motion, no cause could be feigned why the laws of this inequality, adjusted to the motion of the one solar sphere alone, should flow over upon the whole machine. But if, the machine of the world being at rest, the Earth is rotated, the cause of this inequality is easily rendered from the change of the interval between the Sun and the Earth in the same proportion, since the Sun is the minister and helper of all motion.
Fourth, give the argument from the cause of the first motion. Since not only the Earth but also the heaven is of matter, Moses confirming us against Aristotle, neither body is of itself apt for motion. For everything materiate, as such, is apt to rest where it is placed alone, of which more in the theoretical doctrine. The first motion, therefore, must be ascribed either to some animal faculty informing the subject of the motion or assisting it, or to some natural moving power intrinsic. But such a moving faculty, whether animal or natural, is more easily posited in the Earth than in the heaven; for the Earth is globular and full within and narrow, and has a certain form of a heart, the heaven most ample and hollowed out, through which this faculty would have to be diffused in a circle; whereas on the contrary some soul can be placed rooted in the centre of the Earth, according to the nature of its spiritual indivisible essence, and from there, as from the human heart, continued by its species run out into all the parts of its globe; or if a faculty is set over the motion, it can be extended into the axis of the Earth (existing as solid) or into lines parallel to the axis or drawn round the axis, and urging its little body close at hand move it without weakening from a very long diffusion of its powers.
Fifth, finally argue from the organs of motion. The Earth seems to be invited to rotation by her round figure, most apt for rolling. The diurnal motion is made about an axis, which in respect of this first motion becomes immovable; this is true of itself, whether you will say the heaven or the Earth moves. Now this axis wanders through the parts of the outermost sphere of the fixed stars, so that not in all ages are the same fixed stars, but one after another come to this axis; on the contrary, this axis passes through the same parts of the Earth always; therefore this axis is proper to the globe of the Earth, adventitious to the heavens. And consequently, since an axis is so called from a motion, the first motion itself, whose axis this is, will be in the body of the Earth, not in the immense mass of the heavens. The same is demonstrated in the same way also of the middle circle of this motion. A like argument from the organs and from the end considered together, see below in its place.
These three arguments have much hesitation in them: for their elucidation, then, say more clearly what that motion is which you ascribe to the Earth, that by it you may save the appearances of the first motion. It is not fortuitous, not vagrant or trembling or trepidating, not promiscuous in every direction, such as is the disordered motion of a ball of clay which the potter turns between the two palms of his hands, pressing the mass round on every side and preparing it for the wheel, but it is perennial, continuous, in any one revolution uniform by parts and constant; so that the inequality of different revolutions can scarcely, indeed not even scarcely, be perceived.
In respect of the whole globe and its parts its form is that of circular instruments which run in the lathes of metal-workers continually in the same direction, nor is the motion very unlike that of tops, which boys whirl, having thrown them off with cords, when, fixed with their point on a certain point of the floor, they remain there while spinning as if immovable. For the globe of the Earth has two points, on opposite parts of it, which, taken mathematically, are not moved, but through the whole revolution of the Earth rest under two opposite points of the heavens, as does the line joining those points of the Earth with each other. The parts of the surface near these points are turned in small circles and so slowly, the more remote parts in greater circles and so more swiftly, the parts exactly intermediate in the greatest circle and so most swiftly.
What proofs compel you to posit this turning of the Earth uniform and circular? The experience of celestial observations. Brahe used armillae, that is, perfect circles so erected that their axis stood exactly upright upon their planes, was drawn through their centres and directed to the pole of the world. The line of sight, falling on some certain star, proceeded from some one point of the gnomon, touching the extreme margin of the armilla. Attending, therefore, to the passage of the star through a great part of its circuit, he was wont to count the parts of the armilla turned past the immovable mark with the passing star, and to ascribe to the hour a minute and also parts of the minute, from the clocks and water-clocks at hand. Always, therefore, the minutes of time corresponded in a fixed proportion to the quantity of the arcs turned; always the star was seen to go round on the same margin of the armilla, never to depart from it outward, never to be hidden behind it. A uniform and circular motion is, therefore, the only cause of this appearance.
Reason also gives its vote. For since the figure of the Earth is spherical, a motion of revolution other than circular about an axis would not beautifully suit her. And since she is on every side solid, full of herself, and nothing outside occurs against which she may strike, the faculty also, whether animal or natural, the author of this turning, on account of perennity cannot likely be posited otherwise than constant and of a fixed proportion of its forces to the inertia or resistance of matter: no reason, therefore, occurs why the motion itself should not be most uniform according to the type of its cause, apart from the single concourse of moving causes, of which in the theoretical doctrine. For this concourse, according to the measure of its proportion, causes some very slight inequality, as it were by accident.
The rays of the Sun admitted through a hole are seen to tremble on the pavement: is not this an argument of a trembling motion of the Earth? No, for the causes of this trembling are others: first, the ray is let down from on high, but all things raised on high shake and tremble, driven by the shock of the ground, the blasts of winds, sounds also and whatever else happens in the daytime. Next, the ray passes first through the air, and the air is continually and momentarily mingled with exhalations of different kinds, especially those excited by the heat of the Sun by day: by these, therefore, the rays are refracted, with momentary undulations also.
In every local motion it is necessary that, besides that which gives place, something else also have the character of the resting, on which the motion rests, as in the lathe the points, in the top the floor, in flight the air, in swimming the waves, in walking the Earth: I ask what in this turning of the Earth has the character of the resting? Nearly the same as in the boys’ top before it touches the floor. For first the whole globe of the Earth, so far as the first motion goes, remains in its place, while the other parts succeed into the places of those preceding. Next, to come down more to particulars, the same globe along a rectilinear track, parallel to the axis, in which track the axis and poles are also recognised, stands immovable as to this first motion, and again the same whole globe, along the track of the circle midway between the poles, is the subject of this motion, and this motion rests upon that rest, no otherwise than if the globe ran fixed between two immovable points of a lathe. All which things are said of the Earth plainly as must be said of the top, when, flying in the air, it is also being rotated.
If this axis were something real, drawn from one extremity of the heavens to the other and passing through the globe of the Earth, as a globe is really suspended in a lathe, then indeed this form of motion of a globular body about an immovable axis would be credible. It is not necessary that this axis be actually continued as far as the heavens and be something corporeal and hard, for such a motion can nevertheless exist. This is plain from the example of the top already given, while it still flies in the air; for its axis is not overturned, but the point, turned downward in the cast, always remains below, until it touches the floor, the body being rotated meanwhile.
In particular this form of motion is accepted by other astronomers also, although they cannot demonstrate a corporeal axis in the world. For they say that the machine of the whole world is so turned about an imagined axis, resting on nothing but the Earth, by no bond, with no firmament sustaining the poles on the outside, or joining them within to the Earth. But this is much more absurd than that, though it is necessary that one or other of these be true.
You seem to have laid down contradictory things: that this axis remains immovable and yet does not always tend toward the same parts of the sphere of the fixed stars, which gives place to all motions. It will therefore be inclined from one fixed star to another and so be moved. In any one diurnal revolution this axis remains, to all the subtlety of the senses, immovable, but after very many revolutions it is found to have been somewhat inclined: wherefore its inclination, which occurs in single revolutions, is of a quantity plainly insensible.
Whence do you know that the axis of the Earth, produced to the heavens, falls through the whole time of the diurnal revolution always on the same point of the starry heaven, since that track which you call the axis is hidden within the body of the Earth? If this is done by a stile set up on the surface of the Earth, or by any line of sight drawn from the eye to the heavens at an inclination immovable to the surface, the axis also will do the same. For the stile so adhering to the Earth is carried round by the diurnal motion about the axis of the Earth; and if it is not directed to other and other quarters of the heavens, but always to the same point, at an inestimable distance, it remains in its revolution parallel to itself; and consequently the axis also of that revolution will be parallel to it, and so itself also will be always directed to the same point of the heavens.
But this is done, as was said before, by the axes of armillae, and so by the stiles or gnomons of sundials and the lines of sight directed along them by the sights, the manner of properly setting which up will be delivered in the spherical doctrine. They do it by this argument: for either that point of the heavens is visible, having one of the fixed stars, and then on a moonless winter night, 16 hours long, as often as one looked up through the sight, he would always see the same star, and so through the whole year; or there is no notable star at that point, as happens in this age, and then a comparison is to be made of the stars which are near the point which the stile shows, for if one of them is always at an equal interval from the point toward which the stile is directed, now above, now at the side, then below, the stile will certainly be directed to some one of the points of the circle which is described about the star by that interval. Let a second star also be observed; if that also is seen to go round the point of the stile always at an equal interval, then there cannot be more than two points of the heavens to which the stile can be directed, for two circles do not cut each other in more than two points. If a third star is added, not in the same straight line with the two former, doing the same as they, then, all doubt being removed, the stile and the axis of the Earth will be reduced to one certain point of the heavens. For three circles, whose centres are not in the same straight line, do not meet in more than one point.
Is there, then, in a part of the heavens a magnetic power of directing the axis of the Earth toward itself, or in the Earth herself of directing herself to this point of the heavens? No: for the axis is not directed today to any star, and scarcely ever to the stars themselves, but for the most part to empty places of the heavens, which cannot be the subject or source of a magnetic virtue, on account of the extreme thinness of the heavens. Nor in all ages is the axis directed the same way, but gradually to successive places, as already said in the fifth argument. Since, then, in the heavens there is no certain body or part of a body that might receive an active magnetic force, neither will such a force be in the corporeal Earth; it is to be sought in the heavens in that which is nothing corporeal.
What cause, then, can bring it about that the turning of the Earth does not overturn the axis, so that its direction remains invariable through one whole revolution of the Earth? There can be three causes. First, either the form of the revolution itself, bestowed upon the globe, which, being directed toward a certain quarter, not toward all promiscuously, follows by geometrical necessity, so that the axis of this revolution is directed toward the lateral quarters, constant as long as the middle circle of the revolution does not wander from its quarter. Second, or the privative cause of motion, namely the natural inertia of the matter of the globe along the track of the axis, which, to be overturned from its position, would need force; but since no impression of motion is made upon it, it rests, therefore, in its position. Third, or an internal and positive natural faculty in rectilinear fibres, parallel to the axis, of keeping itself in its primeval position. For the faculty of revolution nests in these very filaments of the whole globe, set in a circle about the axis with equal weight on every side, resting on which it turns the body. So, then, in turn the natural constancy of the axis in its direction directs also the quarter of the revolution itself, so that if it were inclined, the circle of motion also must be inclined.
If the axis kept its position by a natural faculty, it would never be inclined. Indeed, for this very reason, because it is a natural cause of direction, it can also in the course of ages be overcome by a stronger extraneous cause, of which it is not the place to say more here, where we treat only of the diurnal motion, in respect of which the axis is not inclined.
Perhaps you have other proofs of this natural faculty also? Indeed there is in the globe of the Earth, along the track of the axis and of the fibres parallel to the axis, a faculty of directing toward itself magnets and all magnetic things, and in magnets there is a mutual faculty of seeking this quarter of the Earth. Since, then, it is manifest that the globe of the Earth is informed by a rectilinear corporeal form along the track of the axis of the first motion, it is not absurd that to the same form is ascribed also this constancy of the direction of the axis toward the same quarter always.
One body cannot have two motions. The Earth has a rectilinear motion downward; therefore she has not a circular motion. First, the whole Earth, as a whole and in respect of her matter, has plainly no motion naturally; for the matter of which the Earth chiefly consists has inertia as its own, repugnant to motion, and that so much the stronger as the quantity of matter is greater, compacted into a narrow space.
Second, although the parts of the Earth are moved with a rectilinear motion downward, yet this happens neither on account of the rectilinear species of the motion, nor on account of the quarter itself in itself, nor on account of the essence of motion. Not the first: for since the parts are inert of themselves, equally with the whole Earth herself, they do not incline themselves toward straightness of motion more than toward circularity, but as any motion makes an impression upon them from its causes, so it overcomes their inertia. Not the second: because quarter is spoken of neither absolutely, nor in respect of the whole machine of the world, but in respect of the Earth alone, wherever she may be. For the parts of the Earth are not moved elsewhere than toward the Earth herself, and that alone is downward. Not the third: for the parts of the Earth are not moved with a rectilinear motion in order to be moved, but in order to rest, namely that they may be united to their whole.
Third, although one body cannot have two contrary motions in one and the same respect and by its own inclination, nothing nevertheless hinders it from having several disparate motions, or contrary ones indeed, but in different respects, depending on disparate forms of the same body, or not by its own inclination but on account of causes either extraneous or assisting. For a little magnetic iron needle has three motions: one of gravity downward, another of direction to the poles, a third of declination toward great continents or neighbouring magnetic things, where the effects are mixed in proportion to the moving principles. So the arm of a standard-bearer is moved at the same time by four motions: the first is of gravity, by reason of matter, by which it comes about that the raised arm sinks down; another is that which the physicians call natural, of nourishment from the supply of juices, spreading itself into a wider space; the third is vital, while the arteries beat; the fourth is animal, while by the force of the soul the arm is raised aloft, holding and waving the standard. All four can be called natural, in that they have their causes either in the soul or in the body, the two essential parts. So a top at one and the same time falls through the air naturally, unwound from the cord, and is rotated about its axis by an extraneous motion conceived through the force of the swing. How much more can there be in the whole Earth herself, as a whole, a motion of revolution about her axis depending on an internal and perennial cause? since the top is actually heavy and falls, the whole Earth, as a whole, is carried by no moment of gravity toward any quarter, the parts of the Earth are heavy indeed in potency in respect of the whole, but are not in act moved toward the Earth, as they already attain the end of the motion of gravity, which is the union of the parts with the whole.
If the Earth by reason of her matter resists circular motion, her diurnal motion will be violent and so cannot be perpetual. Indeed I would not deny that this material inertia of the body of the Earth to motion, and her density, is the seat on which the impetus of rotation is impressed, in the same way as it happens in a top violently spun, in which the more ponderous the matter is, the more successfully the external force falls upon it, the longer the motion impressed by it lasts; but feathers and like bodies, which have no resistance, do not easily conceive motion, nor are they fit for scorpions or slings. But since we call that motion properly violent when some body is moved by an adventitious motion against its nature: the motion which the form of the matter, a faculty or soul, brings upon its body we are not wont to reckon against nature, since nothing is more natural to matter than its form, to a body than its faculty or soul. A magnet by the nature of its matter tends downward, but by the nature of its special corporeal form ascends to another magnet; nor yet is this done violently.
So the running of animals, poising their bodies in the air by impetus, and the leaps of cats or the darts of serpents, are not held violent.
Say finally what it is that brings upon the Earth a circular motion about an immovable axis, an extrinsic cause or an intrinsic, a natural power, or a soul? I say that not only does each of these of itself have its likelihoods, but that all three can concur, and without doubt do concur.
Prove first of the extraneous moving cause. If boys can spin a top toward either certain quarter, with a motion so much the more uniform and constant as the impression made is more exact, so that the top, once set in motion, the impetus conceived, makes very many gyrations, until, struck by the uneven surface of the board and the resistance of the air and overcome by its own weight, with failing motion it gradually falls, why could not God have made at the beginning such an impression upon the Earth, as if from outside, from which all the subsequent rotations even today remain with continued vigour, so that they are now twenty hundred thousand in number, since this rotation is struck by no roughness meeting it from without or density of the aethereal breeze, by no weight or internal gravity; and as to material inertia, it stands in the place of subject for conceiving the impetus and continuing the rotation.
This circumstance also confirms this cause: that as a top is rotated constantly toward that quarter in which it was at first twisted, so the same happens also with the Earth, so that no cause can be given why the Earth should rotate toward this quarter rather than the contrary, except this, that by the Creator at the very beginning she began to be rotated toward this quarter.
Prove of the corporeal faculty. First, it is likely that this very species of the first rotation, continued, was transformed in the Earth, or grew together into such a corporeal faculty, and so grew into the fibres of the Earth, arranged according to the course of the motion itself, so that all the fibres are understood rectilinear, as was said before, and parallel to the axis, but surrounding the axis in a circle, like straight wefts woven into circular warps. For if the species of a motion brought in from outside can be abstracted from the moving cause and impressed on the movable body of a top by the force of the swing or shock and endure in it for some time, notwithstanding that it is only a guest in it, so that the parts of the body, set in motion, urge one another on, as if it were itself a mover, until by the continual blows of the top this species of motion is overcome and, weakened by length of time, gradually dies away: nothing hinders, then, that also the species of that motion with which God the Creator first urged the globe of the Earth should have insinuated itself more closely and durably into the body of the Earth and passed into the circularity of the fibres and as it were into a special corporeal form, no longer a guest in the Earth, as that in the top, but plainly a native, or conqueror and tamer of her matter.
Second, an argument for this can also be that the vigour of this rotation does not slacken, but is found even today of the same swiftness as it was of old, at a like time of the year, namely. But this suits an intrinsic cause rather than an extraneous.