Epitome Astronomiae Copernicanae: Liber IV, part 1
BOOK IV. THE FIRST PART OF THE THEORETICAL DOCTRINE. OF THE SITUATION, ORDER AND MOTION OF THE PARTS OF THE WORLD, OR OF THE SYSTEM OF THE WORLD.
What is the subject of the theoretical doctrine? The proper motions of the planets, which we call second motions, and the planets themselves, the second movables.
In what respect do you call the motions of the planets proper? 1. Because that common motion, the apparent daily motion of the planets as well as of the fixed stars and so of the whole world, with which spherical doctrine is concerned, seems to tend from the rising to the setting, whereas the several motions of the individual planets, far slower, tend in the contrary direction, from the setting to the rising. It is therefore certain that these cannot depend on that common motion of the world with which we have so far dealt, but must be attributed to the planets themselves, and so are proper to the planets in general.
2. Although, however, in these proper motions of the several planets from the setting to the rising there is still something common, not diurnal but annual, which is adventitious and derives its cause from sight alone, apart from the truth of the thing itself; and which sometimes makes the planets in their own motion appear to go back, from the rising, that is, toward the setting; yet because this common element is so implicated in the several periods of the several planets and so variously transformed that at first sight it cannot be discerned what is common to all and what proper to each, therefore the whole compound motion of each planet, as it strikes the eyes, is also called in a special sense proper to that planet; especially since that common element in many planets has its origin not in that common first motion of the whole world but in the proper motion of a single planet.
How many are the parts of the theoretical doctrine? Above, in book one, folio 126, the whole doctrine was divided into three proper parts: the first on the principles from which Copernicus demonstrates the second motions (the matter of book IV); the second on the hand instruments by which these motions are subjected to the eyes, namely eccentric orbs and the like (the matter of book V); the third on the apparent motions of the individual planets themselves and the common accidents of those joined together (the matter of book VI); and a fourth, common to spherical and theoretical doctrine, on the apparent motion of the eighth sphere (the matter of book VII).
What are the hypotheses or principles by which Copernican astronomy saves the appearances in the proper motions of the planets? These are chiefly: 1. That the Sun is placed in the centre of the sphere of the fixed stars (or nearly so), immovable in place. 2. That the several planets move in truth around the Sun, each in its own system, which is composed of several perfect circles turned with a most uniform motion. 3. That the Earth is one of the planets, so that it describes by an annual motion around the Sun an orb midway between the orbs of Mars and Venus. 4. That the proportion of this orb, compared with the diameter of the sphere of the fixed stars, is insensible and therefore like the immense. 5. That the sphere of the Moon is ordered about the Earth as its centre, so that the annual motion around the Sun (and thus from place to place) is common to the whole sphere of the Moon with the Earth.
Do you think these principles are to be retained in this Epitome? Since astronomy has two ends, to save the appearances and to contemplate the genuine form of the structure of the world, of which we treated in book I, folio 119 ff., for the first end all these principles are not needed, but some can be changed and some omitted, and the second must necessarily be emended; for the other end, although most of them are necessary, yet they are not yet sufficient.
Which of these principles can be changed or omitted with the appearances saved? Tycho Brahe demonstrates the appearances with the first and third changed: for he, with the ancients, places the Earth immovable in the centre of the world, but makes the Sun, which is itself the centre of the orbs of the five planets, go around the Earth with the whole system of all its spheres in an annual common motion, while meanwhile in this common system each planet accomplishes its own proper motions. The fourth he omits altogether, exhibiting a sphere of the fixed stars not much greater than the sphere of Saturn.
What do you substitute in place of the second principle, and what do you add concerning the genuine form of the dwelling of the world, or what pertains to the nature of the heavens? Although to each of the planets its own true motions must be left, yet these motions are not moved by themselves nor by the turning of spheres, which are nowhere solid; but the Sun in the centre of the world, turned about the centre and axis of its own body, becomes by this rotation of itself the cause of going round for each of the planets. Further, although the planets in truth become eccentric to the centre of the Sun, there are nevertheless no smaller circles, called epicycles, which by their rotation vary these intervals between the planet and the Sun, but the bodies of the planets themselves, by an inborn force, give occasion to this variation.
What, then, will be the matter of the fourth book? This book IV will contain celestial physics itself, or the form and the principles of the work of the world and the genuine causes of the motions. And this will be that primary office of the astronomer, of which in book I, page 120 ff., namely the demonstration of his hypotheses.
Enumerate the primary parts of book IV. The parts of book IV will chiefly be three: the first on the bodies themselves, the second on the motions of those bodies, the third on the real accidents of the motions.
For the first will teach the conformation of the whole universe, its distinction into parts or chief regions, the place of the Sun in its centre; the number, magnitude and order or situation of the planetary spheres; finally the proportion among themselves of all the bodies of the world. The second will deliver the revolution of the Sun about its own axis and its effect in carrying the planets round, the causes of the proportion among themselves of the motions, that is of the periodic times; the immobility of the centre of the Sun, the annual motion of the centre of the Earth around the Sun; the revolution of the Earth about its own axis and its effect in carrying the Moon round; the aids to the moving of the Moon from the light of the Sun, and what are the causes of the proportions among the day, the month and the year. The third will disclose the causes of the threefold inequality, of altitude, longitude and latitude, in the individual planets, and how those inequalities are doubled in the Moon by the force of illumination from the Sun.
Part One
I. Of the chief parts of the world.
What do you hold to be the disposition of the chief parts of the world? The philosophy of Copernicus counts the chief parts of the world by the distinct regions of the figure of the world. For since in the sphere, the image of God the Creator and the archetype of the world (as was proved in the first book), there are three regions, symbols of the three persons of the Holy Trinity, the centre of the Father, the surface of the Son, and the intermediate of the Holy Spirit: so likewise the chief parts of the world are made three, each in a several region of the sphere, the Sun in the centre, the sphere of the fixed stars in the surface, and finally the system of the planets in the region intermediate between the Sun and the fixed stars.
I thought that the chief parts of the world were to be numbered as Heaven and Earth. Indeed our sight, who are inhabitants of the Earth, cannot show other more notable parts, as was said in book one, folio 122 ff., since we tread the one beneath our feet and are covered by the other, both seeming to be joined and glued together by the common rim of the horizon, like a palace in which stars, clouds, birds, man and the various kinds of earthly living creatures are enclosed. But since we are engaged in a discipline which lays open the causes of things, dispels the deceptions of sight, and raises the mind higher and beyond the bounds of sight, it ought to be a wonder to no one that sight should learn from reason, the disciple from the master, something new which it did not know before: namely that the Earth, considered by itself alone, is not to be reckoned among the primary parts of the great world, but is to be counted with one of the primary ones, namely the planetary region or the movable world, and to hold in it a certain character of a beginning. And that the Sun, set apart from the number of the stars, is to be constituted one of the chief parts of the whole universe. I speak, however, now of the Earth as it is a part of the structure of the world, not of the dignity of the ruling creature which dwells on it.
By what properties do you distinguish these three members of the great world from one another? The perfection of the world consists in light, heat, motion and the harmony of motions, which are analogous to the faculties of the soul: light to the sensitive, heat to the vital and natural, motion to the animal, harmony to the rational. And in the light of the world consists its ornament, in heat life and growth, in motion a kind of action, in harmonies contemplation, in which Aristotle places happiness. Now since to every affection three things necessarily concur, the cause from which, the subject in which, and the form under which: the Sun therefore through all the said affections of the world sustains the part of the efficient, the region of the fixed stars the part of the one that forms, contains and bounds, the intermediate the part of the subject, according to the nature of each affection. In all these ways, therefore, the Sun becomes the chief body of the whole world.
For as to light, the Sun himself is most beautiful and as it were the eye of the world, and also, as the fountain of light or most brilliant torch, he illuminates, paints and adorns the other bodies of the world. The intermediate is not itself luminous but transparent and clear, and a stream through which light is distributed from its source, and in it are the globes and creatures into which the light of the Sun is poured and which enjoy it. The sphere of the fixed stars plays the part of a channel in which this river of light may run down, and is as it were a wall, opaque and illuminated, reflecting and doubling the light of the Sun: you might most rightly compare it to a lantern, which keeps off the winds. So in animals the brain, the seat of the sensitive faculty, communicates all the senses to the whole animal, and by the act of the common sense causes the presence of all those senses, as it were rousing them and bidding them be awake. And in another way: in this comparison the Sun stands as the common sense itself, the globes in the intermediate as the organs of sense, the fixed stars as the objects of sense.
As to heat, the Sun is the hearth of the world, at this hearth the globes in the intermediate warm themselves; the sphere of the fixed stars contains the heat, that it may not flow away, like a kind of wall, skin or garment of the world, to use the flowers of the Psalms of David. The Sun is fire, as the Pythagoreans said, or a stone or glowing mass as Democritus held; the sphere of the fixed stars is ice or a crystalline sphere, comparatively speaking. But if there is, not only of earthly creatures but of the whole ethereal air through the entire amplitude of the world, some vegetating faculty, of which we have a conjecture both from the manifest energy of the Sun in warming and from the physical contemplations on the origin of comets; it is credible that it is rooted in the Sun as in the heart of the world, and from there by the oarage of light runs out together with heat into this most ample space of the world, in the way in which in animals the seat of heat and of the vital faculty is in the heart, and of the vegetative in the liver, whence by the commerce of the spirits these faculties run out into the other members of the body also; the region of the fixed stars here, standing everywhere opposite, assists this vegetation, concentrating the heat, as they say, like a kind of skin of the world.
As to motion, the Sun is the first cause of the motion of the planets of the universe and the first mover, also by reason of his body. In the intermediate the movables range, that is the globes of the planets; the region of the fixed stars furnishes the movables with a place and a kind of base on which the movables may as it were rest, and in comparison with whose immobility in itself motion is understood to be made. So in animals the cerebellum is the seat of the motive faculty, the body and its members are that which is moved; the Earth is the base for the animal body, the body for the arm or head, the arm for the finger, upon which as immovable the motion of each part is made.
Finally, as to the harmony of the motions, the Sun holds that place from which alone the motions of the planets make the appearance of quantities harmonically tempered; the planets themselves, running about in the intermediate, furnish the subject or terms in which the harmonies consist; the sphere of the fixed stars or the zodiac circle furnishes the measures from which the quantity of the apparent motions is known. So also in man it is the intellect which abstracts universals and forms both numbers and proportions, as things which are not outside the intellect; but individuals, received within through the senses, are the foundation of universals, the individual and discrete units, the real terms of numbers and proportions; and memory, finally, distinguished by certain as it were little chambers of quantities and times, after the fashion of the sphere of the fixed stars, is the storehouse and repository of the sensations. And further, of the sensations judgment is made nowhere except in the brain, and nowhere does the affection of joy arise from sensual perception except in the heart.
To the nourishment of animals and plants, therefore, answers the said vegetation, to the vital faculty answers warming, to the animal motion, to the sensitive light, to the rational harmony. Wherefore with the best right the Sun is held to be the heart of the world and the seat of life and of reason, and the chief of the three primary members of the world; and the praises are true in a philosophical sense when the poets celebrate him as king of the stars, the Sidonians, Chaldeans and Persians (by the property of their language, which is also seen in the German) as queen of heaven, the Platonists as king of the intellectual fire.
These three members of the world seem not to answer aptly enough to the three regions of the spherical surface, since the centre is a point, but the Sun a body; so the outermost surface is understood to be continuous, but the region of the fixed stars does not shine all over, being sown here and there with bright points separated from one another; finally the intermediate in the sphere fills the whole space, but in the world what lies between the Sun and the fixed stars does not seem to be wholly constituted in motion. Indeed this very thing indicates the most apt correspondence of the three parts of the world. For since a point could not be clothed or expressed except through some body, and thus the body which is in the centre fell short of the indivisibility of the centre, it was fitting that the sphere of the fixed stars also should fall short of the continuity of the spherical surface and break open into the innumerable most minute points of the fixed stars, and finally that the intermediate itself should not be wholly occupied by motion and the other affections, nor be altogether transparent, but be a little denser, since it could not be altogether empty but had to be filled with some body.
Are the orbs in which the planets are carried solid, and are the intervals between them empty of orbs? Tycho Brahe refutes solid orbs by three reasons: one from the motion of comets, another from unrefracted light, the third from the proportion of the orbs. For if the orbs were solid, comets would not be seen to pass from one orb to another, for they would be hindered by the solidity; but they do pass from one to another, as Brahe demonstrated. From light, moreover, thus: since the orbs are eccentric and the Earth and its surface, in which the eyes are, is not situated in the very centre of each orb, therefore if the orbs were solid, denser, that is, than that most limpid ethereal air, then the rays of the stars would reach our air refracted, as optics teaches, and so the planet would appear irregularly and as it were in places far other than those which could be predicted by the astronomer. The third reason is Brahe’s own, adapted to his principles: these, as also the Copernican, bear witness that Mars becomes at times nearer to the Earth than the Sun is; but Brahe could not believe this change possible if the orbs are solid, since the orb of Mars would have to intersect the orb of the Sun.
What, then, is there in those regions of the planets besides the planets? Nothing but ethereal air, common to the orbs and the intervals alike, most limpid and yielding to the movables no less readily than it yields to the lights of the Sun and stars, so that they can glide down to us.
If it is air, it will be a body with matter and density: its matter will therefore resist the movables somewhat. On the contrary, that ethereal air is thinner than this air of ours, even when it is purest, by an almost immense interval.
How do you prove this? From optics, by refractions: for our air contiguous to the ether causes a refraction of about 30 minutes. Water, contiguous to air, one of about 48 degrees: whence in a manner the proportion of the density of water to air, and of air to ether, is established, by cubic multiplication. For 30′ are contained in 48° almost a hundred times, which in squares is ten thousand, in cubes ten hundred thousand, that is a million. So many times, therefore, is air thinner than water, and ether than air.
The matter of the ether is nevertheless not wholly nothing: the stars will therefore still be hindered by it. So slight a hindrance of motion, so slight a resistance of the ether, we can grant to the movables without inconvenience, since to them, on account of the proper matter of the bodies, some resistance must be granted even before, as will appear below. And what if no resistance of the ether is to be conceded? since it is more credible that the ethereal air which stands nearest around the movable globe accompanies the globe on account of its extreme limpidity.
II. Of the place of the Sun in the centre of the world.
By what arguments do you establish that the Sun is situated in the very centre of the world? Some of these are furnished to us by the most ancient Pythagoreans and the Italian philosophers in Aristotle, book II of On the Heaven, chapter 13, and are drawn from the dignity both of the Sun and of the place, and from the office of the Sun in the world of giving life and light.
Give first the argument from dignity. The reasoning of the Pythagoreans runs thus, word for word from Aristotle: to the most worthy and precious body a more worthy place also is due. Now the Sun (for which they themselves used the word fire, concealing their doctrines according to the custom of the sect) is more worthy than the Earth, and the most precious and worthy of the whole world, as was shown a little before. But the surface and the centre or middle are the two bounds of a sphere. One of these, therefore, belongs to the Sun: not however the surface, for that body which is the chief of the whole world ought above all to be guarded, and the centre is fitted for this, which is why they were accustomed to call it the guardhouse of Jupiter; so it is not fitting that the Earth should be in the middle, for that place belongs to the Sun, and the Earth is carried by an annual motion around the middle.
What did Aristotle reply to this argument? 1. He says that they assume what is not conceded, namely that the middle of magnitude (that is, of the sphere), the middle of the thing (that is, of the body of the world), and the middle of the nature itself (that is, of informing or vivifying) are the same. But as in animals the middle of vivification and the middle of the body are not the same (for the heart is within indeed, but is not equally distant from the extremities), so also we must think of the heavens, and neither fear for the safety of the whole universe, nor place a guard at the centre; rather we must ask what kind of body that heart of the world, or middle of vivification, is, and in what place of the world it is situated. 2. He wishes to show the dissimilarity of the middle of nature from the middle of place. For that middle of nature, or that most worthy and precious body, has the character of a principle; but the middle of place is rather the last, in quantity metaphysically considered, than its first or principle. For what is the middle of a quantity, that is the innermost, is bounded or circumscribed, whereas the bounds are that which bounds or circumscribes. Now what encompasses, bounds and encloses from without is more excellent and worthy than what is bounded within: for matter is among those things which are bounded, limited and contained, but the form or essence of each creature is of the number of those which bound, circumscribe and comprehend. So he thinks he has proved that to the Sun (or, as he understood, to the fire of the Pythagoreans) belongs not so much the middle place of the world as the extremity.
How do you dissolve this refutation of Aristotle? 1. Although it is true that not in all creatures, and least of all in animals, is that in the middle of the whole mass which is the chief of the whole creature, yet when we dispute of the world, nothing is more probable than this very thing. For the figure of the world is round, that of an animal is not. Because animals need instruments extended outside themselves, with which they may stand upon the earth, be moved over it, and receive within themselves food, drink, and the forms of things and sounds taken in from without; the world on the contrary is alone, having nothing outside, resting on itself and, as to the whole, immovable, itself alone all things: there is therefore no cause why the heart of the world should be elsewhere than in the middle, that it, namely the heart, may be equally related to all the outermost parts of the world, being at an equal interval on every side. 2. Further, that he bade us ask what body that chief body of the whole universe is, he was indeed troubled by the riddle of the Pythagoreans, supposing that they were putting forward the element of fire as the chief; yet he did not bid amiss. We too, therefore, following the counsel of Aristotle, choose the Sun, neither the Pythagoreans in their mystical sense nor Aristotle himself opposing. While we inquire in what place of the world this Sun is situated, Copernicus, skilled in the heavens, points out to us the middle; the rest, who show another place for him, are not driven to this by astronomical arguments but by certain others, metaphysical in appearance, drawn from the contemplation of the Earth and its place; the estimation of which arguments is common to us and to them, and by which they do not indicate but themselves also seek the place of the Sun. When, therefore, seeking the place of the Sun in the world we find the centre of the world, we do so on equal terms with Aristotle himself, nor is this refutation of his against us. 3. But that Aristotle ascribes baseness to the centre, directly contradicting the Pythagoreans, he does beyond the nature of figures and beyond the consideration of them, whether geometrical or metaphysical. For above in book one the centre in the sphere was by no means the last but altogether the principle of its most uniform mental genesis, bearing in the adumbration of the Holy Trinity a likeness of God the Father, who is the first of the persons. 4. Finally, to one who estimates physically what are geometrical, it might seem not to be done fittingly, unless what he here disputes about matter and form were taken over by a certain analogy from the very consideration of geometrical figures. For truly in solid quantities the internal corpulence, extending equally in every direction and not itself partaking of any figure, is the genuine image of matter in physical things; but the external figure of the corpulence, composed of certain surfaces bounding the solidity, represents form in physical things. Let this comparison therefore be granted him, but from it it appears that he himself plays on the equivocation of the middle: for when the Pythagoreans spoke of the innermost point of the sphere, he understands by the word middle all the space comprehended within the surface itself. As to space, then, victory must be conceded to him, but a useless one, for as to the centre of the whole space the Pythagoreans and Copernicus prevail. For although the middle, as it is space, does not merit the name of a bound, it does merit it as it is the centre, in which respect it is above all to be counted among the things that form and bound, since above in book one the centre was the origin of the genesis of the sphere, metaphysically weighed.
Prove from the office of the Sun that the centre is due to him. This has partly been done already in the dissolution of the Aristotelian refutation. For 1) if the whole world, which is spherical, needs the light and heat of the Sun equally, it is best therefore that the Sun be in the middle, whence these may be distributed equally to all quarters of the world: which is done more equably and rightly by resting in the centre than by going round it. For if he approached some parts for the sake of warming, he would recede from the opposite and cause vicissitudes, though himself most simple. And it is strange that some, for mockery, use the simile of a light in the middle of a lantern, when the simile is most apt and least suited to deride this opinion, but rather most fit to depict the force of this argument. 2. Of light, moreover, a peculiar demonstration is woven, supposing fitness, not necessity. Imagine the sphere of the fixed stars to be a concave mirror: it is known that an eye placed at the centre of such a mirror beholds itself on every side, and if there is a light at the centre, it is reflected from the concave surface everywhere at right angles, and the reflected rays meet again at the centre: but this can happen at no other point of the concavity than the centre. Therefore, since the Sun is the fountain of light, the eye of the world, the centre will be due to him, that he (himself in the divine symbolism the Father) may contemplate himself in the whole concave surface (which bears the symbol of the Son of God) and be pleased with his image in it, and by illuminating himself may make himself bright, and by warming kindle himself. To this belong the lyric verses:
You who gaze upon your own face, everywhere reflected, navel of the ether, Sun, spring of lightnings flowing through the glassy void, which, reflected, drinks back again.
Copernicus, however, did not wholly bring the Sun into the very centre of the world. It was Copernicus’s intention to show that the node common to all the planetary systems, of which below, is distant from the centre of the Sun by as much as the ancients make the eccentricity of the Sun: which node he himself established as the centre of the world, driven by no astronomical demonstration, but only for the sake of fitness, that that node, and as it were the common centre of the movable orbs, might not differ from the very centre of the world. But if anyone else, using the same fitness, had wished to contend that this is rather to be avoided, that we should make the Sun himself differ from the centre of the world, and that it suffices for that node of the region of the movables to stand nearby, though not exactly in the centre itself: he who had wished to contend this, I say, would have disturbed nothing in Copernican astronomy. So, first, even by this opinion of Copernicus, namely the distance of that node from the Sun, the ultimate arguments for the place of the Sun in the very centre nevertheless remain; and secondly, this opinion of Copernicus, that the node is distant from the centre of the Sun, is not even to be acquiesced in. For that common node of the region of the movables is in the Sun himself, as will be proved below: so by whatever probabilities either the one or the other is referred to the very centre of the sphere of the fixed stars, by the same the remaining one is brought to the same, even with Copernicus himself approving.
III. Of the order of the movable spheres.
How are the planets distinguished from one another? Into primary and secondary; the primary are those whose bodies are carried around the Sun, as will be taught below; the secondary are those whose proper circles are ordered not around the Sun but around one of the primary planets, and to which, besides the proper motion around the body of the primary, there is also the common motion of their primary around the Sun; such Saturn is believed to have, and to carry round with him, two, which sometimes come into view by the aid of the telescope. Such Jupiter has four about him, the Earth one, called the Moon. Of Mars, Venus and Mercury, themselves primary, it is not yet established whether they too have such companions or satellites.
How many planets, then, are to be considered in the theoretical doctrine? Not more than seven; six indeed called primary: 1) Saturn, 2) Jupiter, 3) Mars, 4) the Earth (the Sun to sight), 5) Venus, 6) Mercury, and 7) the one of the secondaries, the Moon, because it alone revolves around the Earth, our dwelling; the other secondaries do not concern us, who are inhabitants of the Earth, nor do we see them without the choicest telescopes.
In what order are the planets disposed among themselves, are they in the same heaven or in different ones? Sight indeed places all of them in that supreme and highest sphere of the fixed stars and supposes that they run about among the fixed stars themselves. But the reason of all times and of men of all schools has persuaded otherwise. For if the centres of all were in the same orb, since we see them more often joined together to the sight, it would happen that one would hinder another, and their motions could not be regular and perennial. The reasoning of Copernicus, however, and of the most ancient Aristarchus, supported by observations, convinces us that the regions of the several planets are distinct from one another and from the fixed stars by immense intervals.
What difference is there here between the reasoning of the ancients and that of Copernicus? 1. The reasoning of the ancients is at least probable, the demonstration of Copernicus, proceeding from its own principles, infers necessarily. 2. They teach only this, that there are not more than one planet in any one sphere; Copernicus adds besides how much each must be raised above the other. 3. The ancients therefore build the heavens one upon another, like bricks in a wall, or, what is more like, like the coats of an onion, the inner sustaining the outer, thinking that all spaces must be filled with orbs and that the upper sphere must be made as great as the lower sphere of known quantity allows; which is a material conformation at least; Copernicus, measuring out to each its spaces from the observations themselves, shows that so great spaces lie between pairs that it is incredible that they should be filled with orbs; so this disposition of his urges the contemplative mind, spurning matter and the contiguity of orbs, to aspire to the search for the formal disposition or archetype to which the intervals were made. 4. The ancients, by their material structure, are compelled to make the planetary or movable world greater by many parts than Copernicus by his formal disposition; Copernicus on the contrary makes the region of the movables moderately wide, but that of the fixed stars, at rest, immense, whereas the ancients make it not much greater than the sphere of Saturn. 5. The ancients do not explain and prove the reason of their disposition, as they wish: Copernicus stands excellently in his reasons.
What do you call the reasons of the disposition of the orbs, and how does Copernicus excel in them? Aristotle teaches, book II of On the Heaven, chapter 10, that nothing is more consonant with reason than that the times of revolution of each planet should correspond to the same altitude or amplitude of the orb. Now for the ancients the highest is also the slowest, namely Saturn, because it has 30 years; whom Jupiter follows in place and in time, having 12 years, and Mars after him, having less than 2 years. But in the rest the reasoning is disturbed for the ancients. For unless you concede to the Earth an annual motion around the Sun, it will come about that the Sun, Venus and Mercury, three distinct planets, have the same annual time of their circuit, to which, nevertheless, they assign different orbs, the upper to the Sun, the middle to Venus, the third to Mercury; and finally they assign the lowest place to the Moon, as she indeed also has the shortest time, the monthly.
Copernicus, however, establishing that the Earth is carried around the Sun, has the same analogy through all the primary planets both of motions and of time. For him the Sun is in the centre of the world and so innermost, lacking a circuit of the centre, that is immovable as to centre and axis: but that the body of the Sun is whirled about an immovable axis was discovered a few years ago, in less than the space of one month. Nearest around him is Mercury in a most narrow orb, which it completes in three months; about this orb is Venus in a wider orb and a longer space of time, namely eight and a half months. Around the heaven of Venus is the Earth with her attendant Moon (for the Moon is a secondary planet, of which no account is taken among the primary) and goes round in the space of twelve months. Afterwards follow Mars, Jupiter, Saturn, as with the ancients, each with his satellites. After Saturn is the sphere of the fixed stars, as it stands at an immense distance, so also wholly at rest.
By what measure does Copernicus mete out the intervals for the several planets? We must here use a proportionate measure, to which the other spheres can be compared and which is nearest connected with us and so in a manner known to us: that is the amplitude of the orb in which the centre of the Earth and of the little orb of the Moon revolve around the Sun, or its semidiameter, or the distance of the Earth from the Sun. This is as it were a ten-foot rod suited to the business; for the Earth is our dwelling, from which we measure the distances of the heavens, and is the middle of the planets and among them, for many reasons, of which below, holds the character of a beginning. The Sun, however, by the indication and judgment of our sight is the chief of the planets, and by the vote of reason weighed above is the very heart of the region of the movables proposed for measuring. So this our rod has two most notable termini, the Earth and the Sun.