Copernicus and Mechanics
Journal of the History of Ideas,
1 (1 )
Jan. 1940, 113-118.
Copernicus overthrew the medieval conception of the solar system by starting from the scanty reports on heliocentric theories in antiquity, by specifying the implications of these geometrically in every detail, and by thus furnishing the exact foundations for ephemerides that far surpassed the exactness of the older tables of planetary movements based on the theory of Ptolemy. 1 His outstanding contribution to astronomy was a mathematico-geometrical one. It is, however, sometimes not sufficiently noticed how far removed Copernicus still is from modern physical and especially mechanical thinking. A few remarks on this point, therefore, may be useful. They refer to the first book of De Revolutionibus Orbium Coelestium (1543), in which Copernicus explains the basic ideas of his theory and where,
1 Cf. Angus Armitage: Copernicus. The Founder
of Modern Astronomy.
consequently, Pythagorean and Scholastic ideas predominate. Ancient and medieval philosophic ideas recede into the background in the following five books (II-VI) in which the mathematical details are explained. 2
(1) Copernicus uses again and again concepts of value in his general arguments. The third sentence of Book I asks the rhetorical question: “What is more beautiful than the sky?... Because of its preeminent excellence most of the philosophers have called it the Visible God.” In chapter 8 he supports the statement of the immobility of the sun in the following way (p. 24, 11. 2 ff.): “Furthermore the condition of immobility is considered more noble and divine than the condition of change and instability which, therefore, 3 is more fitting to the earth than to the universe. I add that it would seem rather absurd to ascribe movement to the containing and locating and not to the contained and located, which is the earth.” In chapter 10 he explains (p. 30, 11. 1 ff.): “The sun is stationed (residet) in the middle of the universe. In this most beautiful temple who could put this lamp in another or better place than the one from which it can illuminate the whole universe at once?” The sun, he continues, therefore is called by some “mind” and “ruler,” and he ends by quoting the chief authority of occult science, alchemy, and Neo-Platonism: “Trismegistus calls the sun the Visible God, Sophocles’ Electra, Him who sees everything. The sun, indeed, sitting on a royal throne rules (gubernat) the family of stars moving around it.”
(2) Copernicus is inclined to apprehend inanimate objects as living beings striving to reach aims. Sometimes he expresses himself in an almost animistic way, more often he gives teleological explanations in the more rational way of Aristotle and the Scholastics. A few sentences after the passage just quoted he remarks (p. 30, line 9): “The Earth conceives from the Sun and is impregnated with annual birth.” In chapter 1 he explains the spherical form of the universe as follows (p. 11): “This form is the most perfect one, does not need any joint (nulla indigua compagine),… and is the most capacious figure… All objects strive (appetant) to be bounded in this way. This is seen in drops of water and other liquids when they wish (cupiunt) to be bounded by themselves.” Gravity he explains in the following way (chap. 9, p. 24, line 25): “I think gravity is nothing else but a natural appetency (appetentia) given to the parts by the divine providence of the maker of the universe in order that they may establish their unity and wholeness (ut in unitatem integritatemque suam se confer-
2. All quotations from Dc Revolutionibus refer to the Thorn edition, 1873 (ed. M. Curtze).
3. The idea that immobility is nobler than movement is Platonic and Pythagorean (cf. the well-known Pythagorean table of values, Diels, Fragmente der Vorsokratiker 45 B 5). Ultimately it goes back to the Eleatic school (Xenophanes, Diels FVS 11 B 26).
ant), by combining in the form of a sphere. It is probable that this affection (affectionem) also belongs to the sun, the moon, and the planets in order that they may, by its efficacy, remain in their roundness (ut eius efficacia in… rotonditate permaneant).” On the phenomenon of terrestrial gravitation he says (chap. 7, p. 19, line 28 : “The element of the earth is the heaviest, and all heavy things are driven towards it, striving (contendentia) to its innermost center.”
(3) Closely related to this teleological conception of nature is the opinion of Copernicus that objects of the same kind exert “sympathetic” influences on each other. In chapter 8 he discusses the fact that the surrounding air rotates with the earth and gives two explanations which he considers to be equally admissible, a medieval-sympathetic one and a modern mechanical one. The air rotates, he explains (p. 22, 11. 18 ff.), “either because it is mixed with earthen and watery matter and, therefore, follows the same nature as the earth (eandem sequatur naturam quam terra), or because the motion of the air is acquired and the air participates in it without resistance, since the air is contiguous to the constantly rotating earth.” A few lines later, discussing loose heavy objects (which rotate with the earth as well), he repeats the “sympathetic” explanation alone (p. 22, line 31) “Since the objects which are depressed by their weight are mainly earthen, there is no doubt that the parts retain the same nature as their whole (eandem servent partes naturam quam suum totum).” It becomes perfectly clear that in the opinion of Copernicus, “equality of nature” is the point that matters in the whole argument when he discusses flames: they participate in the rotation (p. 22, 1. 33) “because this fire is earthly and is nourished mainly by earthen matter.” 4
(4) The teleological, half-animistic conception of nature appears also in his theory of motion, which is based on the Aristotelian distinction of “natural” and “artificial” movements. Copernicus explains the falling of bodies by the Aristotelian theory of “natural place” (locus naturalis, chap. 8, p. 23, 1. 10). He continues (p. 23, 1. 13 ff.): “Rectilinear movement belongs with objects which wander or are expelled from their natural places… Nothing is so contrary to the order of the universe and the form of the world as for a thing to be out of its place (extra bourn suum… esse). Rectilinear motion, therefore, occurs only if things are not rightly ordered (rebus non recte se habentibus).” Obviously Copernicus fully accepts the
4. It may be mentioned that the medical prescriptions of
Copernicus also - he was for a time physician in ordinary to his uncle, the
bishop of Ermiand - show an entirely medieval spirit. For examples cf. M. Curtze,
theory of Aristotle and classical astronomy 5 that celestial bodies move in circles and that this movement is something “natural,” whereas rectilinear motion belongs only to terrestrial bodies and is “artificial,” as it were.
The medieval idea that everything natural is endowed with an, as it were, spiritual power which is lacking in artificial and imperfect objects and processes leads Copernicus to a discussion of centrifugal force which contradicts modern mechanics in a remarkable degree. Already Ptolemy had objected to the rotation of the earth that by it all objects would have to be thrown off the earth. 6 Copernicus has to defend his theory against this objection. He does it as follows (chap. 8, p. 21, 1. 5): “Things governed by nature produce effects contrary to those governed by violence. Things upon which force and impetus are conferred must dissolve and they cannot subsist for a long time; but what is done by nature is rightly ordered (recte se habent) and is preserved in its best composition. Ptolemy, therefore, is wrong in fearing lest the earth and all terrestrial things might be dispersed in a rotation brought about by the efficacy of nature. This is something quite different from art or what human ingenuity can carry on.” Obviously Copernicus thinks centrifugal force appears only in “artificial” not in “natural” rotation.
The modern answer to Ptolemy’s objection, the argument that the effects of centrifugal force may be neglected compared with gravity, would not have been entirely out of the way. Copernicus himself uses the analogous argument against the objection that the revolution of the earth around the sun must bring about parallactic shiftings of the fixed stars. There he argues quite correctly that these cannot be observed (with the insufficient instruments of his period, as we have to add) because of the great distance of the fixed stars (I, chap. 10, p. 30, 1. 24). 7 Certainly positions of stars could already be measured in antiquity, whereas in the time of Copernicus no way was available of measuring centrifugal forces and comparing them quantitatively with gravitation. The lack of methods of measurement rather often has resulted in metaphysical explanations of physical phenomena. At any rate the quoted passages may have shown sufficiently how much Copernicus is imbued with Pythagorean, Aristotelian, and Scholastic metaphysics.
A correct quantitative theory of centrifugal force was developed for the first time by Huyghens, one hundred and twenty years after Copernicus. Galileo, however, already ninety years after Copernicus, discussed the centrifugal force connected with the rotation of the earth in an entirely unmeta-
5. Aristotle, De caelo I, 23; Ptolemy, Almagest III, 3.
6. Almagest I, 7.
7. Copernicus gave the same argument previously in his Commentariolus: M. Curtze, op. cit., p. 6, Quarta petitio. Translated by Edward Rosen, Three Copernican Treatises, 1939, p. 58, assumption 4.
physical way. Certainly his explanation 8 is not yet correct - he thinks the centrifugal force must at any rate be smaller than gravitation, however fast the earth would rotate, and produces a would-be geometrical proof of this assertion - but he knows that the centrifugal effects in question cannot be observed for the reason that they are too small. The idea that “artificial” rotations behave differently from “natural” ones is not even mentioned by him. This is highly important, for in the last consequence the entirely non-mechanical distinction between “natural” and “artificial” movements excludes experimental research on natural objects. Also with Galileo some teleological ideas still persist, but they form nothing but the general background of his explanations. He almost always uses purely mechanical arguments when he proves his single statements and is strongly opposed to explanations of natural phenomena by means of sympathy and antipathy. 9
Copernicus is interested in the exact formulation of the mathematical regularities of celestial movements; he is a Pythagorean, and advances not one real mechanical idea. Galileo, on the other hand, is a mechanist: in his dialogue on the theory of Copernicus he is so little interested in the exact details of the planetary movements that he does not even mention the laws of Kepler. 10 He considers it much more important to support the basic ideas of Copernicus by new observations, to show that there is no fundamental difference between celestial phenomena and terrestrial mechanics and physics, and to refute the pre-mechanical ideas and objections of the Aristotelians of his period.
The difference between Copernicus and Galileo is not a difference of individual psychology only, and even less can it be explained by the mere difference of time. Kepler, who was a contemporary of Galileo, was, as is generally known, at least as Pythagorean and thought at least as teleologically as Copernicus. There rather seems to be a difference between astronomy and mechanics as to their historical evolution and sociological origins. The very first astronomers were Babylonian priests and this connection with priesthood was never quite interrupted; and from antiquity through the Middle Ages up to the end of the sixteenth century, astronomy belonged to the “liberal” arts, as contrasted with the “mechanical” ones. This might explain why metaphysical, Pythagorean and teleological ideas could persist in astronomy until Copernicus and Kepler. It is scarcely mere chance that Copernicus starts his work (I, p. 9) with a eulogy of astronomy “which is the chief of the liberal arts, is most worthy of free men, and rests upon almost all kinds of mathematics.” And it is not mere chance that, by enumerating these, Copernicus gives mechanics as the last one. For me-
8. Diabogo sopra i due
9. Cf. Dialogo, Ed. naz., VII, 436, 17 ff.; Discorsi, Ed. naz., VIII, 116.
10. The dialogue appeared in 1632, Kepler published his laws in 1609 and 1619.
chanics belonged to the “mechanical arts,” to those which required the use not only of head and tongue, but also of hands, and therefore were left to lower-class people. It may be that in the modern era the experimental method and the elimination of teleological and animistic by causal thinking originated in those ranks of mechanicians and craftsmen. Certainly scientific mechanics and physics did not appear in modern times before the way of thinking of the craftsmen was adopted by academically trained scholars of the upper class, as happened in the period of Galileo. A more extensive inquiry, however, than could be given in this short note on Copernicus, would be necessary to verify this sociological explanation.
International Institute of Social Research