On the Principles of Astronomy

Johannes Kepler

After an Old Engraving

What is astronomy? It is the science of treating of the causes of those celestial appearances which we who live on the earth observe and which mark the changes of times and seasons; by the studying of which we are able to predict for the future the face of the heavens, that is, the stellar phenomena, and to assign fixed dates for those which have occurred in the past.

Why is it called astronomy? From the law (nomos) or governance of the stars (astra), that is, of the motions in which the stars move, just as economy is named from the law of domestic affairs (oiconomia) and paedonomy (paidonomia) from the ruling of youths.

What is the relation of this science to the other sciences? 1) It is a branch of physics because it investigates the causes of natural objects and events, and because among its subjects are the motions of the heavenly bodies, and because it has the same end as physics, to inquire into the conformation of the world and its parts.

2) Astronomy is the soul of geography and hydrography, for the various appearances of the sky in various districts and regions of the earth and sea are known only by astronomy.

3) Chronology is dependent upon it, because the movements of the heavenly bodies prescribe seasons and years and date the histories.

4) Meteorology is also its subordinate, for the stars move and influence this sublunary nature and even men themselves.

5) It includes a large part of optics, because it has a subject in common with that; that is, the light of the heavenly bodies, and because it corrects many errors of sight in regard to the character of the earth and its motions.

6) It is, however, subordinate to the general subject of mathematics and uses arithmetic and geometry as its two wings, studying the extent and form of the bodies and motions of the universe and computing the periods, by these means expediting its demonstrations and reducing them to use and practical value.

How many, then, are the branches of astronomical study? The departments of the study of astronomy are five; historical, in the matter [p.312] of observations, optical as to the hypothesis, physical as to the causes of the hypotheses, arithmetical as to the tables and calculations, mechanical as to its instruments.

Since we must begin with appearances, explain how the world seems to be made up? The world is commonly thought, accepting the testimony of the eyes, to be an immense structure consisting of two parts, the earth and the sky.

What do men imagine concerning the figure of the earth? The earth seems to be a broad plane extending in a circle in every direction around the spectator. And from this appearance of a plane bounded by a great circle the appellation, orbis terrarum, the circle of the earth, has arisen, and has been taken over by the Scripture and among other nations.

What do men imagine to be the center of the earth? Each nation, unless it has become familiar with the notion of the circle, thinks by the instinct of nature and the error of vision that its country is in the center or middle of this plane circle. So the common people among the Jews believe still that Jerusalem, the earliest home of their race, is situated at the center of the world.

What do men think about the waters? Since men proceeding as far as possible in any direction finally came upon the ocean, some have thought that the earth is like a disc swimming in the waters, and that the waters are held up by the lower part of the sky, whence poets have called the ocean, the father of all things. Others believe that a strip of land surrounds the ocean which keeps the water from flowing away, and these suppose there is land under the water, saying that the water is held up by the earth. Besides these there are still others who, since the ocean seems higher than the land if it is looked at from the edge of the shore, believe that the earth is, as it were, sunk in the waters and supernaturally guarded by the omnipotence of God lest the waters rushing in from the deep should overwhelm it.

What do men imagine to be under both the land and the waters? There has been great discussion among men marveling concerning the foundation which could bear up the great mass of the earth so that it should remain for so many centuries firm and immovable and should not sink; and Heraclitus among the early philosophers, and Lactantius among the ecclesiastics said that it reached down to the lowest root of things. [p.313]

How about the other part of the world, the sky and its extent? Men have thought that the sky was not much larger than the earth, and indeed was connected with the earth and the ocean at the circumference of the circle, so that it bounded the earth; and that anyone going that far, if it could be done, would run up against the sky, blocking further progress. With this idea of men the Scriptures also agreed.

So also the poets said that Mt. Atlas, a lofty mountain on the farthest short of Africa, bore up the sky on his shoulders, and Homer placed the Aethiopeans at the extremities of the rising and setting sun, thinking that because of the contiguity of the earth and sky there, the sun was so close to them that it burned their skin.

What form do they ascribe to the sky? The eyes ascribe to the sky the shape of a tent, extending over our heads and beyond the sun, moon and stars, or rather the shape of an arch overspanning the terrestrial plane, with a long curve, so that the part of the sky just over the head of the spectator is much nearer to him than the part that touches the mountains.

What have men conceived in regard to the motion of the sky? Whether the sky moves or stands still is not apparent to the sight because the tenuity of its substance escapes the eyes, unless indeed those things appear to stand still in which the eye can perceive no variation. But the changing positions of the sun, moon and stars in relation to the ends of the earth was apparent to the eyes. For the sun seems to emerge from an opening between the sky and the immovable mountains and ocean, as if coming out of a chamber, and having traversed the vault of the sky seems to sink again in the opposite region; so also the moon, and the planets, and the whole host of stars proceed as if strictly marshalled and drawn up in line, first one and then the other marching along, each in his order and place.

And so, since the ocean lies beyond the extreme lands, the mass of men have thought that the sun plunges into the ocean and is extinguished, and from the opposite region a new sun issues forth daily from the ocean. The poets have used this figure in their creations. But, indeed, there have been even philosophers who have declared that on the farthest shores of Lusitania could be heard the roar of the ocean extinguishing the flames of the sun, as Strabo recounts.

I understand the forms of the sky and the earth and the atmosphere surrounding the earth, also the place of the earth in the universe; now I would ask what causes the stars to seem to rise daily from one part of the horizon and to sink in the opposite part; the motion of the sky or of the earth? The astronomy of Copernicus shows that our sight has led us astray in regard to this motion; for the stars do not actually come up from beyond the mountains and climb toward the zenith, but rather the mountains which surround us and which are a part of the surface of the earth are revolved along with the whole globe about its axis from west to east and by this revolution the immovable stars of the east are disclosed to us one after the other, and those of the west are obscured, so the stars are not passing over us, but the vertical point is moving through the fixed stars.

You say that by this marvelous hypothesis may be explained satisfactorily all the phenomena of the first motion and the spherical theory. Just so, and that is the scope of this section, to demonstrate in fact what has been suggested in words.

How do you expect to be able to prove this absurd hypothesis, and by what arguments? It is possible to demonstrate that this first motion results from the revolution of the earth about its axis, while the heavenly bodies are at rest (as far as this first motion is concerned), by seven kinds of arguments: 1) from the subject of the motion; 2) from the velocity of the motion; 3) from the equableness of the motion; 4) from the cause of the motion, or the moving principle; 5) from the motive instruments, that is, the axis and the poles; 6) from the object of the first motion; and 7) from the indications or results.

Demonstrate it then from the subject of the motion. Nature does not seek difficult means when she can use simple ones. Now, by the rotation of the earth, a very small body, about its axis, toward the east, the same thing is accomplished as by the rotation of the immense universe about its axis toward the west. Just as it is more likely that a man’s head turns in the auditorium than that the auditorium is turned about his head, so it is more credible that the earth is rotating from west to east, than that the rest of the machine of the universe is revolved from east to west, since in both cases the same thing results,

If the first motion is in the heavenly bodies, then they are subject to two motions, one common to the whole universe, the other particular to each sphere; but it is much more probable that the two motions should be distinct in regard to their subjects, so that the second set of motions, which is multifold, should belong to each sphere, and the first, which is single, should belong to the single body of the earth, and to it alone. [p.315]

Why cannot the whole machinery of the universe be moved? The universe is either infinite or finite. Suppose it to be the former, according to the opinion of William Gilbert, who thinks that the omnipotence of God is illustrated in this that the universe extends outward infinitely, so that the infinite power of the creator would be recognized from the infinite extent of the creation. Although this may be refuted by metaphysical arguments, no argument on either side can be drawn from astronomy, in which trust is placed rather in the evidence of the senses than in abstract reasonings not dependent on observation. But supposing this universe to be infinite, Aristotle has shown that the whole universe should not be moved about in a revolution since it is the whole.

But let the universe be finite; then there is nothing outside the universe which would locate the universe but should remain quiet itself. Where there is nothing that rests there is no motion. For 1) motion is the separation of a movable thing from its place and its transfer to another place: 2) the motion of a machine about an axis and quiescent poles cannot be grasped by the mind where there is no thing in respect to which the poles remain still.