Copernicus was born in Poland on 19 February, 1473, to a noble and influential family. When his father died ten years later, he was placed under the patronage of his maternal uncle, who would later become the Prince-Bishop of Warmia. Copernicus discovered his passion for mathematics and astronomy while studying at the University of Krakow from 1491-1495. It was likely there that he was first introduced to the problem of logical inconsistencies between the Aristotelian worldview and the Ptolemaic model, perhaps the most notable being the latter’s use of equants while the former held that celestial motion should be uniform circular motion.
During the next eight years, Copernicus studied both canon law and medicine in Italy, while also pursuing his interests in astronomy. When he returned to Warmia in 1503, he had very likely already begun seriously considering the heliocentric hypothesis. From that time until his uncle’s death in 1512, Copernicus served as his secretary, assisting with legal/political and economic matters, as well as acting as his physician. Afterwards, he moved to Frombork, where he worked in a similar capacity at the cathedral for the rest of his life.
Sometime before 1514, Copernicus distributed a brief outline of his heliocentric hypothesis to a number of close associates, a document which is now referred to as the Commentariolus. The Commentariolus contained seven basic postulates, ranging from there being no one centre of circular motion (e.g., the Moon orbiting the Earth, while the Earth and other planets orbit the Sun), to postulates pertaining to a fixed sphere of stars that remains unchanged and at a distance so great that the Earth-Sun distance is imperceptible in comparison, while the Earth spins once daily and orbits the Sun once a year, causing the apparent motion of the Sun with respect to the fixed stars, and finally to a postulate that the apparent retrograde motion of the planets was actually due to the Earth’s uniform motion about the Sun. Thus, the Commentariolus contained all the basic theoretical elements of heliocentrism, which, Copernicus purported, could explain the apparent motions of the Sun, the Moon, the stars and the planets, through a system of only 34 circles.
Over the next 30 years, in addition to his regular duties to the Frombork Cathedral, Copernicus privately pursued a quantitative heliocentric model, collecting many observations as well as developing the mathematical details of his theory. After the Commentariolus, word of Copernicus’ theory did spread and, following a series of lectures in 1533 by Pope Clement VII’s secretary, which outlined the theory for the Pope and a number of his Cardinals, Copernicus received requests from Rome to communicate his work. Despite similar urgings from all over Europe, Copernicus delayed publishing his book, De revolutionibus orbium coelestium, until 1543, the same year that he died.
In his preface to the De revolutionibus, Copernicus argued that ancient astronomy had failed to produce a model that did not compromise its own principles (e.g. Aristotle’s uniform circular motion vs Ptolemy’s equant), and that the geocentric hypothesis had proven incapable of leading to a single discernable description of the shape of the universe and its symmetries. In comparing the incongruous aspects of models that had been used by geocentrists, Copernicus conjured imagery likening the result to Frankenstein’s monster:
With them it is as though an artist were to gather the hands, feet, head and other members for his images from diverse models, each part excellently drawn, but not related to a single body, and since they in no way match each other, the result would be monster rather than man (Kuhn, 1957).
In essence, Copernicus argued that geocentrism should be discarded because it was unable to produce a single, self-consistent physical model of the universe based upon well-defined first principles, which led to a sufficiently accurate description of the celestial phenomena. He concluded that there must be a fundamental flaw in the approach because invariably the models it produced were monstrosities. This was his justification for proposing a heliocentric model.
The heliocentrists who would eventually follow Copernicus—in particular, Kepler and Galileo—did so because they saw in the heliocentric system the potential to explain many of the phenomena more naturally. For example, in the geocentric models there was no known reason why the deferents of Mercury and Venus should be tied to the Sun, so that those two planets would orbit a point somewhere along the line connecting the Earth and the Sun. Similarly, at least at a qualitative level in the heliocentric system the retrograde motion of the three outer planets could be explained without the need for epicycles.
However, there is little accuracy in a seven circle (six planets, plus the Moon) Sun-centred (except for the Moon) system, and many of the benefits of the heliocentric system that we so easily identify today were not truly benefits of Copernicus’ theory. In practice, Copernicus employed just as many epicycles and eccentrics as there had been in the Ptolemaic model, and his model was no more accurate. With as many epicycles in Copernicus’ system, one could hardly argue that it gave a more “natural” explanation of planetary retrogression.
Learning Activity
Today, we tend to think of Copernicus’ explanation of retrograde motion as “more natural,” since it was qualitatively described as an effect due to parallax rather than an actual physical motion about a point bearing no other significance, and for a reason that was left unexplained. However, Copernicus’ model involved just as much of this unexplained motion around otherwise insignificant points, and therefore can hardly be considered “more natural” in that regard. A simple exercise serves to make this point clear. In the Copernican model, the Earth did not really orbit the Sun; the Earth orbited an otherwise insignificant point that followed an epicycle around a deferent that was centred on the Sun. On a piece of paper, draw these three circles:
- beginning with the Earth’s orbit, followed by the point at the centre and a smaller epicycle for that point to move along;
- then draw the slightly larger deferent to that epicycle, still within the Earth’s orbit;
- finally, draw a point at the centre of the deferent and label it with an “S” for Sun.
Having drawn the deferent along which the epicycle of the imaginary point that the Earth orbits in Copernicus’ “heliocentric” model, you should be cured of any illusion that planetary motion within the Copernican system was somehow “more natural.” However, in case you are not, you should also note that while the apparent retrograde motions of the planets were not described as a consequence of actual motion along an epicycle in the Copernican model, the planets nevertheless did follow epicycles around deferents, and these were all situated with the moving centre of Earth’s orbit eccentric to them.
The physics that Copernicus’ universe (see Figure 3-4) was based upon was every bit as Aristotelian as the physics behind the Ptolemaic model, and both had to make unexplained modifications in order to accurately describe the phenomena. For instance, both models assumed the Earth was situated at the centre of gravity. That’s right: the Earth was a universal centre of gravity even in the Copernican model; therefore, while Copernicus was bothered by the fact that Ptolemy had incorporated the equant into his theory, causing the circular motion to be not truly uniform, a geocentrist might have been equally troubled by Copernicus’ requirement that the centre of gravity in the universe was constantly in motion.
One further aspect of modern heliocentrism should be discussed in contrast to Copernicus’ model. Today, we understand that the seasons occur on Earth because the axis of Earth’s rotation is tilted with respect to its orbital plane. As the Earth orbits the Sun, apart from its very slow precession, the Earth’s rotational axis maintains its orientation, moving always parallel to itself. This is due to the Earth’s inertia: since nothing is causing the Earth’s rotational axis to dramatically change direction, it pretty much stays pointed in the same direction. When one’s hemisphere (north or south) is pointed towards the Sun, it is summer; and when one’s hemisphere is pointed away from the Sun, it is winter. The seasons are thus explained as a consequence of Newton’s law of inertia, which in this case tells us that the angle and rate of Earth’s spin should not change unless some force were to cause that to happen.
Copernicus was not a student of Newtonian physics, but of Aristotelian physics, which is both very different and largely wrong. In this instance, according to Aristotelian physics Copernicus expected that the Earth should be carried around the Sun within its own solid crystalline rotating sphere. From this perspective, he had expected that the Earth’s axis should spin 360° as it went around the Sun, e.g. like a person on a merry-go-round looking outward will spin 360° as it rotates. In order to compensate for this, and to explain the seasons, Copernicus had to introduce a counter-rotation. By turning the tilted Earth 360° in the opposite direction as it went around the Sun—just as a person on a merry-go-round would need to do in order to keep their eyes trained always in one particular direction as it spins around—Copernicus explained the seasons. Thus, Copernicus added a third motion to the Earth, on top of its orbit and daily spin.
Copernicus was not a creative, revolutionary thinker. He believed that the universe was pretty much as described by Aristotle, but with the Sun at the centre of things rather than the Earth. He based this proposal on the fact that the Earth-centred models had invariably turned out to be monstrosities, and he believed he could fix that with a heliocentric model that respected all the old rules of Aristotelian physics. But it turns out that the geocentric approach was only part of the problem with the Ptolemaic system, and that new physics would be required as well before an accurate explanation of the observed celestial motions could be found.
While Copernicus did not produce a more accurate model than Ptolemy, and did not even begin to imagine the new physics required to get there, he had done his part by recognising that the Ptolemaic model bore the signs of a fundamentally flawed theory: it was indeed a monstrosity carefully crafted by bringing together mathematically beautiful parts such as Apollonius’ theorem (Module 2), which somehow failed to fit together properly when constrained by observation, so that the result appeared more like a monster than a man—and one which, even so, lacked an acceptable degree of precision. Furthermore, although Copernicus himself had failed to reconcile all the potential benefits of his central hypothesis (e.g., explaining the seasons and retrograde motion) with his mathematical theory, that potential to explain nevertheless remained. It would provide a heuristic basis to others who would later take up Copernicus’ hypothesis and his criticism and complete the revolution that he had begun.
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