The sixteenth century saw major changes in astronomy and in particular in cosmology. These changes was go on to have a major influence on the development of the theory of mechanics in the seventeenth century. To really understand we need to go back to Aristotle and his systems of astronomy and cosmology.
For Aristotle (384–322 BCE) the cosmos was divided into two spheres by the orbit of the Moon around the Earth. Everything under that orbit, the sublunar sphere, was terrestrial, and everything about it, the supralunar sphere, was celestial. The terrestrial sphere was made up of the four elements–water, earth, air, fire–and was subjected to change and decay. The celestial sphere was made up of the fifth element–aether or the quintessence–and was eternal and unchanging.
In the terrestrial sphere there was both natural and unnatural movement. Natural movement was straight down towards the centre of the Earth, which explained why the Earth was a sphere and was the result of matter returning to its natural place. All other movement was violent or unnatural and had a cause. We have dealt extensively with Aristotle’s theories of motion in other episodes and don’t need to repeat them here. In the celestial sphere there was only natural motion and this was in uniform, circular motion, a theory like the four elements taken over from Plato (c. 428–348 BCE), who had taken them from Empedocles (c. 494–c. 434 BCE). In Aristotle’s cosmology this uniform, circular motion was also homocentric, meaning that it all took place around a common centre, namely the Earth.
Now it is very obvious, when one observes the planets for any length of time, as the Babylonians and the Greek had done, that they do not move in a uniform, circular fashion around the Earth but appear to proceed in one direction, then pause before looping back in the other direction, once again pausing and then continuing in the original direction. We now know that this so-called retrograde motion is actually an illusion created in our heliocentric system when the observed planet is overtaken by the Earth on its faster orbit.
The Ancient Greeks, however, convinced that the cosmos was geocentric did not have this simple explanation for the observed, bizarre behaviour of the planets.
To explain the retrograde motion astronomically, the mathematician and astronomer Eudoxus (c. 390–c. 340 BCE) created the geometrical, homocentric spheres model, which required twenty-seven spheres to map the movements of the then seven known planets. Callippus (c. 370–c. 300 BCE) a student of Eudoxus improved this model by adding more spheres bringing the total to thirty-four. The systems of Eudoxus and Callippus were almost certainly just mathematical models for the purpose of prediction and were not considered by their creators to reflect reality.
Aristotle adopted the homocentric spheres model of Eudoxus and Callippus and also increased the number of spheres to fifty-five, but later thought that there were only forty-seven or forty-nine. Unlike the two mathematicians Aristotle though that the homocentric spheres model reflected the reality in the celestial sphere. For Aristotle the nested spheres that carried the planets were real and were made of aether. These were his famous crystalline spheres. In Aristotle’s model the spheres are actually in contact with intervening spheres connecting the spheres of two neighbouring planets and the spheres of Saturn being connected to the sphere of the fixed stars. Outside of the sphere of the fixed stars was the unmoved mover, who sets the whole apparatus in motion like a giant friction drive toy. The sphere of the fixed stars drives Saturn, which in turn drives Jupiter, which drives Mars and so on down to the Moon. So, what drove this whole machine? Aristotle conceptualised the unmoved mover as a being that functioned as an object of love and desire for the soul that animated the body of the outermost sphere of fixed, the primum mobile. Hence, the expression, love makes the world go round.
It is important to note that because the celestial sphere was eternal and unchanging, Aristotle thought that comets were a sublunar meteorological phenomenon within the atmosphere. Opposed to Aristotle the Stoics did not divide the cosmos into two but regarded it as one, the total being filled with pneuma. For the Stoics, comets were supralunar, so the celestial sphere, which contained no crystalline spheres was subject to change.
The homocentric model of planetary motion had various problems explaining observed astronomical phenomena and so it got replaced by deferent-epicycle model first suggested by Apollonius (c. 240–c. 190 BCE), developed by Hipparchus (C. 190–c. 120 BCE) and then brought to completion by Ptolemaeus (fl. 150 CE) in his Mathēmatikē Syntaxis. The deferent-epicycle model contradicted Aristotle’s cosmological homocentric principle but Ptolemaeus, in his Planetary Hypotheses actually embedded each of the planetary deferent-epicycle models in a crystalline sphere, like Aristotle producing a set of nested spheres.
In the High Middle Ages, Albertus Magnus (c. 1200–1280) and his pupil Thomas Aquinas (c. 1225–1274) made the philosophy of Aristotle acceptable to the Catholic Church, who then adopted it, including his cosmology, as their official description of the physical world. Aristotle’s unmoved mover being obviously the Christian God. However, they didn’t adopt his astronomical model, preferring instead that of Ptolemaeus. As already noted above this contradicted Aristotle’s central cosmological dogma of homocentricity. However, this was not a problem for the scholastics, who took the philosophical cosmology as describing reality, whereas the mathematical astronomy was merely a tool for calculating the position of the celestial bodies for calendars, horoscope and other astronomical and astrological purposes.
For the next two centuries things ticked along quite smoothly with a couple of suggestions being thrown up and discussed concerning celestial motion. One theory going back to Aristotle’s unmoved mover was that the planets were animated bodies with souls, where animated means filled with life, running on eternally, so to speak. Another suggestion from the theological side was that each planet had an angel, who pushed it around its orbit. A third model that I’ve never really investigated, but which goes back to the Stoics, was that the heavens were fluid and the planets swam around their orbits like fish. A variation on the animated theme.
Unfortunately, things started to go awry in the sixteenth century. Already in the late fifteenth century Paolo dal Pozzo Toscanelli (1397–1482) traced the path of a comet across the heavens concluding that it was supralunar.
Then inn the 1530s there was a series of spectacular comets, which attracted the attention of the new class of European astronomers and their observations led to new developments. A fairly large group of well trained astronomers began to speculate on the nature of comets and came to the conclusion that they were not sublunar are stipulated by Aristotle but supralunar, that is in the celestial sphere, and as a consequence the celestial sphere was definitely not eternal and unchanging. It is not just chance that these debates too place during a major renaissance of Stoic philosophy.
A debate had begun and the next comets were eagerly awaited. Unfortunately, when the next spectacular comet appeared over Europe in 1556, one generation of capable astronomers was already dead and the next one was still in its childhood (Tycho was ten and Mästlin was six years old) or in the case of Kepler not yet born. However, the 1570s did not disappoint, delivering up a supernova in 1572 and a spectacular comet in 1577.
Both were observed extensively by astronomers throughout Europe and as accurate as possible parallax measurements were made. The resounding conclusion of the leading astronomers of the age, Tycho Brahe (1546–1601), Michael Mästlin (1550–1631) and Thaddaeus Hagecius ab Hayek (1525–1600) and above all the leading Catholic Church astronomer, Christoph Clavius (1538–1612) all confirmed that both celestial phenomena were clearly supralunar. Aristotle was wrong, the heavens were not eternal and unchanging.
There was, however, another important nail in the coffin of Aristotle’s carefully constructed cosmological structure. If the observed comet was supralunar, then given the path it had been observed to follow, either it had smashed its way through several crystalline spheres or those crystalline spheres simply didn’t exist.
The situation was made even more complex by the fact that Copernicus (1473–1543) had argued not unconvincingly that the cosmos wasn’t actually geocentric at all, but rather heliocentric. To counter the problem that there was no evidence that the Earth actually moved, Tycho Brahe then suggested that the cosmos was a geo-heliocentric system. The debate raged and went into overdrive, when at the end of the first decade of the seventeenth century, the first telescopic astronomical observations showed that at least Venus orbited the Sun, excluding a pure geocentric system, but still allowing either a heliocentric or a geo-heliocentric one, and the Moon was shown to be like the Earth and not a perfect sphere made of aether, as stated by Aristotle.
All of these developments led to two very pertinent questions, firstly if the crystalline spheres don’t exist and the heavenly bodies are earth like and not made of aether, what keeps them in the heavens and what causes them to orbit the same? On the first question, Cesare Cremonini (1550 – 1631), the hard core Aristotelian, Paduan professor of philosophy and one of Galileo’s drinking mates and sparing partner argued quite correctly, that if the moon was really a large lump of rock as Galileo claimed then he, Galileo, would have to develop a new physics to explain why the moon didn’t fall on the earth. He Cremonini would wait until that new physics existed before accepting Galileo’s discoveries.
I don’t know whether Cremonini was aware that he was repeating an argument that had first been broached in the fifth century BCE by Anaxagoras (c. 500–c. 428) who had first suggested that the planet were made of stone and according to Plutarch “Anaxagoras is said to have predicted that if the heavenly bodies should be loosened by some slip or shake, one of them might be torn away, and might plunge and fall to earth.” This concept is said to have occurred to Anaxagoras when a spectacularly large meteorite, described as brown in colour and the size of a wagon load. fell to earth near Aegospotami, in 467 BCE.
Following all the developments in the sixteenth century and their consequences, in one thing Cremonini was totally correct, not only Galileo but all of the natural philosophers in the seventeenth century would have to work on a new celestial mechanic which was desperately needed to replace that of Aristotle and that is, as we shall see in further episodes exactly what happened.