2009 is the International Year of Astronomy and it was chosen as such because of three anniversaries, firstly it is forty years since the first moon landing and secondly it is four hundred years since Kepler published his Astronomia Nova with his first two laws of planetary motion and in the same year the first ever telescopic astronomical observations were made; the latter were made by several European astronomers but it is the observations of the Tuscan mathematician Galileo Galilei that everyone knows and remembers because he was the first to publish. Galileo famously published the first account of his observations in a small pamphlet titled Sidereus Nuncius, which translates both as Starry Messenger and the Starry Message. This publication was the start of the path that was to lead Galileo into conflict with the Roman Catholic Church and eventual to his trial and condemnation. In discussions on Galileo and his struggle with the Church many strange and wild claims are made concerning the Sidereus Nuncius and what exactly it does or doesn’t prove; in what follows I sketch the contents of his treatise and explain their consequence for the astronomical theories at the beginning of the 17th century.
Before starting on the Sidereus Nuncius it is first necessary to outline the mainstream views held on astronomy by those qualified to have such views in or around 1610. In 1610 the majority of European astronomers held a complex of views on matters celestial that were an uneasy marriage of Aristotelian cosmology, Ptolemaic astronomy and Christian belief. This combination of theories was not without problems as Aristotelian cosmology and Ptolemaic astronomy were actually contradictory in several central points and both, as heathen philosophical creations, were not exactly attuned to Christian theology. Copernican heliocentric cosmology was only accepted by a very small minority of astronomical experts, including Galileo, as there were very serious empirical physical problems that needed to be solved before a moving Earth could envisaged; problems that would first be solved during the 17th century starting with Galileo’s own work on the laws of fall. Unfortunately there was also no form of empirical proof for heliocentrism. The alternative heliocentric-geocentric system of Tycho Brahe which combined the positive aspects of Copernican astronomy, his explanation of planetary retrograde motion, his determination of the planetary order etc., whilst eliminating the problems of terrestrial motion had at this point very few adherents. The discoveries made by Galileo with his telescope were thus to be measured against the claims of the Aristotelian, Ptolemaic, Christian synthesis.
The Sidereus Nuncius contains three major new discoveries made by Galileo with his telescopes, the fact that the moon was physically like the earth, that lighted patches in the night sky such as the Milky Way resolved into stars when viewed with the telescope and the real sensation that Jupiter had four moons of its own.
Through accurate drawings of the moons surface as seen through his telescope and convincing argumentation Galileo was able to demonstrate that the moon’s surface was three dimensional with mountains and valleys just like the earth. It is interesting in this context that it appears to be his formal training as an artist that enabled Galileo to reach this conclusion. Thomas Harriot who had observed and drawn the moon telescopically before Galileo had only perceived it as a flat two-dimensional surface and only after he had read the Sidereus did his moon maps gain an extra dimension. This discovery didn’t have immediate consequences for the Ptolemaic astronomy but was a serious blow for the Aristotelian cosmology. Aristotle had claimed that everything below the moon consisted of combinations of the four elements, fire, air water and earth, was impermanent and was subject to change and corruption. However everything in the heavens, including the moon, consisted of a fifth element, the quintessence, was permanent and incorruptible. Galileo’s discovery of the similarity between earth and moon seriously damaged this claim. Interestingly Stoic philosophy which was much more prevalent than Aristotelian philosophy in the late classical period did not accept the Aristotelian division between the sub-lunar and supra-lunar realms but regarded both as equal in their constituents. The Stoic philosophy had been enjoying a strong revival in the 15th century so the loss of the Aristotelian cosmology on this point was not necessarily such a big disaster. Also the chief proponent and defender of the Ptolemaic astronomy at this time, Christoph Clavius had proved himself very flexible when it came to conflicts between the Aristotelian and Ptolemaic systems. If necessary he was prepared to re-interpret, modify or abandon aspects of the Aristotelian system.
Galileo’s second big discovery, that there were thousands of stars in the heavens that could not be seen with the naked eye was not actually new. The French flyer published in 1608, which described the visit of the Thai Ambassador to the Court of Prince Maurice of Nassau in Den Haag also reports the first public appearance of the telescope as Lipperhey, its inventor, demonstrated it to the Prince. The description contains the following passage:
…& even the stars which normally are not visible for us, because of the scanty proportion and sight of our eyes, can be seen with this instrument.
This discovery had no direct consequences for either the Ptolemaic astronomy or the Aristotelian cosmology but serious ones for the Christian theology. Christian theology had a very strong teleological element that claimed that mankind was God’s creation and had dominion over the whole world and everything in the world had been placed there purposefully for mankind’s use. What function could stars possess that could not be perceived with the naked eye? If the other problems posed by the new astronomy had not rolled over Christianity so rapidly I’m sure that the wily theologians would have been able to resolve this problem.
Galileo made a second claim concerning the stars. He said that his telescope could resolve the planets to hard edged discs but not the stars therefore the stars must be much further distant, therefore proving Copernicus right who had argued that the stars were so far distant that it was not possible to detect stellar parallax. Interestingly Simon Marius, the Ansbacher Court Astronomer, who was making his telescopic observations at the same time as Galileo, but did not publish them till 1614, claimed that the stars did resolve to hard points thereby disproving Copernicus and confirming Tycho, whose helio-geocentric system Marius supported. We now know that the quality of the lens in the early telescopes was so poor that it was totally impossible to make any definitive statement concerning stellar resolution and that the two rivals saw what they wanted to see.
Galileo’s third discovery is the one that caused the greatest sensation and which he believed offered the most support for Copernicanism, the discovery of the four largest moons of Jupiter. As a footnote Marius discovered them exactly one day later than Galileo. This was again a serious blow for the Aristotelian cosmology that stipulated that all of the heavenly bodies rotated around a single common centre, that is were homocentric. The moons of Jupiter revolved around Jupiter and not around the earth or any other central world point. This was less of a problem for the Ptolemaic astronomy as it violated this principle itself. In the Ptolemaic system planets were carried on small circles (epicycles) whose middle points were carried on large circles (deferents) whose centre was the earth (it wasn’t actually but that needn’t bother us here). Aristotelian opponents of the Ptolemaic astronomy pointed out that the planets were definitely not revolving around the centre of the world. This objection had been at the centre of the cosmology of the Islamic philosopher Averroes in the 12th century and had entered Europe with his philosophy in the High Middle Ages. Averroists proposed a return to the homocentric astronomy of Aristotle a call that was echoed by the Italian astronomer Fracastoro and others in the 16th century. Clavius regarded the homocentrists as at least as great a threat to his Ptolemaic astronomy as the heliocentrists and this was one of the Aristotelian principles that he was more than prepared to abandon.
It should be clear by now that the Sidereus Nuncius although causing a great stir throughout Europe in no way proved conclusive in the struggle between the competing systems of the world as they stood when it was published in 1610, however our story does not end here.
In the next three years the pioneers of telescopic astronomy made two further major discoveries, sunspots and the phases of Venus. The first of these was principally the same as the lunar surface it was further proof that the heaven were indeed corruptible. The sunspots also demonstrated that the sun rotated on its axis providing on the one hand indirect support for the concept of terrestrial diurnal rotation and at the same time support for the astro-physics of Kepler that required a rotating sun as driving force for his world system. The second of these new discoveries was devastating for the Ptolemaic astronomy. All three systems the geocentric, the heliocentric and the helio-geocentric required Venus and Mercury to display phases like the moon because of their proximity to the sun. Those observed independently by various astronomers in those years definitely ruled out the Ptolemaic system. This led the majority of European astronomers to adopt the helio-geocentric system of Tycho by 1620 as the physical problems associated with a moving earth were still a long way from being solved.