Copernican Revolution

The Almagest

The idea of heliocentrism - a Sun-centered Universe - can be traced back to Aristarchus of Samos, a Hellenistic author writing in the 3rd century BC, who may in turn have been drawing on even older concepts in Pythagoreanism. Ancient heliocentrism was, however, eclipsed by the geocentric model presented by Ptolemy in the Almagest^[1] and accepted in Aristotelianism.

Martianus Capella (5th century CE) expressed the opinion that the planets Venus and Mercury did not go about the Earth but instead circled the Sun.^[2] Capella's model was discussed in the Early Middle Ages by various anonymous 9th-century commentators^[3] and Copernicus mentions him as an influence on his own work.^[4] Macrobius (420 CE) described a heliocentric model.^[5] John Scotus Eriugena (815-877 CE) proposed a model reminiscent of that from Tycho Brahe.^[5]

European scholars were well aware of the problems with Ptolemaic astronomy by the 13th century. The debate was precipitated by the reception by Averroes's criticism of Ptolemy, and it was again revived by the recovery of Ptolemy's text and its translation into Latin in the mid-15th century.^[a] Otto E. Neugebauer in 1957 argued that the debate in 15th-century Latin scholarship must also have been informed by the criticism of Ptolemy produced after Averroes, by the Ilkhanid-era (13th to 14th centuries) Persian school of astronomy associated with the Maragheh observatory (especially the works of Al-Urdi, Al-Tusi and Ibn al-Shatir).^[7]

The state of the question as received by Copernicus is summarized in the Theoricae novae planetarum by Georg von Peuerbach, compiled from lecture notes by Peuerbach's student Regiomontanus in 1454 but printed only in 1472. Peuerbach attempts to give a new, mathematically more elegant presentation of Ptolemy's system, but he does not arrive at heliocentrism. Regiomontanus himself was the teacher of Domenico Maria Novara da Ferrara, who was in turn the teacher of Copernicus.

There is a possibility that Regiomontanus already arrived at a theory of heliocentrism before his death in 1476, as he paid particular attention to the heliocentric theory of Aristarchus in a later work, and mentions the "motion of the Earth" in a letter.^[8]

Purging the Equant

Copernicus studied at Bologna University during 1496–1501, where he became the assistant of Domenico Maria Novara da Ferrara. He is known to have studied the Epitome in Almagestum Ptolemei by Peuerbach and Regiomontanus (printed in Venice in 1496) and to have performed observations of lunar motions on 9 March 1497. An early short work, Commentariolus, written some time before 1514, circulated in a limited number of copies among his acquaintances.^[9][10]

In De revolutionibus orbium coelestium, published in 1543, Copernicus attempted to align his work as closely as possible with Ptolemaic tradition. A comparison of his work with the Almagest shows that he followed Ptolemy's methods^[11] and even his order of presentation.^[12] Yet, in order to purge astronomy of the equant - which violated the theological and philosophical ideal that all celestial motion must be perfect and uniform^[13] - Copernicus challenged Ptolemy’s geocentrism, an orthodoxy that had prevailed for over a millennium. Copernicus' heliostatic model (with a stationary Sun located near, though not precisely at, the mathematical center of the heavens^[14][15][16]) retained several false Ptolemaic assumptions such as the planets' circular orbits, epicycles, and uniform speeds,^[17] but also included accurate ideas such as:

• The Earth is one of several planets revolving around the Sun in a determined order.
• The Earth has three motions: daily rotation, annual revolution, and annual tilting of its axis.
• Retrograde motion of the planets is explained by the Earth's motion.
• The distance from the Earth to the Sun is small compared to the distance from the Sun to the stars.

In The Sleepwalkers, Arthur Koestler wrote that De revolutionibus orbium coelestium "was and is an all-time worst-seller."^[18] Discovering a first edition of De revolutionibus that had been extensively annotated by the leading teacher of astronomy in Europe in the 1540s - which seemed to contradict Koestler - astronomer and science historian Owen Gingerich spent three decades tracking down and personally examining all existing first and second editions of Copernicus' major work. He not only established that De revolutionibus was widely read by 16th century astronomers but also what they thought of it:

Reinhold and his many followers admired Copernicus for a quite different aesthetic idea, the elimination of the equant. Copernicus devoted the great majority of De revolutionibus to showing how this could be done. While he had eliminated all of Ptolemy's major epicycles, merging them all into the Earth's orbit, he then introduced a series of little epicycles to replace the equant, one per planet. Because this made the motion uniform in each Copernican circle, the anti-equant aesthetic was satisfied. My Copernican census eventually helped to establish that the majority of sixteenth-century astronomers thought eliminating the equant was Copernicus' big achievement, because it satisfied the ancient aesthetic principle that eternal celestial motions should be uniform and circular or compounded of uniform and circular parts.^[19]

Copernicus' challenge reached 16th-century astronomers but failed to displace the dominance of Ptolemy's geocentrism, which only fell out of favor among astronomers after Galileo's telescopic observations of 1610.^[20] But Copernicanism did gain a handful of supporters in the 16th century. Thomas Digges^[21] and Giordano Bruno^[22] used Copernicus' new estimate of the distance to the stars to argue for an indefinitely extended or even infinite universe in opposition to the ancient orthodoxy of celestial spheres. William Gilbert also argued (correctly) that Copernicus was right about the Earth rotating on its axis (instead of an outer "shell" of rotating stars) while also arguing (incorrectly) that the mechanism of the Earth's rotation is magnetism.^[23]

Protestant Attacks

Copernicus was a canon, a lifelong official of the Catholic Church. Even before his death in 1543 and during the following 70 years (until 1610), his model faced withering criticism from Protestant leaders who were locked in combat with the Church, were often animated by a fierce anti-clericalism and typically adopted a literalist approach to Scripture. Protestant leaders Martin Luther and Philip Melanchthon both attacked Copernicus. Luther famously cited the Book of Joshua to prove the sun moves and reportedly called Copernicus a "fool."^[24] His colleague Melanchthon urged governments to repress the "absurd" theory.^[25] Meanwhile, the Catholic Church indirectly used Copernican mathematics in its reform of the Gregorian calendar in 1582 and otherwise, until 1610, remained officially silent on either the merits or demerits of Copernicanism.^[26]

The Telescope

In 1610 Galileo Galilei observed with his telescope that Venus showed phases, despite remaining near the Sun in Earth's sky (first image). This proved that it orbits the Sun and not Earth, as predicted by Copernicus' heliostatic model and disproved the then conventional geocentric model (second image).

Galileo Galilei, sometimes referred to as the "father of modern observational astronomy,"^[27] developed his own telescope with enough magnification to allow him to study Venus and discover that it has phases like a moon. His improvements to the telescope, astronomical observations, and support for Copernicanism were all integral to the Copernican Revolution.

Based on the designs of Hans Lippershey, Galileo designed his own telescope which, in the following year, he had improved to 30× magnification.^[28] Using this new instrument, Galileo made a number of astronomical observations which he published in the Sidereus Nuncius in 1610. In this book, he described the surface of the Moon as rough, uneven, and imperfect. He also noted that "the boundary dividing the bright from the dark part does not form a uniformly oval line, as would happen in a perfectly spherical solid, but is marked by an uneven, rough, and very sinuous line, as the figure shows."^[29] These observations challenged Aristotle's claim that the Moon was a perfect sphere and the larger idea that the heavens were perfect and unchanging.

Galileo's next astronomical discovery would prove to be a surprising one. While observing Jupiter over the course of several days, he noticed four stars close to Jupiter whose positions were changing in a way that would be impossible if they were fixed stars. After much observation, he concluded these four stars were orbiting the planet Jupiter and were in fact moons, not stars.^[30] This was a radical discovery because, according to Aristotelian cosmology, all heavenly bodies revolve around the Earth and a planet with moons obviously contradicted that popular belief.^[31] While contradicting Aristotelian belief, it supported Copernican cosmology which stated that Earth is a planet like all others.^[32]

In 1610, Galileo observed that Venus had a full set of phases, similar to the phases of the moon we can observe from Earth. This was explainable by the Copernican or Tychonic systems which said that all phases of Venus would be visible due to the nature of its orbit around the Sun, unlike the Ptolemaic system which stated only some of Venus's phases would be visible. Due to Galileo's observations of Venus, Ptolemy's system became highly suspect and the majority of leading astronomers subsequently converted to various heliocentric models, making his discovery one of the most influential in the transition from geocentrism to heliocentrism.^[33]

The Trials of Galileo

A panel of the Roman Inquisition condemned Galileo's work in defense of heliocentrism in 1616. A second trial in 1632 led to Galileo's house arrest and a ban on his books.^[34]