Copernican heliocentrism


Copernican heliocentrism is the astronomical model developed by Nicolaus Copernicus and published in 1543. This model positioned the Sun near the center of the Universe, motionless, with Earth and the other planets orbiting around it in circular paths, modified by epicycles, and at uniform speeds. The Copernican model challenged the geocentric model of Ptolemy that had prevailed for centuries, which had placed Earth at the center of the Universe.
Although Copernicus had circulated an outline of his own theory to colleagues sometime before 1514, he did not decide to publish it until he was urged to do so later by his pupil Rheticus. His model was an alternative to the longstanding Ptolemaic model that purged astronomy of the equant in order to satisfy the theological and philosophical ideal that all celestial motion must be perfect and uniform, preserving the metaphysical implications of a mathematically ordered cosmos. His heliostatic model retained several false Ptolemaic assumptions such as the planets' circular orbits, epicycles, and uniform speeds, while at the same time using accurate ideas such as:
  • The Earth is one of several planets revolving around a stationary 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.
The Copernican model was later replaced by Kepler's laws of planetary motion.

Background

Antiquity

was one of the first to hypothesize movement of the Earth, probably inspired by Pythagoras's theories about a spherical, moving globe. In the 3rd century BCE, Aristarchus of Samos proposed what was, so far as is known, the first serious model of a heliocentric Solar System, having developed some of Heraclides Ponticus's theories. Though his original text has been lost, a reference in Archimedes's book The Sand Reckoner describes a work in which Aristarchus advanced the heliocentric model. Archimedes wrote:
It is a common misconception that the heliocentric view was rejected by the contemporaries of Aristarchus. This is the result of Gilles Ménage's translation of a passage from Plutarch's On the Apparent Face in the Orb of the Moon. Plutarch reported that Cleanthes as a worshiper of the Sun and opponent to the heliocentric model, was jokingly told by Aristarchus that he should be charged with impiety. Ménage, shortly after the trials of Galileo and Giordano Bruno, amended an accusative with a nominative, and vice versa, so that the impiety accusation fell over the heliocentric sustainer. The resulting misconception of an isolated and persecuted Aristarchus is still transmitted today.

Ptolemaic system

The prevailing astronomical model of the cosmos in Europe in the 1,400 years leading up to the 16th century was the Ptolemaic System, a geocentric model created by Claudius Ptolemy in his Almagest, dating from about 150 AD. Throughout the Middle Ages it was spoken of as the authoritative text on astronomy, although its author remained a little understood figure frequently mistaken as one of the Ptolemaic rulers of Egypt. The Ptolemaic system drew on many previous theories that viewed Earth as a stationary center of the universe. Stars were embedded in a large outer sphere which rotated relatively rapidly, while the planets dwelt in smaller spheres between—a separate one for each planet. To account for apparent anomalies in this view, such as the apparent retrograde motion of the planets, a system of deferents and epicycles was used. The planet was said to revolve in a small circle about a center, which itself revolved in a larger circle about a center on or near the Earth.
A complementary theory to Ptolemy's employed homocentric spheres: the spheres within which the planets rotated could themselves rotate somewhat. This theory predated Ptolemy. Also popular with astronomers were variations such as eccentrics—by which the rotational axis was offset and not completely at the center. The planets were also made to have exhibit irregular motions that deviated from a uniform and circular path. The eccentrics of the planets motions were analyzed to have made reverse motions over periods of observations. This retrograde motion created the foundation for why these particular pathways became known as epicycles.
Ptolemy's unique contribution to this theory was the equant—a point about which the center of a planet's epicycle moved with uniform angular velocity, but which was offset from the center of its deferent. This violated one of the fundamental principles of Aristotelian cosmology—namely, that the motions of the planets should be explained in terms of uniform circular motion, and was considered a serious defect by many medieval astronomers.

Aryabhata

In 499 CE, the Indian astronomer and mathematician Aryabhata, influenced by Greek astronomy, propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbits of planets are elliptical. Aryabhata's followers were particularly strong in South India, where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them.

Middle Ages

Islamic astronomers

Several Islamic astronomers questioned the Earth's apparent immobility and centrality within the universe. Some accepted that the Earth rotates around its axis, such as Al-Sijzi, who invented an astrolabe based on a belief held by some of his contemporaries "that the motion we see is due to the Earth's movement and not to that of the sky". That others besides Al-Sijzi held this view is further confirmed by a reference from an Arabic work in the 13th century which states: "According to the geometers , the earth is in constant circular motion, and what appears to be the motion of the heavens is actually due to the motion of the earth and not the stars".
In the 12th century, Nur ad-Din al-Bitruji proposed a complete alternative to the Ptolemaic system. He declared the Ptolemaic system as an imaginary model, successful at predicting planetary positions but not real or physical. Al-Btiruji's alternative system spread through most of Europe during the 13th century. Mathematical techniques developed in the 13th to 14th centuries by the Arab and Persian astronomers Mu'ayyad al-Din al-Urdi, Nasir al-Din al-Tusi, and Ibn al-Shatir for geocentric models of planetary motions closely resemble some of the techniques used later by Copernicus in his heliocentric models.

European astronomers post-Ptolemy

expressed the opinion that the planets Venus and Mercury did not go about the Earth but instead circled the Sun. Capella's model was discussed in the Early Middle Ages by various anonymous 9th-century commentators and Copernicus mentions him as an influence on his own work. Macrobius described a heliocentric model. John Scotus Eriugena proposed a model reminiscent of that from Tycho Brahe.
Since the 13th century, European scholars were well aware of problems with Ptolemaic astronomy. 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. 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 Persian school of astronomy associated with the Maragheh observatory. It has been argued that Copernicus could have independently discovered the Tusi couple or took the idea from Proclus's Commentary on the First Book of Euclid, which Copernicus cited. Another possible source for Copernicus's knowledge of this mathematical device is the Questiones de Spera of Nicole Oresme, who described how a reciprocating linear motion of a celestial body could be produced by a combination of circular motions similar to those proposed by al-Tusi.
In Copernicus's day, the most up-to-date version of the Ptolemaic system was that of Georg von Peuerbach and his student Regiomontanus. The state of the question as received by Copernicus is summarized in the Theoricae novae planetarum by Peuerbach, compiled from lecture notes by Regiomontanus in 1454, but not printed until 1472. Peuerbach attempts to give a new, mathematically more elegant presentation of Ptolemy's system, but he does not arrive at heliocentrism. Regiomontanus was the teacher of Domenico Maria Novara da Ferrara, who was in turn the teacher of Copernicus. There is a possibility that Regiomontanus had 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 late work and mentions the "motion of the Earth" in a letter.
The state of knowledge on planetary theory received by Copernicus is summarized in Peuerbach's Theoricae Novae Planetarum. By 1470, the accuracy of observations by the Vienna school of astronomy, of which Peuerbach and Regiomontanus were members, was high enough to make the eventual development of heliocentrism inevitable, and indeed it is possible that Regiomontanus did arrive at an explicit theory of heliocentrism before his death in 1476, some 30 years before Copernicus.

Copernican theory

Copernicus's major work, De revolutionibus orbium coelestium, was a compendium of six books published during the year of his death, though he had arrived at his theory several decades earlier. The work marks the beginning of the shift away from a geocentric universe with the Earth at its center. Copernicus held that the Earth is another planet revolving around the fixed Sun once a year and turning on its axis once a day. But while Copernicus put the Sun at the center of the celestial spheres, he did not put it at the exact center of the universe, but near it. Copernicus's system used only uniform circular motions, correcting what was seen by 16th century European astronomers as the chief inelegance in Ptolemy's system.
The Copernican model replaced Ptolemy's equant circles with more epicycles. 1,500 years of Ptolemy's model helped to create a more accurate estimate of the planets' motions for Copernicus. That is the main reason that Copernicus's system had even more epicycles than Ptolemy's. The more epicycles proved to have more accurate measurements of how the planets were truly positioned, "although not enough to get excited about". The Copernican system can be summarized in several propositions, as Copernicus himself did in his early Commentariolus that he handed only to friends, probably in the 1510s. The "little commentary" was never printed. Its existence was only known indirectly until a copy was discovered in Stockholm around 1880, and another in Vienna a few years later.
The major features of Copernican theory are:
  1. Heavenly motions are uniform, eternal, and circular or compounded of several circles.
  2. The center of the universe is near the Sun.
  3. Around the Sun, in order, are Mercury, Venus, the Earth and Moon, Mars, Jupiter, Saturn, and the fixed stars.
  4. The Earth has three motions: daily rotation, annual revolution, and annual tilting of its axis.
  5. Retrograde motion of the planets is explained by the Earth's motion, which in short was also influenced by planets and other celestial bodies around Earth.
  6. The distance from the Earth to the Sun is small compared to the distance to the stars.
Inspiration came to Copernicus not from observation of the planets, but from reading two authors, Cicero and Plutarch. In Cicero's writings, Copernicus found an account of the theory of Hicetas. Plutarch provided an account of the Pythagoreans Heraclides Ponticus, Philolaus, and Ecphantes. These authors had proposed a moving Earth, which did not revolve around a central Sun. Copernicus cited Aristarchus and Philolaus in an early manuscript of his book which survives, stating: "Philolaus believed in the mobility of the earth, and some even say that Aristarchus of Samos was of that opinion". For unknown reasons, Copernicus did not include this passage in the publication of his book.
Copernicus used what is now known as the Urdi lemma and the Tusi couple in the same planetary models as found in Arabic sources. Furthermore, the exact replacement of the equant by two epicycles used by Copernicus in the Commentariolus was found in an earlier work by al-Shatir. Al-Shatir's lunar and Mercury models are also identical to those of Copernicus. This has led some scholars to argue that Copernicus must have had access to some yet to be identified work on the ideas of those earlier astronomers. However, no likely candidate for this conjectured work has come to light, and other scholars have argued that Copernicus could well have developed these ideas independently of the late Islamic tradition. Nevertheless, Copernicus cited some of the Islamic astronomers whose theories and observations he used in De Revolutionibus, namely al-Battani, Thabit ibn Qurra, al-Zarqali, Averroes, and al-Bitruji. It has been suggested that the idea of the Tusi couple may have arrived in Europe leaving few manuscript traces, since it could have occurred without the translation of any Arabic text into Latin. One possible route of transmission may have been through Byzantine science; Gregory Chioniades translated some of al-Tusi's works from Arabic into Byzantine Greek. Several Byzantine Greek manuscripts containing the Tusi-couple are still extant in Italy.