The Astronomical Revolution

Copernicus - Kepler - Borelli

Alexandre Koyre

• Editorial: Routledge
• Año: 2013
• ISBN:
  9781135028336
• Edición: 1
• Categoría: Philosophie
• Categoría: Geschichte & Theorie der Philosophie

Originally published in English in 1973. This volume traces the development of the revolution which so drastically altered man's view of the universe in the sixteenth and seventeenth centuries. The "astronomical revolution" was accomplished in three stages, each linked with the work of one man. With Copernicus, the sun became the centre of the universe. With Kepler, celestial dynamics replaced the kinematics of circles and spheres used by Copernicus. With Borelli the unification of celestial and terrestrial physics was completed by abandonment of the circle in favour the straight line to infinity.

II
KEPLER AND THE NEW ASTRONOMY

Introduction

‘Mihi non minus admirandae videntur
occasiones quibus homines in cognitionem
rerum coelestium deveniunt, quam ipsa
Natura rerum coelestium.’

In the galaxy of great minds whose combined efforts have produced that which it is customary to call the ‘scientific revolution of the seventeenth century’, Johann Kepler occupies a unique position. It could be said, that he was both in advance of, and behind, his contemporaries. He was, in fact, the first to overcome, at least partially, the obsession with circularity from which Galileo was never able to free himself, and to divest astronomy of the cumbrous apparatus of spheres and circles which Copernicus had rigidly maintained. Consequently, from the purely scientific viewpoint, he, far more even than Copernicus who technically was a Ptolemaean or even a rather strict Hipparchan, was the true founder of the New Astronomy. Yet, it was he who, with all the power at his command, opposed Giordano Bruno's attempt to make the Universe infinite¹; and believed with the strength of iron—or rather with the strength of ice, for he thought it was composed of icr—in the existence of a celestial vault enclosing the Universe and containing the fixed stars.

He, too, was the first to conceive and conscientiously follow, if not realize, a programme directed to the scientific unification of the starry Universe and the sublunary Universe, and substitute celestial dynamics for the kinematics of circles and the metaphysics of spheres in traditional astronomy. He it was who based this dynamics on solar astro-biology by ascribing to the Sun a spiritual, perhaps even intellective, driving force, and to the Earth not only a spiritual, but also a sensory driving force. He based his defence of heliocentrism on a conception of the Universe which, by an inspiration both Pythagorean and Christian, saw therein an expression of the divine Trinity, and caused the concept of the Cosmos, hereafter condemned, to shine with supreme brilliance.…

Johann Kepler is a veritable Janus; and that is why a study of his ideas is so attractive, yet so difficult. If we may judge by the bibliography of Kepler,² it is more difficult than attractive. It is well-known that we lack an exhaustive study of Kepler.³ This is a rather serious deficiency, for Kepler was not only a genius of the first rank, who has left a deep impression on all the branches of knowledge he touched— mathematics, optics, astronomy—he was also an influence who played a decisive part in the development of science. Without Kepler, without the strenuous work he undertook, without his years of laborious and wearisome calculations, the progress of astronomy would have been delayed for a century; without Kepler, there would have been no Newton.⁴

Now, as regards these calculations, this work which no-one would have done in his stead, for no-one amongst his contemporaries understood its importance,⁵ would Kepler have undertaken it in order to sustain him in his isolation had not his scientific passion as an astronomer been exceeded by his ardour as a metaphysician and even his faith as a believer?

Naturally, I shall not attempt a detailed consideration of Kepler's scientific work; I shall restrict myself to that of Kepler the astronomer.⁶ The direction and deep inspiration of this work consist, according to Kepler, in the substitution of a ‘philosophy’ or ‘celestial physics’ for the ‘theology’ or ‘metaphysics’ of Aristotle⁷; in the demonstration that ‘celestial physics’ is to be elaborated by geometry and computation; in the proof that ‘celestial physics’ and ‘terrestrial physics’ do not constitute two ‘physics’, but one only.⁸

Clearly, this represents a complete overthrow of the structure and meaning traditionally ascribed to the science of celestial bodies, and the question naturally arises: How did Kepler come to adopt such a revolutionary attitude? By way of answer we could suggest: Kepler asked himself—a quo moventur planetae (what is it that makes the planets move)? He gave this question a dynamic significance, which was something no-one had done before.

In astronomy of the Middle Ages and Antiquity, this problem, which seems quite natural, does not arise; or, if one prefers, it is resolved before it arises. In the traditional cosmological model, the planets as such do not move, but are moved by, or with, the spheres and celestial orbs in which they are set. The spheres, being made of a very special, imponderable material and offering no resistance to motion (particularly circular motion), are moved in virtue of their own proper natures, or even by ‘spirits’ or ‘intelligences’ which are attached to them. The causes of motion in this instance are trans-physical.⁹

On the other hand, the problem of the causes of planetary motion has no meaning for mathematical astronomy: the circles and spheres which appear in this astronomy are mere adjuncts to computation, and there is no claim for their physical reality. Celestial motions belong to the realm of pure kinematics: astronomy is one thing, physics is something quite different.

The advent of Copernican Astronomy changed nothing, or practically nothing, in this state of affairs. There was scarcely any dynamic problem for Copernicus, because in his view the planets were still transported by the celestial spheres, which, like the Earth, revolved simply as a result of their spherical shape. Even Tycho Brahe and Giordano Bruno, who no longer believed in planetary spheres, did not raise the decisive question. Tycho Brahe wavered between belief in planetary spirits and a purely computational attitude; Giordano Bruno, on the other hand, frankly adopted an animistic conception. Kepler was the first, and the only one up to the time of Descartes, to ask for a physical explanation.

The reply that I have outlined above proves therefore to be right, but the question then arises: Why did Kepler ask for this explanation?

I believe that he did so for a highly metaphysical reason, namely, that he divided the visible Universe quite differently from the way in which his predecessors had done. He did not counter-balance the Earth by the heavens, nor the latter by the planets (notwithstanding his veneration for the Sun), but, pursuing a suggestion of Copernicus,¹⁰ he counter-balanced the motionless Universe (the Sun,¹¹ space and the fixed stars), en bloc, by the moving Universe, which comprises the planets and Earth, and which thereby acquired in his mind a unity and similarity in nature and structure that rendered it subject to the same physical laws.

Furthermore—but here we come to the psychological aspect, the very structure of Kepler's genius, a structure that we can try to understand, though it would be in vain to try and explain it—Kepler always raised questions that nobody else raised, and sought answers on matters where certainly nobody else saw any problem. He was firmly convinced that an answer must be forthcoming to every reasonable question.

Whilst his predecessors accepted the cosmological state of affairs as the ultimate fact beyond which it was impossible to enquire, and at most believed (as did Copernicus) that they had discovered its specific law of order,¹² Kepler boldly sought the underlying reason. The Cosmos was not formed by chance; it was created by God; and God, assuredly, did not create it temere, haphazardly, but on the contrary was guided by rational considerations and followed a perfect architectural plan.¹³ Consequently, Kepler considered it perfectly reasonable to seek the architectonic principles which determined the structure and composition of the Cosmos; and not to limit himself, as did Copernicus, to exposing the principle on which the solar system is arranged, namely, that the planets complete their course round the Sun in a time which increases the further they are away. Kepler asked himself the further questions: Why are the superior planets slower in their motion than the inferior planets? and why is there a fixed number of planets, namely, six and not four, eleven or fourteen?

Let us be quite clear on the matter! These questions are not concerned with God's purpose in creating the Universe. There is no intention of developing a cosmo-theology centered on the needs of Man, of which there are many examples in history. Although it was quite clear to Kepler that the Universe was created for man, and that man forms the axiological centre of the world and so determines to a very great extent its architecture,¹⁴ nevertheless, Kepler was looking for something quite different from a teleological explanation. In the first place, he was concerned with finding the constructional laws (he called them the ‘archetypal’ laws) which, in the mind of the Creator, directed the creation of the Universe. In Kepler's view, these laws could only be mathematical, or indeed, to be more precise, geometrical ones. In the next place, he was concerned to find the physical (dynamic) means used by the Divine Architect, or Engineer, to keep his construction together, or to set it in motion.

This was by no means a preposterous undertaking in Kepler's estimation. Indeed, why should not an earthly mathematician be capable of penetrating, at least partially, the mind of the Divine Mathematician?¹⁵ Was not Man, that is to say his soul, created in the image and likeness of God?

It is this attitude which is revealed in the Mysterium Cosmo-graphicum, Kepler's first writing; and this very same attitude is found again in his last great works, the Harmonice Mundi and the Epitome Astronomia Copernicanae.¹⁶ It is practically certain tha...