Epitome Astronomiae Copernicanae: Liber I, part 1
On the principles of astronomy in general and of spherical doctrine in particular.
What is astronomy? It is the science that sets forth the causes of the things which appear to us, dwelling on the Earth, in the heavens and the stars, and which produce the changing of the seasons; once these are understood, we may be able to foretell the face of the heavens, that is, the celestial appearances, for the future, and to assign the fixed times of those that are past.
Whence is astronomy so called? From the law or governance of the stars, that is, of the motions by which the stars are moved, as economy is named from the management of a household, and a pedagogue from the guiding of boys.
What relationship has this science with the others? First, it is a part of physics, because it inquires into the causes of natural things and events, because the motions of the heavenly bodies are among its subjects, and because one of its aims is to search out the structure of the edifice of the world and of its parts.
Second, astronomy is the soul of geography and hydrography, or navigation. For the different things that happen from the heavens in the different places and regions of the lands and the ocean are judged by astronomy alone.
Third, it has chronology subordinate to it, because the celestial motions order the times and the civil years and mark out histories.
Fourth, it has meteorology subordinate to it. For the stars move and stir the nature below the moon, and in a certain manner men themselves.
Fifth, it embraces a great part of optics, because it has a subject in common with it, the light of the heavenly bodies, and because it detects many deceptions of sight concerning the forms of the world and of its motions.
Sixth, it nevertheless belongs to the class of the mathematical disciplines, and uses geometry and arithmetic as its two wings, considering the quantities and figures of the bodies and motions of the world, numbering the times, and by these despatching its demonstrations and bringing the whole speculation to use, that is, to practice.
How manifold, then, is the care and office of the astronomer? The parts of the astronomical office are chiefly five: the historical, concerning observations; the optical, concerning hypotheses; the physical, concerning the causes of the hypotheses; the arithmetical, concerning tables and calculation; the mechanical, concerning instruments.
How do they differ from one another? Although none of them can do without the geometrical demonstrations, which serve theory, and the numbers, which serve practice, since these are, as it were, the common speech of geometers, yet the first three belong more to theory, the last two more to practice.
Describe to me the first of them, the historical. At the outset the historical part records what the face of the world appears to us to be, and what changes in it daily, what yearly or over longer revolutions of time, what things appear different in different places of land and sea, and what the same. The rarer or more notable events, such as eclipses of the Sun and Moon and remarkable conjunctions, it draws from historical monuments; the finer observations of the individual stars it gathers into one, taken from the books of trustworthy craftsmen, from Hipparchus, Ptolemy, Albategnius, Arzachel and others whom these cite, adding also what the present age has observed; in which work Tycho Brahe, beyond all others, laboured with incredible diligence, leaving behind his own observations of thirty-eight years, most copious, almost continuous, and most worthy of trust.
Observations of this kind, therefore, ought to be skilfully compared with one another and arranged in definite classes according to definite revolutions of time, so that like may be fitted to like, much in the way that Aristotle, when about to explain the nature of animals, first composed a most skilful history of animals, reviewing in summary, for all the species ranged under the same genus, what they had in common.
Describe also the second part of the astronomical office. The second part, the optical, having considered these varieties of observations and the agreement of the diverse ones in certain respects, strives to penetrate to the causes by which it comes about that to the eyes of men appearances far different from the truth arise, which appearances the astronomers call apparentiae, in Greek phainomena. Here, according as each man is strong in ability, so he saves a greater number of the diversities of appearances and accounts for them by some one form of motions, or figure of bodies, always like itself, adapting his whole method of demonstration to the laws and theorems, both geometrical and optical, the latter being subordinate to geometry; and it happens that one man comes nearer than another to the very nature of things by devising such forms of motions. And so, since in this difficult and blind pursuit of causes it befalls those who consult nature to stray from the truth in some members of their opinions, and yet nonetheless to save the celestial appearances by them, the custom has prevailed of calling the opinion of each of the more famous craftsmen, by which he explains the causes of the celestial appearances, hypotheses; because the astronomer is wont to say that, this or that being posited or supposed, which he himself affirms of the world, it follows by the necessity of geometrical demonstrations that those many things appeared which are contained in the historical record spoken of, and which appeared in his own time.
Thus today three forms of hypotheses are put forward, those of Ptolemy, of Copernicus and of Tycho Brahe.
To these two former parts there belongs in common also the contemplation of the nature and properties of light, that is, the practice of the doctrine of refractions.
Of the causes of hypotheses.
What is the third part of the astronomical office? The third part, the physical, is commonly not thought necessary to the astronomer, although it most of all belongs to the end of this part of philosophy, and can be completed by no one but the astronomer. For astronomers ought not to have mere licence to feign anything whatever without reason; rather it is needful that you also be able to render probable causes for your hypotheses, which you offer as the true causes of the appearances, and thus that you first establish the principles of your astronomy in a higher science, namely physics or metaphysics, yet not shut out from those geometrical, physical or metaphysical arguments which are supplied to you by the very course of your own discipline concerning matters pertaining to those higher disciplines, provided you admit no begging of the question. For in this way it comes about that the astronomer, having attained so far his purpose, having devised causes of motions agreeable to reason and apt to produce all that the history of observations holds, now brings into one view what he had earlier established piece by piece, and, setting aside the end hitherto proposed (which was the demonstration of the phenomena and the benefit to common life that flows from it), aims himself at a higher end, to the great rejoicing of those who philosophize, and refers all his doctrines to that end, again by geometrical and by physical arguments, namely to set before the eyes the genuine form and disposition, or adornment, of the whole world. And so this is the very book of nature, in which God the Founder has in part declared and portrayed His essence and His Will toward man, in a certain wordless kind of writing.
What does the astronomer treat in the fourth place? The fourth and fifth parts are referred to that lower end, namely to the benefit in common life. For the fourth, which I have called arithmetical, applies the discovered causes of the motions to numbers, teaching the method by which for any time, past or future, the appearing face of the heavens and the configuration of the stars may be computed. Hence arise the astronomical Tables, by which the method just mentioned is made easier and shorter; wherefore the Greeks call them handy canons. Such are the Tables of Ptolemy, which the Alphonsine Tables corrected some four hundred years ago, and the Tables of Copernicus some eighty years ago, which Reinhold, having made them more exact and fuller, called the Prutenic. But to put the crown upon all the tables of the predecessors, since they are erroneous, the Rudolphine Tables promise, so named by Tycho Brahe and begun by him, which now await the light.
This part, therefore, furnishes to chronologers, astrologers, weather-watchers, physicians, sailors and farmers the principles necessary to each one’s art.
Of instruments, the material sphere and theories.
Set forth also the fifth part of the astronomical office. The fifth part, the mechanical, does for instruments nearly what the fourth does for numbers, and it is manifold. For first it serves the historical part, since, in order that observations, which are the foundations of astronomy, may be of exquisite fineness and sufficient exactness, the eyes must be helped by instruments, with which they may both take their aim more surely and count more minutely and without illusion; in this kind there exists the book of Tycho Brahe called the Mechanics of Restored Astronomy, displaying a great number of most exquisite instruments, whose equipment wins the highest credit for the observations made by them and recorded by Brahe.
As for the parts that follow, either models must be made of wood, metal, paper and the like, by which we may express the astronomical hypotheses and set them before the eyes, for teaching the unlearned and beginners, or to lighten the labour of computation, and in this class are the material sphere and the theories; or we prepare amusements for the great or the wealthy, to which end serve the celestial automata, imitating the heaven itself in some measure by an artificial motion put within them, in which, besides mere delight, there is sometimes also a use, especially when the sky is clouded; or again we build observing instruments, akin to those first mentioned, but for the sake of common and popular use. Hence has arisen an endless variety of instruments, the skill of mechanics’ hands vying with the most ingenious demonstrations of geometers. Yet they agree chiefly in this: since the celestial motions and appearances are spherical and curvilinear, while for us dwellers on the earth the face of the horizon lying beneath is a plane, and walls are erected perpendicular to it, and finally the bodies we handle are bounded either by pure planes or at least by mixtures of the straight and the curved, those instruments are easier for us to handle in which the curved is changed into the straight; such are, among the principal ones, the rules engraved with the divisions of the circle, called geometrical squares, the astrolabes, and, on account of the plane walls of buildings, the sundials, an instrument of use that lies open very widely by land and sea, which human life can scarcely any longer do without.
This use serves chiefly geography and navigation, because geographical charts have the celestial circles projected upon a plane.
To which part chiefly does this little book refer? There is none of these five parts which this Epitome does not touch upon. For it runs through, in summary fashion, the variety of the celestial appearances, and, that their causes may be rendered, it explains the hypotheses, chiefly that of Copernicus but also that of Tycho Brahe, by figures and suitable instruments, the sphere and the theories; it interposes physical and metaphysical disputations on their truth, sets before the eyes the inner idea of the whole universe, describes and defines the terms of the art, the circles, arcs, lines and angles which are to be used in astronomy; it propounds the doctrine of the eclipses of the Sun and Moon and of the configurations of the planets with one another and with the luminaries (in which consists the efficacy of the planets upon these lower things); it teaches the form of the calculation by which both the Rudolphine Tables and the very positions of the planets without tables, and finally the laws of constructing instruments, have been computed or can be computed.
Since, then, we must begin from the appearances, explain what sort of thing the world seems to be. The world, with the eyes leading the way, is commonly held to be a vast edifice consisting chiefly of two parts, Heaven and Earth.
What do men think of the figure of the Earth? The Earth seems to stretch out in a circle in the widest plane in every direction around the spectator, whence Homer calls it the broad-wayed earth. And from this plane, spread out in a very wide circle, has flowed the name Orb of the Lands, used as much in the Sacred Writings as among the other nations.
What do they suppose to be the middle of the orb of the Earth? Every nation, if it has not been instructed in the knowledge of the globe, supposes by the instinct of nature and of sight that its own country is at the centre or middle of this plane circle. The common people of the Jews believe even today that Jerusalem, the ancient country of their race, is situated in the middle of the world.
But what seems to men concerning the waters? Because, however far men can advance over the Earth, the ocean at last meets them, some think that the Earth floats upon the waters like a disc, and that the waters are contained by the lower part of the heaven as by a vessel, whence the poets make the ocean the father of things. Others, on the contrary, conceive beyond the ocean that surrounds the lands another rim of the earth, which holds the ocean that it may not flow away, and for the same cause they put the earth beneath it, saying that the waters are sustained by the lands. Besides these there are a third sort, to whom, since the ocean seems higher than the lands if one looks at it from the shores, the earth is therefore thought to be founded, sunk as it were among the waters, and preserved supernaturally by the omnipotence of God, lest the waters hanging over from on high should return and cover it.
But what do the common people suppose is laid beneath both, the Earth, namely, and the Waters? When men think about the foundation on which so great a mass of lands rests, so that it stands firm and immovable through so many ages now and does not slip downward, much darkness is poured about them; and there were among the early philosophers Heraclitus, and among ecclesiastical writers Lactantius, who said that it is extended downward by infinite roots.
What, finally, of the other part of the world, namely the heaven and its magnitude? Men think that the heaven is not much greater than the orb of the earth, being joined at its outermost circle to the lands and the ocean, so that it bounds the lands, and for those who advanced so far, if it could be done, the extremities of heaven would be met with, hindering further progress. To this imagination of men Scripture also accommodates itself: If you shall be scattered to the ends of heaven, from thence will I gather you again. His going out is from the end of heaven, and his circuit even to the end thereof.
Thus the poets said that Mount Atlas, the highest on the farthest shore of Africa, bears the heaven on its shoulders, and Homer places the Ethiopians at the extremities of the rising and the setting, supposing that the Sun, because of this contiguity of heaven and Earth, comes so near to them as to burn their skin.
What figure do they think the heaven has? The eyes ascribe to the heaven the figure of a tent, spread above our heads and above the Sun, Moon and stars, or rather the figure of a vault resting on the plane of the earth, with an arch gently steep, so that the part of the heaven above the head is much nearer to the spectator than the parts adjoining the mountains.
What do men conceive about motion? Whether the heaven moves or rests is not discerned by sight, since the thinness of its substance escapes the eyes, except that things which present no variety to the eyes seem to be at rest. The Sun, Moon and stars are seen to fall upon the eyes in one position after another at the extremities of the lands. For the Sun seems with a kind of effort to emerge between the heaven and the motionless mountains or ocean and, as it were, to come out of a chamber, and, having traversed the vault of heaven in the same way, to hide itself again in the opposite quarter; so likewise the Moon, the planets and the whole remaining host of stars, as if they advanced in a drawn-up and well-guarded line, this one going before, that one departing later, each in its own order.
Therefore, since the ocean meets one beyond the last lands, the common sort of men think that the Sun is plunged into the ocean and quenched, and that daily a new one comes out of the ocean in the opposite quarter. This, then, the poets imitate in their fictions. Indeed even certain philosophers have reported that on the farthest shores of Lusitania one hears the hissing of the ocean quenching the flames of the Sun, as Strabo relates.
What does the common people determine about the magnitude of the stars? Their magnitude is not always thought the same. For the Sun, when it rises or sets, appears huge, and so at its going forth and the beginning of its course it is compared to a giant (Psalm 19). So the Moon, when she rises with full face, seems to equal the round or bottom of a great vessel. When the Sun and Moon are nearer the zenith, they show about the breadth of a human face, equal to each other and of the figure of a plane circular disc. So too the size of the constellations or asterisms seems immense near the mountains, so that they are scarcely recognized after they have risen high, on account of their contemptible narrowness.
Of Orion Virgil says: how great is Orion, when he walks on foot through the mightiest pools of mid Nereus, cleaving his way, and overtops the waves with his shoulder.
But the little bodies of the several stars are taken as points. They differ nevertheless among themselves in brightness and in the breadth of diffused light, and each one differs from itself with changes of the air; some even seem to shoot across very swiftly and fall to the earth.
But those who observe these matters more carefully than the common run of men, what else do they detect in the stars with the eyes that is worth noting? First, it is easy to distinguish between spurious stars and true ones. For the former are momentary, and are not seen in the place from which they rush before they begin to shoot across, nor after the shooting has ceased are they seen any longer in the place to which they came, whereas, on the contrary, the true stars are everlasting. That motion of shooting is also most swift, and therefore unlike the other motions, which do not so strike the eyes. From this swiftness of motion we rightly gather that they are kindled and quenched not among the true stars in the aether, but in the innermost region of the world; and so we give them the name of shooting stars or falling stars, and, excluding them from astronomy, as having no law of motion, we send them over to physics.
Next, among those stars also which are durable to the senses, this difference has been detected: that the greatest part of them keep always the same order among themselves, and, this being so preserved, rise and set daily in the same places within the space of a century; but a few others, that is, five, and the Moon as sixth and the Sun himself as seventh, although they too rise and set daily with the former, yet when many days are compared are found not to keep the same position either among themselves or in relation to those former stars, nor do they always rise and set in the same places of the horizon. Therefore the former are called fixed, the latter erratic, or planets.
In each kind some spurious bodies mingle again: among the planets, the hairy stars, commonly comets; among the fixed, stars so like the rest that they are scarcely distinguished by the most skilled astronomers. They are distinguished, however, by this, that in the end they too disappear and, not being seen to set and return as the others do, they have no rule of their appearing, no order, and are seen very rarely, after intervals of many years and even centuries. Since these show some regularity of motion and are not distinguished by their regions from the true stars, and yet perish and arise, like the shooting stars, they belong by equal right to physics and to astronomy.
Thirdly, to this difference of the stars in motion there also follows a difference in the clearness of light. For the Sun tempers the day, the Moon the night, with their light, and cause shadows. The other wandering stars also have for the most part a clear, shining and full light, and an evident one, which appears even in bright dawn, when the fixed stars are then hidden. But comets by their beard, hair or tail, and some new fixed stars by their unusual brightness at the beginning of their light, are distinguished from the perennial ones.
Fourthly, the planets also surpass the fixed stars in magnitude for the most part.
Fifthly, the planets do not twinkle as the fixed stars do, but either not at all, or little, or in another and different manner, which can easily be discerned.
Sixthly, the planets do not always keep the same visible magnitude, as the fixed stars do. The diameters of the Sun and Moon are found by delicate instruments to grow and diminish; the Moon and Venus also change their aspect; the other five, when opposite the Sun, spread about them the largest light and appear greatest, at other times smaller.
I would have you explain more clearly this difference between the motion of the fixed stars and that of the planets, as it strikes the eyes. Whereas any fixed star, within the lifetime of one man, daily crosses the face of the Earth along the same track, the planets on the contrary pass at times in a low and short arc, soon in a long one near the zenith, notably changing the points of their risings and settings.
This alternation is first very conspicuous in the Sun, whom Venus and Mercury perpetually accompany. For in summer, for those who turn their faces to the south, the Sun rises on the left and behind, passes very high, and from on high strikes the lands and rouses the heat and lengthens the day, and sets again on the right behind the back; in winter all is contrary, and both his rising and his setting can be seen with the face unmoved. This alternation the Moon undergoes in the space of a month, Mars in two years, Jupiter in twelve years, Saturn in thirty.
How many kinds of motion, then, are to be considered by the astronomer? Two. The first and the second.
What do they call the first motion? The daily motion, in Greek the diurnal, which makes the fixed stars, planets, Moon and Sun, and also whatever else is in the heavens, rise from one quarter of the horizon and set in the opposite; which motion for us in this part of the world, turning our faces to the east, goes from the rising toward the right. By this solitary motion, if the second motion did not intervene, any star could rise on each day at one and the same point of the horizon and climb always to the same height above the visible plane of the horizon, and again sink at one and the same point of the horizon.
What do they call the second motions? Those proper and slower motions of the individual planets, which bring it about that the planets do not rise and set each day at the same point of the horizon as the fixed stars do, nor, on reaching the middle of the heavens, always attain the same height, nor keep the same position either among themselves or in relation to the fixed stars.
And these motions, for us in this part of the world, when we turn our faces to the west, go from the setting toward the left. They go, I say, although they are not so obvious to the eyes as the first, which alone insinuates itself into the untrained experience of men and disturbs the contemplation of the second, carrying the second movables away with itself. But the second are understood to occur thus, if you abstract the first in thought and compare with one another the evening observations of several days.
How many parts of astronomy are there, then? Two. The spherical doctrine and the theoretical doctrine.
Whence are they named? The spherical is named from the instrument, namely the armillary sphere, which it uses to explain the first motion. The theoretical, from the theories: so are called the plane instruments which we use to explain the second motions.
In how many books does the whole Epitome consist? Seven. Three on the spherical doctrine, three others on the theoretical doctrine, and one on both together, or on the motion of the eighth sphere.
Review the arguments of the books on the spherical doctrine. This first, after the general matters thus far explained, next has the principles by which the spherical doctrine was delivered by Copernicus, and the proofs of them and the solutions of the contrary arguments. The second explains the origin, causes and divisions of the circles and the other parts of the sphere, by which instrument the spherical doctrine ought to be taught. The third delivers the spherical doctrine itself in its parts.
Tell the arguments of the following books that belong to the theoretical doctrine. For the theoretical doctrine, after the method of the spherical doctrine, three books likewise have been made: the fourth, fifth and sixth.
For the fourth has the principles by which Copernicus delivered the theoretical doctrine, reviewing and proving the disposition of the whole world and its inner parts and the motions of the parts, and dissolving objections. The fifth sets forth the origin and causes of the eccentric circles in the theories, because we need these figures for despatching the theoretical doctrine, where the form of the true motion of the planets is explained in general. The sixth despatches the theoretical doctrine itself on the apparent motions of the planets, first of each separately, then also as compared with one another.
What are the parts of this first book that remains, on the principles of the spherical doctrine? Five. The first is on the figure and magnitude of the earth and of the ocean, and the method of measuring it. The second, on the outermost sphere of the fixed stars and the figures of it and of the stars. The third, on the nature and height of the air, poured around the lands and ocean, and of the breeze diffused through the whole aether, and on the difference between the two. The fourth, on the place of the Earth within the inner embrace of the sphere of the fixed stars. The fifth, on the turning, whirling or revolving of the globe of the Earth, uniform about an immovable axis.