Chapter 3
The Copernican Controversy (1543–1609)
Copernicus’s Innovation
In 1543, the Polish astronomer Nicolaus Copernicus published an epoch-making book, On the Revolutions of the Heavenly Spheres. It so happened that he died the same year; in fact, he received the first copies of the printed book on his deathbed. (Some people are keen to point out that Copernicus both published and perished!)
Be that as it may, in his book Copernicus elaborated the idea that, rather than standing still at the center of the universe, with all the heavenly bodies revolving around it, the Earth moves by rotating on its own axis daily, and by revolving around the Sun once a year. Copernicus was trying to replace the traditional geostatic and geocentric theory with a geokinetic and heliocentric theory.
As in the geostatic view, for Copernicus the Earth was spherical and the universe was finite and spherical; the fixed stars were attached to the celestial sphere and equidistant from the center. However, the celestial sphere was motionless and did not revolve around the Earth with westward diurnal rotation; instead, the diurnal rotation belonged to the Earth, and its direction was eastward, resulting in the observational appearance of the whole universe rotating westward. Copernicus also gave the Earth a second motion, an orbital revolution around the Sun with a period of one year, and also in an eastward direction. That is, the annual motion was shifted from the Sun to the Earth (with the direction remaining unchanged), thus making the Earth a planet, rather than the Sun. This terrestrial orbital revolution meant that the Earth was located off center, the center of the universe being instead the Sun. Moreover, the other five planets continued to be planets, but their orbits were centered on the Sun rather than on the Earth. Around the Sun there thus revolved six planets in the order: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. The Moon remained a body that circles the Earth eastward once a month.
What Copernicus accomplished was to update an idea which had been advanced in various forms by the Pythagoreans, by Aristarchus of Samos (about 310–250 bc), and by other astronomers in ancient Greece, but which had been almost universally rejected. In a sense, Copernicus’s accomplishment was to give a new argument in support of this old idea that had been considered and rejected earlier. His theory was not primarily based on new observational evidence, but was essentially a novel and detailed reinterpretation of available data. He demonstrated in quantitative detail that the known facts about the motions of the heavenly bodies (especially the planets) could be explained more simply and more coherently if the Sun rather than the Earth is assumed to be at the center, and the Earth is taken to be the third planet circling the Sun yearly and spinning daily on its own axis.
For example, there are thousands fewer moving parts in the geokinetic system than in the geostatic one; for the apparent daily motion of all heavenly bodies around the Earth is explained by the Earth’s axial rotation, so there is only one thing moving daily (the Earth), rather than thousands of stars. Thus, insofar as simplicity depends on the number of moving parts, the geokinetic arrangement is simpler than the geostatic system.
A similar point can be made in regard to the number of directions of motion. Fewer are needed in the Copernican than in the Ptolemaic system. In the geostatic system there are two opposite directions, but in the geokinetic system all bodies rotate or revolve in the same direction. That is, in the geostatic system, while all the heavenly bodies revolved around the Earth with the diurnal motion from east to west, the seven planets (Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn) simultaneously also revolved around it from west to east, each in a different period of time. However, in the geokinetic system, only one direction of motion is needed. For, if the apparent diurnal motion from east to west is explained by attributing to the Earth an axial rotation, then the direction of the latter has to be reversed (west to east). On the other hand, if the apparent revolution of the Sun from west to east is explained by attributing to the Earth an orbital revolution around the Sun, then the same direction has to be retained; this is easy to understand by visualizing the situation, as follows. In Figure 4, S represents the Sun; the small circle is the Earth’s heliocentric orbit, which the Earth traverses in a counterclockwise (or west-to-east) direction; the large circle represents the stellar sphere, which is regarded as motionless. The key issue is that, as the Earth actually moves through points E1, E2, E3, E4, etc. in its orbit around the Sun, from the Earth the Sun appears to move through a corresponding set of points (S1, S2, S3, S4, etc.) on the stellar sphere; but the direction of motion is the same, counterclockwise (or west-to-east).
Figure 4. Direction of annual motion in Copernican system
The explanatory coherence of the Copernican theory—its capacity to explain many phenomena without having to add artificial and ad hoc assumptions—derived from its ability to explain the various known facts about the motions and orbits of the planets by means of basic principles concerning the motion of the Earth. By contrast, in the geostatic system, the thesis of a motionless central Earth had to be combined with a whole series of unrelated assumptions in order to explain what is observed to happen.
The best example of explanatory coherence is the Copernican explanation of retrograde planetary motion and of planetary variation in brightness. Careful observation revealed that no planet moved at a uniform rate in its orbit, but that its speed appeared to vary. Moreover, although the Sun and Moon always moved eastward in their apparent planetary revolutions, the other five planets were periodically seen to slow down, stop, reverse course, and briefly move westward relative to the fixed stars; this reversed movement was called retrogression or retrograde planetary motion. Finally, during retrogression planets appeared brighter, as if they were nearer the Earth than at other times.
These observations meant that a planet could not be simply attached to a rotating heavenly sphere, for in that case neither the distance nor the direction of revolution nor the speed should change. In the geostatic system, the device most commonly used to explain retrograde motion and variation in brightness and speed was a mechanism consisting of deferents and epicycles (Figure 5).
Figure 5. Deferents and epicycles in Ptolemaic system
A deferent was defined as a geocentric circle whose circumference (ABCD) rotated around the Earth (E). An epicycle was defined as a circle (FGHI) whose center (A) lay on the circumference of the deferent, and whose circumference rotated in the same direction as the deferent. The planet was located on the circumference of the epicycle. Thus, when the rotation of the epicycle carried the planet on the far side (F) of the epicycle from the Earth, its distance was the sum of the radii of the deferent and the epicycle; whereas when the epicyclic rotation carried the planet on the near side (H) of the epicycle from the Earth, its distance was the difference between the two radii. Thus, in its geocentric revolution, the distance of the planet from the Earth changed by an amount equal to the diameter of the epicycle. This difference accounted for the variation in brightness.
Moreover, the planet’s motion was the result of its motion along the epicycle and the motion of the center of the epicycle along the deferent. Thus, when the planet was on the far side (F), its speed was the sum of the deferent speed and the epicycle speed; then its speed was faster than its average speed. But when the planet was on the near side (H) of the deferent, its speed was the difference between the two; in this case, if the epicycle speed was greater than that of the deferent, the planet appeared to move backwards (clockwise or westward). Retrograde planetary motion then resulted.
For each planet, the relative sizes of deferent and epicycle and their relative rates of rotation could be adjusted so that their combination yielded mathematically the observed details about retrogression and changes in brightness and speed. For example, if the planet was observed to retrogress twice while revolving through its complete orbit once, then the epicycle was assumed to rotate twice as fast as the deferent; this yielded a path which in reality was looped; but from the Earth (E) the loop was not seen, and instead the planet would appear brighter and retrogressing near B and D.
The framework of deferents and epicycles was a very powerful instrument for the analysis of planetary motion. There was much more that an astronomer could do besides adjusting the relative sizes and speeds of a deferent and its epicycle. For example, one could add a second epicycle on the first epicycle; one could make the center of the deferent different from the center of the Earth, in which case the deferent was called an eccentric; and one could even make the center of the deferent move in some way, perhaps in a small circle around the Earth’s center. For many centuries before Copernicus, such calculations, adjustments, and refinements involving deferents, epicycles, and eccentrics constituted the primary theoretical and mathematical task of planetary astronomy. This enhanced the power of the theory, but it also rendered the whole system more complicated and ad hoc. Moreover, the physical reality of deferents, epicycles, and eccentrics became increasingly unclear.
In the Copernican system, the observed phenomena are explained without the need for Ptolemaic ad hoc postulation and construction of epicycles; instead, they are direct consequences of the relative motion between the Earth and the other planets as they all rotate around the Sun. When the Earth and another planet reach points in their orbits which are on the same side of the Sun and are thus at the minimum distance from one another, their different orbital speeds make the other planet seem to move backward (westward).
If the other planet is a superior one, i.e., one with an orbit larger than the Earth’s, this retrograde motion is seen when the planet’s apparent position on the celestial sphere is opposite to that of the Sun; the Earth’s faster orbital motion toward the east leaves behind the other planet, which thus appears to move toward the west relative to the fixed stars. For example, in Figure 6a, while the Earth moves through points E1 to E11 along the smaller orbit, the superior planet moves along its bigger orbit through a corresponding set of points (P1 to P11). The latter make up a smaller portion of its orbit due to its slower orbital speed. Thus, the apparent position of the superior planet against the background of the fixed stars changes in the order S1 to S11; and in this sequence, the part S4, S5, S6, S7, and S8 is retrograde.
Figure 6. Copernican explanation of retrograde planetary motion
If the other planet is an inferior one, with an orbit smaller that the Earth’s, then at certain times the Earth’s slower orbital motion enables the other planet to overtake the Earth relative to the fixed stars, thus generating the appearance that the planet is moving in a direction opposite to its usual one. That is, as Figure 6b shows, while the Earth moves counterclockwise through points E1 to E9 along the bigger orbit, the inferior planet moves also counterclockwise along the smaller orbit through a corresponding set of points (P1 to P9); then the apparent position of the inferior planet along the stellar sphere changes in the order S1 to S9, and the part S2, S3, S4, S5, S6, S7, and S8 is clockwise, or retrograde.
Despite these advantages of the geokinetic theory from the points of view of simplicity and explanatory coherence, as a proof of the Earth’s motion, Copernicus’s argument was far from conclusive. Notice first that his argument is a hypothetical one. It is based on the claim that if the Earth were in motion, then the observed phenomena would result; but from this it does not follow that the Earth is in motion; all we would be entitled to infer is that the Earth’s motion offers an explanation of observed facts, and one that was simpler and more coherent than the geostatic explanation. This does provide two reasons for preferring the geokinetic idea, but they are not decisive reasons. They would be decisive only in the absence of reasons for rejecting the idea. One has to look at counter-arguments—and there were plenty of them.
The Anti-Copernican Arguments
The arguments against the Earth’s motion can be classified into various groups, depending on the branch of learning or type of principle from which they stemmed. In fact, these objections reflected the various traditional beliefs which contradicted or seemed to contradict the Copernican system. The objections were epistemological, philosophical, theological, religious, physical, mechanical, astronomical, and empirical.
Two sets of issues can also be distinguished, each applying to both the ancient and the Copernican view. The ancient view, recall, contained two main parts: the geostatic thesis that the Earth is motionless, and the geocentric thesis that the Earth is located at the center of the universe. These are independent of each other because there is no contradiction in holding that the Earth is devoid of any motion but is located slightly off the center of the universe; this is what some ancient thinkers speculated in order to account for some of the specific details of the apparent motions of the heavenly bodies. Conversely, there is no incompatibility in holding that the Earth is located at the center of the universe but moves by performing a simple daily axial rotation; in fact, this is precisely the kind of compromise position which other thinkers conceived of. Similarly, in regard to the Copernican view, the Earth’s daily axial rotation and its annual orbital revolution are distinct, in the sense that one could admit terrestrial rotation but deny orbital revolution; this could be done by letting the Earth rotate at the center of the universe. On the other hand, if one lets the Earth revolve annually around the Sun then it would be absurd to deny terrestrial rotation because this would mean that the apparent daily motion of the heavens was actual, and thus we would have a situation where the Earth was going around the Sun once a year while the whole universe was revolving around the Earth once a day.
Let’s begin with the empirical objections, to underscore the fact that the opposition to Copernicanism was neither all mindless nor simply religious. However, to set the stage for the empirical details, it is best to begin with an argument which is empirical in the sense of involving observation and sense experience, but which does so in such a way that what we really have is an epistemological objection.
The argument was aptly called the objection from the deception of the senses. To understand the deception involved, note that Copernicus did not claim that he could feel, see, or otherwise perceive the Earth’s motion by means of the senses. Like everyone else, Copernicus’s senses told him that the Earth is at rest. Therefore, if his theory were true, then the human senses would not be reporting the truth, or would be “lying” to us. But it was regarded as absurd that the senses should deceive us about such a basic phenomenon as the state of rest or motion of the terrestrial globe on which we live. In other words, the geokinetic theory seemed to be in flat contradiction with direct sense experience and to violate a fundamental epistemological principle: that under normal conditions the senses are reliable and provide us with one of the best instruments to learn the truth about reality.
One could begin trying to answer this difficulty by saying that deceptions of the senses are neither unknown nor uncommon; this is shown, for example, by a straight stick half immersed in water that appears bent, or by the shore appearing to move away from a ship to an observer standing on the ship and looking at the shore. Still, the difference is that these perceptual illusions involve relatively minor and secondary experiences, whereas to live all one’s life on a moving globe without noticing it would be a gigantic and radical deception. Moreover, it was added, the former illusions are corrigible, since we have other ways of discovering what really happens, whereas there is no way of correcting the perception of the Earth being at rest. This difficulty may be labeled an epistemological objection because the real issue is whether the Earth’s motion ought to be (directly) observable, and whether the human senses ought to be capable of directly revealing the fundamental features of physical reality.
This general empirical objection is in a sense the reverse side of the coin of the fundamental advantage of the geostatic system. (The same applies, of course, to all the other anti-geokinetic objections, which may thus be easily turned into pro-geostatic arguments.) The most basic and important argument in favor of the geostatic view was taken from direct observation, which testifies to the correctness of the geostatic thesis: our visual experience reveals that the heavenly bodies move around the Earth every day, a point which is most easily observable for the Sun, whose rising in the east and setting in the west generates the cycle of night and day; further, according to our kinesthetic sense the Earth is felt to be at rest; the argument here was simply that the Earth must be standing still because our sense experience shows this.
The other empirical objections to Copernicanism were more specific, and were based primarily on effects in the heavens which ought to be observed in a Copernican universe, but which in fact were not. These specific empirical difficulties may therefore be also called the astronomical objections.
We can start with the objection from the Earth–heaven dichotomy. It argued that, if Copernicus were right, then the Earth would share many physical properties with the other heavenly bodies, especially the planets, since the Earth would itself be a planet, the third one circling the Sun. However, as we have seen (Chapter 2), it was widely believed that whereas the heavenly bodies were weightless, luminous, changeless, and made of the element aether, terrestrial bodies were dark, subject to constant changes, and made of parts that had weight (earth and water) or levity (air and fire). Now, before the invention of the telescope this belief had considerable empirical support.
The issue of the phases of the planet Venus was the basis of another objection. If the Copernican system were correct, then this planet should exhibit phases similar to those of the Moon but with a different period; yet none were visible (before the telescope). The reason why Venus would have to show such phases stems from the fact that, in the Copernican system, it is the second planet circling the Sun, the Earth is the third, and these two planets have different periods of revolution. Therefore, the relative positions of the Sun, Venus, and the Earth would be changing periodically, and corresponding changes would occur in the amount of Venus’s surface visible from the Earth: when Venus is on the far side of the Sun from the Earth, its entire hemisphere lit by the Sun is visible from the Earth, and the planet should appear as a disk full of light (like a full moon, though much smaller); when Venus comes between the Sun and the Earth, none of its hemisphere lit by the Sun is visible from the Earth, and the planet would be invisible (as in the case of a new moon); and at intermediate locations, when the three bodies are so positioned that the line connecting them forms a noticeable angle, then different amounts would be visible, giving Venus an appearance ranging from nearly fully lit, to half lit, to a crescent shape.
This objection assumes that Venus is dark and opaque, and not intrinsically luminous. However, this is a reasonable assumption for Copernicanism, according to which Venus shares these optical properties with the Earth, the Moon, and the other planets. By contrast, in the Ptolemaic system, Venus is intrinsically luminous, like all other heavenly bodies; thus, its appearance would never show phases, despite its constant location between the Earth and the Sun. It follows that the objection from the phases of Venus is indeed a difficulty for Copernicanism, but not for the Ptolemaic system.
The apparent brightness and size of the planet Mars involved another problematic issue. In the Copernican system, this planet revolves in the next outer orbit (M1–M2 in Figure 7) after the Earth’s orbit (E1–E2). Since they also revolve at different rates, they are relatively close to each other when their orbital revolutions align both on the same side of the Sun (E1 and M1, or E2 and M2), and relatively far when they are on opposite sides of the Sun (E1 and M2, or E2 and M1). This variation in distance between the Earth and Mars is considerable; according to some estimates, it was supposed to be eightfold. This change in distance would cause a corresponding variation in the apparent size of Mars when seen from the Earth, and an even greater change in brightness, since the intensity of light varies as the square of the distance. Now, the difficulty was that, although Mars did indeed exhibit a noticeable change in brightness with periodic regularity, this change was not nearly as much as it should be; further, there was practically no variation in apparent size (before the telescope).
Figure 7. Copernican variations in appearance of Mars
Again, in the Ptolemaic system the distance between the motionless central Earth and Mars also changed, due to its epicycles; and so its apparent size and brightness should also undergo some changes. However, the appearance of Mars did not present a serious difficulty for the geostatic view, as it does for Copernicanism, because in the Ptolemaic system, the relevant quantities (distance, epicycle, and so on) could be adjusted to correspond to the actual observations, whereas in the Copernican system the variation could be derived from other elements of the system, because of its greater coherence.
A final empirical astronomical argument was based on the fact that observation revealed no change in the apparent position of the fixed stars; this is commonly known as the objection from stellar parallax, a term that denotes a change in the apparent position of an observed object due to a change in the location of the observer. At its simplest level, the apparent position of a star may be thought of as its location on the celestial sphere, which in a sense is its position relative to all the other stars (also located on that sphere); or, from the viewpoint of the Copernican system, it may be conceived as measured by the angular position of the star above the plane of the Earth’s orbit (the so-called plane of the ecliptic). Now, if the Earth were revolving around the Sun, then in the course of a year its position in space would change by a considerable amount, defined by the size of the Earth’s orbit; therefore, a terrestrial observer looking at the same star at six-month intervals would be observing it from different positions, the difference being a distance equal to the diameter of the Earth’s orbit; consequently, the same star should appear as having shifted its position either on the celestial sphere or in terms of its angular distance above the plane of the Earth’s orbit. It follows that if Copernicanism were correct, we should be able to see stellar parallaxes with a periodic regularity of one year. Yet none were observed.
For example, in Figure 8, let ANBO represent the Earth’s orbit; line AB a diameter of the Earth’s orbit; ABC a line in the plane of that orbit; CEH a portion of the celestial sphere; H a fixed star whose position, when observed from point B, may be defined in terms of the angle HBC. However, six months later, when star H is observed from point A in the Earth’s orbit, the star’s position may be defined in terms of angle HAC; and this angle (HAC) is smaller than the previous one (HBC). That is, when observed at six-month intervals, the same star H would appear to shift its position, appearing sometimes higher and sometimes lower above the plane of the ecliptic.
Figure 8. Annual stellar parallax in Copernican system
As we shall see later, the first three of these empirical astronomical objections were not answered until Galileo’s telescopic discoveries, and stellar parallax was not detected until much later, in 1838, by German astronomer Friedrich Bessel (1784–1846). In fact, the magnitude of parallax varies inversely as the distance of the observed object; and the stars are so far away that their parallax is exceedingly small; so for about two centuries telescopes were not sufficiently powerful to make the fine discriminations required. One may then begin to sympathize with Copernicus’s contemporaries, including Galileo, who initially found his idea very hard to accept.
Besides, there were many other reasons for their opposition to Copernicanism. The next group of objections may be labeled mechanical or physical, in the sense that they are based directly or indirectly on a number of principles of the branch of physics which today we call mechanics, which studies how bodies move. We will look first at four objections which hinge indirectly (though crucially) on the laws of motion, and later at two others where the appeal to such physical principles is direct and explicit.
The objection from vertical fall began with the fact that bodies fall vertically. This is something that everyone can easily observe by looking at rainfall when there is no disturbing wind; or by throwing a small rock directly upwards, and noticing that it falls back to the place from which it was thrown; or by dropping a rock from the top of a building or tower and observing that it moves perpendicularly downwards, landing directly below. It was argued that this could not happen if the Earth were rotating; for, while the body was falling through the air, the ground below would move a considerable distance to the east (due to the Earth’s axial rotation), and although the building and person would be carried along, the unattached falling body would be left behind; so that on a rotating Earth the body would land to the west of where it was dropped, and it would appear to be falling along a westward slanted path. Since this is not seen, but rather bodies are observed to fall vertically, it was concluded that the Earth does not rotate.
An analogous argument was advanced by the objection from east–west gunshots. The relevant observation here was that, when ejected with equal force, projectiles range equal distances to the east and to the west. This can be most easily observed by throwing a rock with the same exertion in both opposite directions in turn, and measuring the two distances; one could also use bow and arrow, so as to have a slightly better measure of the propulsive force; or one could use a gun, and shoot it first to the east and then to the west with the same amount of charge. Now, the argument claimed that on a rotating Earth such projectiles should instead range further toward the west than toward the east, because in its westward flight the projectile would be moving against the Earth’s rotation, which would carry the place of ejection and the ejector some extra distance to the east; whereas, in its eastward flight, the projectile would be traveling in the same direction as the ejector, due to the latter being carried eastward by the Earth’s rotation; therefore, on a rotating Earth the westward projectiles would range further by a distance equal to the amount of the Earth’s motion, while the eastward ones would fall short by the same amount. Again, since observation reveals that this is not so, it supposedly follows that the Earth does not rotate.
Of course, today these arguments can be refuted. However, their refutation requires knowledge of at least two fundamental principles of mechanics, to which Galileo himself contributed. One is the law of conservation of momentum, or more simply the principle of conservation of motion, according to which the motion acquired by a body is conserved unless an external force interferes with it. The other is the principle of superposition, which specifies how motions in different directions are to be combined with each other to yield a resultant motion. The point that needs to be stressed is that, since the phenomena to which these two objections appealed to are indeed true, the issues they raised were about how bodies would or could move on a rotating Earth, and the resolution of these issues depended on the possession of more accurate mechanical principles. The next objection raised these same issues, but also the question of what the facts of the case really were; however, to establish these facts was not so easy as it might seem.
The objection from the ship’s mast experiment referred to an experiment to be made on a ship, and it then drew an analogy between the Earth and the ship. The experiment consisted of dropping a rock from the top of a ship’s mast, both when the ship is motionless and when it is advancing forward, and then checking the place where the rock hits the deck. It was asserted that the experiment yielded different results in the two cases: that when the ship was standing still the rock fell to the foot of the mast, but that when the ship was moving forward the rock hit the deck some distance toward the back. Then the moving ship was compared to a portion of land on a rotating Earth, and a tower on the Earth was regarded as the analogue of the ship’s mast. From this it was inferred that, if the Earth were rotating (eastward), then a rock dropped from a tower would land to the west of the foot of the tower, just as on a ship moving forward it falls toward the back; however, since the rock can be observed to land at the foot of the tower, they concluded that the Earth must be standing still.
This objection partly involves the empirical issue of exactly what happens when the experiment is made on a moving ship. If the experiment is properly made, the result will be that the rock still falls at the foot of the mast. However, it is easy to get the wrong result due to extraneous causes, such as wind and the rocking motion which the boat is likely to have in addition to its forward motion. Therefore, it is not surprising that there were common reports of the experiment having been made and having yielded anti-Copernican results. Nor is it surprising that, as we shall see later, when Galileo tried to refute the objection, although he disputed the results of the actual experiment, claiming to have performed it, he emphasized a more theoretical answer in terms of the principles of conservation and superposition of motion. These principles are needed to determine what will happen to the horizontal motion the rock had before it was dropped from the mast of the moving ship, and how it is to be combined with the new vertical motion of fall it acquires.
The last one of the indirectly mechanical objections to be discussed here is the objection from the extruding power of whirling, or, as we might say today, the centrifugal-force argument. The basis of this objection was the fact that in a rotating system, or in motion along a curve, bodies have a tendency to move away from the center of rotation or of the curve. For example, if one is in a vehicle traveling at a high rate of speed, whenever the vehicle makes a turn one experiences a force pushing one away from the center of the curve defined by the turn: if the vehicle turns right, one experiences a push to the left, and vice versa. Or you could tie a small pail of water at the end of a string and whirl the pail in a vertical circle; now suppose a small hole is made in the bottom of the pail; as the pail is whirled you would see water rushing out of the hole always in a direction away from your hand. Then the argument called attention to the fact that, if the Earth rotates, bodies on its surface are traveling in circles around its axis at different speeds depending on the latitude, the greatest speed being about 1000 miles per hour at the equator. This sounds like a very high rate of speed, which would generate such a strong extruding power that all bodies would fly off the Earth’s surface, and the Earth itself might disintegrate. Since this obviously does not happen, it was concluded that the Earth must not be rotating.
This objection raised issues whose resolution involved the correct laws of centrifugal force. At the time, however, these laws were not known, and so this objection was a very strong one. Next, we come to the objections according to which the conflict with physical principles was so explicit that the Earth’s motion seemed a straightforward physical impossibility.
One of these objections was the natural-motion argument. It claimed that the Earth’s motion (whether of axial rotation or orbital revolution) is physically impossible because the natural motion of earthly bodies (rocks and water) is to move in a straight line toward the center of the universe. The context of this argument was the science of physics which I elaborated earlier (Chapter 2) as part of the geostatic world view: it contrasted natural motion to violent motion; it postulated three basic types of natural motion; and it attributed each type to one or more of the basic elements: circular motion around the center of the universe was attributed to aether; straight motion away from the center of the universe was ascribed to the elements air and fire; and straight motion toward the center of the universe was given to the elements earth and water. Thus, unlike natural circular motion, which can last forever, straight natural motion, especially straight-downwards, cannot be everlasting since, once the center (of the universe) is reached, the body will no longer have any natural tendency to move. Now, the terrestrial globe on which we live is essentially the collection of all things made of the elements earth and water, which have collected at the center (of the universe) or as close to it as possible. Therefore, this whole collection cannot move around the center (in an orbital revolution as Copernicus would have it), because such a motion would be unnatural, could not last forever, and would in any case be overcome by the tendency to move naturally in a straight line toward the center; further, for the same reasons, once at the center, the whole collection could not even acquire any axial rotation.
The Copernican system was also deemed physically impossible because it was in direct violation of the principle according to which every simple body can have one and only one natural motion. This principle was another aspect of the laws of motion of Aristotelian physics, whereas Copernicanism seemed to attribute to the Earth at least three natural motions: the revolution of the whole Earth in an orbit around the Sun, the rotation of the Earth around its own axis, and the downwards motion of parts of the Earth in free fall.
Just as the last two objections are essentially unanswerable as long as one accepts the two principles of traditional physics just mentioned, they are easily answerable by rejecting these two principles. However, rejecting them is easier said than done since, to be effective, the rejection should be accompanied by the formulation of some alternatives. In short, what was really required was the construction of a new science of motion, a new physics, which Copernicus did not provide. In fact, the alternatives were such cornerstones of modern physics as the law of inertia, the law of gravitational force, and the law of conservation of (linear and angular) momentum. For example, according to the law of inertia, the natural motion of all bodies is uniform and rectilinear; and according to the law of gravitation, all bodies attract each other with a force that makes them accelerate toward each other, or diverge from their natural inertial motion in a measurable way. Thus, the Earth’s orbital motion becomes a forced motion under the influence of the Sun’s gravitational attraction; the axial rotation of the whole Earth becomes a type of natural motion in accordance with conservation laws; and the downward fall of heavy bodies near the Earth’s surface becomes a forced motion under the influence of the Earth’s gravitational attraction.
Finally, there were theological and religious objections. One of these appealed to the authority of the Bible and may be labeled the biblical or scriptural objection. It claimed that the idea of the Earth moving is heretical or at least erroneous because it conflicts with many biblical passages which state or imply that the Earth stands still. For example, Psalm 104:5 says that the Lord “laid the foundations of the earth, that it should not be removed for ever”; and this seems to say rather explicitly that the Earth is motionless. Other passages were less explicit, but they seemed to attribute motion to the Sun, and thus to presuppose the geostatic system. For example, Ecclesiastes 1:5 states that “the sun also riseth, and the sun goeth down, and hasteth to the place where he ariseth.” And Joshua 10:12–13 describes the following miracle, which presupposes that the Sun (not the Earth) normally moves: “Then spake Joshua to the Lord in the day when the Lord delivered up the Amorites before the children of Israel, and he said in the sight of Israel, ‘Sun, stand thou still upon Gibeon; and thou, Moon, in the valley of Ajalon’. And the sun stood still, and the moon staid, until the people had avenged themselves upon their enemies.”
The biblical objection had greater appeal to those (like Protestants) who took a literal interpretation of the Bible more seriously. However, for those (like Catholics) less inclined in this direction, the same conclusion could be reinforced by appeal to the consensus of Church Fathers; these were the saints, theologians, and churchmen who had played an influential and formative role in the establishment and development of Christianity. The argument claimed that all Church Fathers were unanimous in interpreting relevant biblical passages (such as those just mentioned) in accordance with the geostatic view; therefore, the geostatic system is binding on all believers, and to claim otherwise (as Copernicus did) is erroneous or heretical.
A third theological-sounding objection was based crucially on the idea that God is all-powerful, and it may be labeled the divine-omnipotence argument.1 One of its most famous proponents was Pope Urban VIII; it was, in fact, his favorite anti-Copernican objection. A version of the argument is stated, without criticism, at the end of Galileo’s Dialogue, and this formulation got him into trouble with Church authorities and played a role in the trial of 1633, as we shall see later.
One version of the argument claimed that since God is all-powerful, he could have created any one of a number of worlds, for example one in which the Earth is motionless; therefore, regardless of how much evidence there is supporting the Earth’s motion, we can never assert that this must be so, for that would be to want to limit God’s power to do otherwise. Another version seemed to argue that divine omnipotence implies that God could have created a world in which the evidence suggests a moving Earth despite its being motionless. The argument is not purely theological, but also raises issues of a logical, methodological, and epistemological nature. Moreover, some versions of the argument may very well be essentially correct and unanswerable; for example, part of the argument seems to suggest that scientific knowledge of the physical world is contingently true rather than necessarily true. On the other hand, the argument was also taken to suggest a general skeptical doubt about physical theories, as well as a specific difficulty for the Copernican theory; and these suggestions are controversial and questionable.
In summary, the idea updated by Copernicus was vulnerable to a host of counter-arguments and to considerable counter-evidence. the Earth’s motion seemed epistemologically absurd because it flatly contradicted direct sense experience, and thus undermined the normal procedure in the search for truth. It seemed empirically and astronomically untrue because it had astronomical consequences that were not seen to happen. It seemed a physical impossibility because it was thought to have consequences that contradicted the most incontrovertible mechanical phenomena, and because it directly violated many of the most basic principles of the available physics. And it seemed religiously heretical, erroneous, or suspect because it conflicted with the words of the Bible, with the biblical interpretations of the Church Fathers, and with the basic theological idea of an omnipotent God.
Copernicus was aware of many of these difficulties. He realized that his novel argument did not conclusively prove the Earth’s motion, and that there were many counter-arguments of apparently greater strength. Although his motivation was complex and is not yet completely understood and continues to be the subject of serious research, his awareness of the counter-arguments was an important reason why he delayed the publication of his book until he was almost on his deathbed.
Responses to Copernicanism
In light of the many objections, a common response to Copernicanism was to regard the Earth’s motion as a mere instrument of mathematical calculation and observational prediction, rather than a description of physical reality. This may be labeled the instrumentalist interpretation of Copernicanism, and was popularized by an anonymous foreword preceding Copernicus’s own preface in the printed Revolutions. This foreword was written and inserted without his approval or knowledge by one of the editors supervising the book’s publication—Andreas Osiander. It is unlikely that Copernicus would have endorsed this interpretation since it is clear from the book that, although he was aware of the difficulties, he treated the Earth’s motion as a description of physical reality (capable of being true and false), and not as a mere instrument of calculation and prediction (limited to being more or less convenient). In short, Copernicus subscribed to a realist interpretation of the geokinetic theory (in accordance with the doctrine of epistemological realism), and not to an instrumentalist interpretation (in accordance with epistemological instrumentalism).
The fact that Osiander’s foreword had not been authorized by Copernicus soon became public knowledge among experts, but many scholars adopted the instrumentalist interpretation as the only way out of the difficulties.
One different response was that of Danish astronomer Tycho Brahe (1546–1601). He decided to collect new data by means of systematic naked-eye observations and the construction of new instruments. The scope, range, accuracy, and precision of his observations were unprecedented. Partly on the basis of his observational data, Tycho constructed a new theory different from both the Ptolemaic and the Copernican ones. In the Tychonic system, the Earth was still motionless at the center of the universe; the stellar sphere still had the westward diurnal motion around the Earth; and the Sun still had the eastward annual motion around the Earth. But the other planets revolved in orbits centered at the Sun, so that the system was to that extent heliocentric; but the Sun carried the whole Solar System around the motionless Earth. Moreover, Tycho did away with the solid spheres that, in some other versions of the geostatic system, carried the planets in their orbits; they no longer fit properly in the new arrangement, since some of the orbits of the heavenly bodies intersected, so that the spheres would have had to interpenetrate one another.
On a different note, the Italian philosopher and theologian Giordano Bruno (1548–1600) undertook a multifaceted defense of Copernicanism that addressed epistemological, metaphysical, theological, and empirical issues. Bruno’s defense was in some ways similar to, but in important ways different from, Galileo’s; it embodied more of a muddled confusion than a proper synthesis of such issues. In any case, for complex reasons, Bruno’s contribution remained largely unknown, disregarded, or unappreciated; not the least of these reasons was the fact that he was burned at the stake by the Inquisition, after a long trial for heresy, lasting 8 years, during which he behaved in a defiant manner.2
One of Tycho’s assistants inherited his data, and analyzed them in a deeper and more systematic and sophisticated manner. He was the German mathematician and astronomer Johannes Kepler (1571–1630). He rejected Tycho’s compromise and was committed to the Copernican system, in part for aesthetic and metaphysical reasons. But Kepler also had a strong empirical orientation, and so he spent his life analyzing Tycho’s observational data. The result was the strengthening of some key Copernican theses, such as the Earth’s motion, and the refinement or revision of others. In fact, Kepler discovered that the planets revolve around the Sun in elliptical rather than circular orbits, with the Sun located at one of the two foci of these ellipses.
Unfortunately, Galileo never did pay the proper attention to Kepler’s writings, and so ignored or neglected the elliptical nature of planetary orbits; the reasons for this neglect remain unclear or controversial, but there is no question that Galileo was turned off by the metaphysical flavor of Kepler’s thought.3 Nor did Galileo think much of the instrumentalist interpretation of the Copernican theory. Still less did he find acceptable Tycho’s compromise of combining geostatic and heliocentric elements. However, Galileo did devise his own response to the Copernican controversy, and that is our main focus in this book. And we will explore not only how he responded, but why he responded the way he did. His intellectual motivation is at least as important as his behavior. To begin with, we should consider his early stance toward Copernicanism.
Galileo’s Indirect Pursuit of Copernicanism
Galileo’s earliest reference to Copernicus is found in a work entitled On Motion, written probably in the period 1589–92 while he was a professor at the University of Pisa, but left unpublished by him. The context is one in which Galileo is arguing, against Aristotle, that when the motion of a body changes from one direction into the opposite direction, there does not have to be a state of rest at the turning point. One of Galileo’s several arguments is that oscillatory motion along a straight line can be generated by combining two continuous circular motions: consider two equal circles such that the center of each is located on the circumference of the other, and such that they rotate in opposite directions at different rates; then one can adjust these rates of rotation so that there is a point on the circumference of the faster rotating circle which moves back and forth along a straight line (as seen from a location outside both circles). Galileo explicitly credits Copernicus’s work On the Revolutions as the source of this demonstration.4
This reference is important for two reasons. First, it shows that Galileo was intimately acquainted with Copernicus’s masterwork at the beginning of his career. Second, it is even more important for what Galileo does not say: Copernican astronomy is nowhere in sight, and so he must not have been impressed by the cogency or conclusiveness of Copernicus’s arguments for the geokinetic thesis. Other passages in Galileo’s On Motion do have a connection with the Earth’s rotation, but that connection is indirect and to appreciate it we must wait for other clues, which will be elaborated presently.
Galileo’s first explicit discussion of Copernican astronomy is found in a letter he wrote to Jacopo Mazzoni, dated May 30, 1597. Mazzoni had been a senior colleague and good friend of Galileo’s during his time at the University of Pisa. Mazzoni was an eclectic philosopher with wide interests, who held anti-Aristotelian ideas on the motion of falling bodies and their speed of fall; and these ideas overlapped with Galileo’s own. In 1597, Mazzoni had just published a book containing a critical comparison of Plato and Aristotle. Galileo, who was then at the University of Padua, had just read the book and was writing to congratulate the old friend and to express his gratification at the fact that they seemed to agree about many things. However, the book also contained an anti-Copernican argument, and most of Galileo’s letter is a lengthy analysis and refutation of that argument.
Mazzoni had argued as follows. If the Earth revolves around the Sun, then this off-center location would imply that terrestrial observers would not always see exactly half of the stellar sphere; rather, they would see less than half at midnight and more than half at noon. Because of the immense size of the stellar sphere, the difference would be very small; but it should be noticeable, because on the Earth by climbing a tall mountain (such as Mount Caucasus) one can notice a comparable difference in the visual horizon. However, we always see exactly half of the stellar sphere. It follows that the Earth is not located off-center revolving around the Sun.
Galileo’s refutation is the following. If the Earth revolves around the Sun, the difference in visibility of the stellar sphere between midnight and noon would be equal to that caused on the Earth by climbing a mountain whose height is 1 and 1/7 miles. On the Earth, the difference in visibility of the stellar sphere resulting from climbing such a mountain is 1 degree and 32 minutes on each side. These quantities are based on the traditional estimates of astronomical distances, which are: distance between Earth and the Sun = 1216 Earth radii; radius of the stellar sphere = 45,225 Earth radii; and Earth radius = 3035 miles. However, the Copernican estimate of the size of the stellar sphere is much greater. Thus, the difference in stellar horizon would be much less than 1 degree and 32 minutes; and that would be unlikely to be noticeable.
The most relevant and important aspect of Galileo’s letter to Mazzoni is that it constitutes an explicit defense of Copernicanism from an astronomical objection. Moreover, the core of Galileo’s reasoning is mathematical or quantitative, and so what we have here is a mathematical defense. Combining these two aspects, we might say that Galileo is exhibiting a mathematical appreciation of Copernicanism, and this is in accordance with his earlier reference to Copernicus in On Motion, discussed earlier. However, it is unclear that we can describe Galileo’s attitude any more precisely than is conveyed by the vague notion of what I am labeling “appreciation.”
In fact, Galileo described his stance in a similarly vague and unclear manner in the introductory part of this long letter. There, he reminded Mazzoni that in the first years of their friendship (1589–92) they often engaged in amicable debates on astronomical topics, and that, for the sake of the argument, he (Galileo) would then take the Copernican side. However, now (in 1597) Galileo had some feelings towards the topic of the Earth’s motion and location, which he did not have earlier.
More clues about Galileo’s attitude toward Copernicanism are found in a letter he wrote to Kepler a few months thereafter (August 4, 1597). The letter was occasioned by the fact that Galileo had just received a copy of Kepler’s book The Secret of the Universe, published the previous year, and he wanted to thank Kepler. Galileo said that he had only read the introduction, but planned to read the rest. Then he added several clarifications about his own stance toward Copernicanism.
Galileo indicates that he is engaged in a program of physical research that fits well with Copernicanism, but not at all with the Aristotelian Ptolemaic view. We might say that he is pursuing the physical side of Copernicanism, physical by contrast with Copernicus’s own astronomical motivation, or with the metaphysical flavor of Kepler’s own book. Galileo is obviously referring to the sort of theory of motion which he had been working on for some time and part of which is recorded in his work On Motion. In particular, he claims to have found that many physical phenomena cannot be explained by the geostatic theory, but are explicable on the basis of the geokinetic hypothesis. He does not explicitly mention the phenomenon of the tides, but there can be no doubt that he had the tides in mind.
Another clarification is this. Galileo explicitly says that he is in possession not only of some positive evidence or constructive reasons favoring Copernicanism, but also of criticisms or refutations of counter-arguments and objections. Thus, he is implicitly suggesting that the negative aspect of the investigation is also essential, namely that the elaboration of Copernicanism must include a serious and careful critical component. In fact, a few months earlier, in his letter to Mazzoni, Galileo had just conducted one exercise in such a critical defense. However, he is clear that this is just one example of many. Indeed, if one examines the historical context, one can identify these other anti-Copernican arguments to be the mechanical objections to the Earth’s motion that had recently been advanced by Tycho Brahe, in books published in 1588 and 1596.
Finally, it is very revealing that Galileo expresses a fear to publish his Copernican inclinations and pursuits. This expression is an explicit comment on his view of the strength of the arguments in support of Copernicanism. He obviously does not think that the Copernican arguments are conclusive or even strong enough to convince someone who, unlike Kepler, is not already favorably inclined.
Now that we have a general description of the type and of the strength of Galileo’s stance and rationale, let us see whether we can identify some of the constructive arguments more precisely. I have already mentioned that one of them was probably the tidal argument; but, since he did not elaborate it until later, we shall postpone discussion of it. Instead, let us see whether we can identify any others that were demonstrably in his mind in the earlier period. I believe one can be found in the section dealing with the Earth being motionless of his Treatise on the Sphere, or Cosmography.
It is well known that this Cosmography, which Galileo never published, was not meant to be an original contribution to knowledge, but a concise and elementary introduction to spherical astronomy for beginning students; and it is likely that he used it in both his university courses and his private tutoring. Moreover, it cannot be denied that the work is generally conservative, Aristotelian, and Ptolemaic in content. In regard to its form, however, it has some original and interesting elements. One is the methodological and epistemological introduction, where Galileo explicitly discusses the method of hypothesis, giving the following examples of hypothetical assumptions: that the sky is spherical, that it moves circularly, that the Earth stands still, and that it is located at the center. The other aspect is the impersonal, informational, and noncommittal style which Galileo uses as he discusses the details of traditional astronomy. The Copernican system is explicitly mentioned only once, and this brings us to the passage referred to earlier.
Unlike other sections, this one, entitled “That the Earth Is Motionless,” begins with an admission that this is a controversial question. However, he is quick to add that nevertheless he is primarily going to present some arguments for the geostatic thesis of Aristotle and Ptolemy. Then he goes on to argue why the Earth cannot have rectilinear motion. After that he takes up the question of its possible rotation, concerning which we have the following very important passage:
But, that it may move circularly has more verisimilitude, and therefore some have believed it; they have been moved principally by their considering it almost impossible that the whole universe except the earth should experience a rotation from east to west in the period of 24 hours, and hence they have believed that it is rather the earth which undergoes a rotation from west to east during such a time.5
He does not say whether or why he rejects this argument. Instead, in his usual impersonal style he goes on to summarize the traditional objections from falling bodies, birds, clouds, ship’s mast experiment, and whirling.
The argument in the passage quoted above is important because it is obviously an appeal to Galileo’s own doctrine of natural and neutral motions, which he had elaborated at least as early as 1589–92 in his work On Motion.6 This is the theory according to which there are three basic types of motions: natural motion, or motion where the object approaches its natural place; violent motion, or motion where the object recedes from its natural place; and neutral motion, or motion which is neither natural nor violent. For Galileo, examples of neutral motions would be the rotation of a homogeneous sphere at the center of the universe or the rotation of a sphere around its center of gravity even if the sphere is located elsewhere. Finally, he thinks that to start a body moving with neutral motion, a force as small as you like is sufficient. Given this doctrine, it presumably follows that the Earth would have axial rotation even in an otherwise Ptolemaic universe.
Galileo explicitly recognized this type of argument as one favorable to Copernicus both in the History and Demonstrations Concerning Sunspots, where he argued in support of solar rotation, and in the Dialogue, where he gave it at the beginning of the Second Day in support of terrestrial rotation.7 So it is likely that he had made the connection even in the pre-1609 period.
Finally, some items of negative evidence are also revealing of Galileo’s early stance toward Copernicanism. On May 4, 1600, Tycho wrote a letter to Galileo, but he never answered it. Moreover, Galileo also did not answer the letter dated October 13, 1597 which Kepler wrote after receiving his. Kepler was asking to be informed of Galileo’s Copernican arguments and wanted him to make certain observations to try to detect stellar parallax. There were undoubtedly external factors that contributed to Galileo’s lack of cooperation, such as Kepler being a “heretic,” namely a Protestant. Moreover, as already mentioned, Galileo found Kepler’s approach too metaphysical. And here we can add a more internal and methodological reason: Galileo’s lack of interest in pursuing the astronomical side of Copernicanism, and his dissatisfaction with the strength of the pro-Copernican arguments.
We may summarize the pre-telescopic period by saying that Galileo’s attitude toward Copernicanism was one of partial, qualified, and indirect pursuit. Such pursuit was based largely on its compatibility with his own new physics of motion. However, he neither believed nor accepted Copernicanism as true. Indeed, as he confessed later, he was much more impressed by the observational astronomical objections against it, and deemed them to be strong and unanswerable.