What was new about the astronomy of Copernicus and what arguments did Galileo use to support it?

Anna Dadayan

What was new about the astronomy of Copernicus and what arguments did Galileo use to support it?

Nicolas Copernicus is often regarded as one of the key personalities in the history of astronomy. The publication of his work On the Revolutions of the Heavenly Spheres in 1543 is generally considered to be the beginning of the Scientific Revolution, the historical event that gave birth to modern science. Copernicus’s heliocentric system subsequently caused major changes in astronomy by influencing other prominent thinkers of that period, in particular, Galileo Galilei, who is renowned for his contribution to the establishment of the modern vision of the universe. The purpose of this essay is to investigate the Copernican heliocentric system, identifying its differences from the prevailing view at that time: the Ptolemaic system, and examine how Galileo supported Copernicus’s ideas.

It is essential to outline the historical context in which Copernicus created his astronomical system. The period under review is known as the Scientific Renaissance, (“rebirth”, French), which is a European phenomenon that began in fifteenth- century Italy and is often considered to be a bridge that connected the Middle Ages and the Modern World. One of the main components of this historical period was humanism, a philosophical movement, characterized by its profound interest in Antiquity and revival of classical learning. Rediscovery of Ancient Greek texts was a crucial factor that contributed to the establishment of the movement. Apart from that, it is essential to emphasize the importance of courts at this time. They became a new center of scientific activity and played an important role in encouraging discoveries in science by providing patronage to natural philosophers. (Moran, 2008) Astronomy was of particular importance to princes in this period, ‘rulers needed astrological predictions, which required knowledge of planetary positions, and physicians used the stars to plot the likely course of a disease. ‘(Donahue, 2008, p. 563)

Nicolas Copernicus, as a genuine representative of humanism, had diverse interests and pursued careers in a variety of disciplines. As a humanist, he dedicated himself to reading ancient texts, but instead of just repeating the work of the Greeks, in particular, Ptolemy’s work, he used their methods to create his own concepts. (Dear, 2009) In order to understand the novelty of his ideas we need to examine the Ptolemaic system, the dominating vision of the cosmos at that time.

The roots of Ptolemy’s astronomy can be traced back to the times of Plato, when the idea of change as something imperfect and negative came into existence. Subsequently, the Ancient Greek, who considered the heavens to be perfect started to regard them as stable, unchanging. (Henry, 2001) Aristotelian physics, the core of medieval education, was based on the idea of radical division of the universe into the terrestrial and celestial realms, which consisted of different elements and had distinct qualities. They had also different “natural motions”, while the terrestrial realm was characterized by straight, linear motion, for the heavens the motion was considered to be uniform and circular. (Lloyd, 1968)

Ptolemy, a Greco-Roman astronomer of the second century AD, devised his astronomical constructions using the knowledge of his predecessors. His purpose was not to discuss the physical reality, but to create a system that could be used by people of other professions, such as physicians and philosophers, to calculate the motions of heavenly bodies at any time. (http://microcosmos.uchicago.edu/ptolemy/astronomy.html) Ptolemy’s astronomical system was geocentric: the Earth was placed in the center of the universe and the Sun, Moon and the other planets were moving around it. It used epicycles, according to which the planets moved in a smaller circle, which in turn rotated around the larger circle. The equant, another important element in Ptolemy’s system, assumed that the planets were seen to be moving with a uniform speed from a particular point, the equant point, which was not located in the center of the construction and was opposite the eccentric the Earth. (http://galileo.rice.edu/sci/theories/ptolemaic_system.html). Thus, his models allowed him to explain the retrograde motion of the heavenly bodies and the variations in their speed. Despite the fact that Ptolemy intended to conform to the physical laws of Aristotle, his construction, the equant, in particular, violated the perfect circular motion. It is often considered to be the primary reason that motivated Copernicus to create his own system of the universe. So, what changes did he make to the Ptolemaic vision of the cosmos?

First of all, the Copernican system was heliocentric: according to Copernicus, the Sun was located in the center of the universe and the Earth moved around it in a circular motion. To support his model, Copernicus made a «typical humanistic move – canvassed the authorities», he stated in his work that many important figures of the Classical World, Hicetas, for instance, believed in the heliocentric system of the universe. (Dear, 2009, pp. 34-35) In order to get rid of the equant, Copernicus had to add extra epicycles, which were used to account for the speed variations of the planets’ motion in his system. (Henry, 2001) What is remarkable about Copernicus’s model is that it provided a convincing argument concerning the retrograde motion, which was explained by the annual motion of the Earth. Apart from that, it also set the order of the planets, and, what is more, its mathematical constructions could also be used by astronomers within the geocentric system. (http://galileo.rice.edu/sci/theories/copernican_system.html#1).

Copernicus’s new system, however, was very similar to the Ptolemaic model, as it still used epicycles and was based on circular motion. His work was praised by his contemporaries primarily because of his achievement to construct a sophisticated system of astronomy, using the methods of Ptolemy, which could be effectively used to gather astronomical data. (Dear, 2009) Nevertheless, Copernicus’s system was not accepted at that time, since it could not answer several questions and generated further problems. First of all, it could not be explained in terms of familiar experience and was incompatible with Aristotelian physics, the core concept of which was based on the idea that the Earth was the center of the universe. Furthermore, there was also a problem related to the absence of stellar parallax, the necessary evidence of the Earth’s movement (http://galileo.rice.edu/sci/theories/copernican_system.html#1).

It was Galileo, an Italian natural philosopher of the seventeenth century, who commenced the controversy between the two systems of the universe, a hundred years later since the appearance of Copernicus’s model, and managed to support the heliocentric system with a number of arguments. Being a mathematician, he was convinced that mathematics was the foundation of science and, in contrast to Copernicus, was against Aristotelianism and its concepts of natural and violent motions. Galileo is known to have dedicated his work called Sidereal Messenger (1610), which gave a description of his astronomical discoveries, to the Grand Duke of Tuscany, who subsequently became Galileo’s patron. Later, in 1633 he published his work, the Dialogue, which provided a comparison of the two systems of the universe and showed that the case for the Copernican model was more convincing. (Finocchiaro, 2009)

So, what arguments did Galileo use to support the heliocentric system of Copernicus? After perfecting his telescope Galileo was able to observe the sky in unprecedented detail, and, while examining the surface of the Moon, he could see that it was rough and imperfect. Thus, Galileo practically destroyed the hoary myth of its divinity and perfection. His discoveries showed that the Moon and the Earth were in many respects similar, which Galileo used as an argument for heliocentrism, since “if the solid and the earthlike Moon could move about the Earth, the bright and moonlike Earth could move about the Sun” (Swerdlow, 1998, p. 250) Among his other discoveries were the phases of Venus, showing that it was going around the Sun, the satellites of Jupiter, which in turn illustrated that other planets could also have moons and that the Earth was not the center of the motion of all heavenly bodies. Apart from that, Galileo also observed sunspots, the variation of speed of which could be explained by the motion of the Earth. (http://galileo.rice.edu/sci/theories/copernican_system.html#1) Galileo’s research allowed him to see the infinite number of stars, which, in contrast with the planets, were barely magnified with the telescope. The fact that they were more distant from the Earth than previously assumed could explain «the one purely astronomical objection to the motion of the Earth about the Sun»: absence of stellar parallax. (Swerdlow, 1998, p. 252) In addition to this, Galileo had other arguments supporting Copernicanism, such as the erroneous theory that tides were caused by the Earth’s movements. (Swerdlow, 1998)

Galileo faced opposition from some clerics and other natural philosophers, who used the Bible to argue against his ideas. In his letter to the Grand Duchess Christina (1615), which deals with the relations between science and religion, he also gives additional arguments supporting Copernicanism. He argues that the biblical passages should not be interpreted literally, as they are abstract and were written in the manner “to accommodate them to the capacities of the common people”, because, otherwise, it would have caused confusion and would have prevented them from understanding the sacred message. According to him, the Bible did not oppose Copernicanism, it just considered the question of the motion of heavenly bodies irrelevant to salvation. He backs his argument quoting Cesare Baronio, who said: «The Bible teaches us how to go to heaven, not how the heavens go». Also, Galileo claims that «the holy Bible and the phenomena of nature proceed alike from the divine word», therefore his observations should not be considered contradictory to God’s order. (http://www.fordham.edu/halsall/mod/galileo-tuscany.asp)

To conclude, I would like underline that the difference between the system of the universe created by Copernicus and Ptolemy was that the former placed the Sun, not the Earth at the center of the construction. However, as Copernicus himself stated, the idea of heliocentrism was not new and had had its supporters in Antiquity. Furthermore, he rejected the concept of the equant, as it violated the idea of Aristotle’s perfect circular motion, and therefore had to add more epicycles to his model in order to explain the anomalistic behavior of heavenly bodies. Despite these changes, Copernicus, as a humanist, imitated Ptolemy’s work and used his methods, and therefore the two systems were still similar in many respects. Galileo’s unprecedented research undermined Aristotle’s physics, as it destroyed the idea of the perfection of heavens and his discoveries – the surface of the Moon, the phases of Venus, the stars, sunspots and the moons of Jupiter- were used to support Copernicus’s heliocentric system.

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