Copernican heliocentrism

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Split from main article Nicolaus Copernicus in order to concentrate on his work:

Contents 1 Earlier theories 2 The Ptolemaic system 3 Copernican theory 4 De revolutionibus orbium coelestium 5 Acceptance of Copernican Heliocentrism 6 Notes

[edit] Earlier theories

Much has been written about earlier heliocentric theories. Philolaus (4th century BC) was one of the first to hypothesize movement of the Earth, probably inspired by Pythagoras' theories about a spherical Globe.

Aristarchus of Samos in the 3rd century BC had developed some theories of Heraclides Ponticus (speaking of a revolution by Earth on its axis) to propose what was, so far as is known, the first serious model of a heliocentric solar system. His work about a heliocentric system has not survived, so one may only speculate about what led him to his conclusions. It is notable that, according to Plutarch, a contemporary of Aristarchus accused him of impiety for "putting the Earth in motion."

The 14th-century Arab astronomer ibn al-Shatir developed mathematical techniques similar to those used by Copernicus, and it has been suggested that Copernicus might have been influenced by them.

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 reasons unknown (although possibly out of reluctance to quote pre-Christian sources), he did not include this passage in the publication of his book. Inspiration came to Copernicus not from observation of the planets, but from reading two authors. In Cicero he 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, however, revolve around a central sun. Copernicus did not attribute his inspiration to Aristarchus as is sometimes stated. When Copernicus' book was published, it contained an unauthorized preface by the Lutheran theologian Andreas Osiander. This cleric stated that Copernicus wrote his heliocentric account of the earth's movement as a mere mathematical hypothesis, not as an account that contained truth or even probability. Since Copernicus' hypothesis was believed to contradict the Old Testament account of the sun's movement around the earth (Joshua 10:13), this was apparently written to soften any religious backlash against the book. However, there is no evidence that Copernicus himself considered the heliocentric model as merely mathematically convenient, separate from reality.

It has been argued that in developing the mathematics of heliocentrism Copernicus drew on, not just the Greek, but the Islamic tradition of mathematics and astronomy, especially the works of Nasir al-Din Tusi, Mu’ayyad al-Din al-‘Urdi and ibn al-Shatir.

[edit] The Ptolemaic system

The prevailing theory in Europe as Copernicus was writing was that created by Ptolemy in his Almagest, dating from about 150 A.D.. 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 to this view, such as the retrograde motion observed in many planets, a system of epicycles was used, by which a planet rotated on a small axis while also rotating on a larger axis around 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 (it was first devised by Eudoxus of Cnidus; by the time of Copernicus it was associated with Averroes). Also popular with astronomers were variations such as eccentrics — by which the rotational axis was offset and not completely at the center.

Ptolemy's unique contribution to this theory was the idea of an equant — a complicated addition which specified that, when measuring the rotation of the Sun, one sometimes used the central axis of the universe, but sometimes a different axis. This had an overall effect of making certain orbits "wobble," a fact that would greatly bother Copernicus (such wobbling rendered implausible the idea of material "spheres" in which the planets rotated). In the end, after all these complications, the astronomers could still not get observation and theory to match up exactly. In Copernicus' day, the most up-to-date version of the Ptolemaic system was that of Peurbach (1423-1461) and Regiomontanus (1436-1476).

[edit] Copernican theory

Copernicus' major theory was published in the book, De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres) during the year of his death, 1543, though he had arrived at his theory several decades earlier.

The book marks the beginning of the shift away from a geocentric (and anthropocentric) 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. He arrived at the correct order of the known planets and explained the precession of the equinoxes correctly by a slow change in the position of the Earth's rotational axis. He also gave a clear account of the cause of the seasons: that the Earth's axis is not perpendicular to the plane of its orbit. He added another motion to the Earth, by which the axis is kept pointed throughout the year at the same place in the heavens; since Galileo Galilei, it has been recognized that for the Earth not to point to the same place would have been a motion.

Copernicus also replaced Ptolemy's equant circles with more epicycles. This is the main source of the statement that Copernicus' system had even more epicycles than Ptolemy's. With this change, Copernicus' system showed only uniform circular motions, correcting what he saw as the chief inelegance in Ptolemy's system. But while Copernicus put the Sun at the center of the celestial spheres, he did not put it at the exact centre of the universe, but near it.

Copernicus' system was not experimentally better than Ptolemy's model. Copernicus was aware of this and could not present any observational "proof" in his manuscript, relying instead on arguments about what would be a more complete and elegant system. From publication until about 1700, few astronomers were convinced by the Copernican system, though the book was relatively widely circulated (around 500 copies of the first and second editions^[1] have survived, which is a large number by the scientific standards of the time). Many astronomers, however, accepted some aspects of the theory at the expense of others, and his model did have a large influence on later scientists such as Galileo and Johannes Kepler, who adopted, championed and (especially in Kepler's case) sought to improve it. Galileo's observation of the phases of Venus produced the first observational evidence for Copernicus' theory. Similarly, Galileo's observation of the moons of Jupiter proved that the solar system contained bodies that did not orbit Earth.

The Copernican system can be summarized in several propositions, as Copernicus himself collected them in a Compendium of De revolutionibus that was found and published in 1878.

The major parts of Copernican theory are:

1. Heavenly motions are uniform, eternal, and circular or compounded of several circles (epicycles).
2. The center of the universe is near the Sun.
3. Around the Sun, in order, are Mercury, Venus, 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.
6. The distance from the Earth to the sun is small compared to the distance to the stars.

Whether these propositions were "revolutionary" or "conservative" was a topic of debate in the late twentieth century. Thomas Kuhn argued that Copernicus only transferred "some properties to the sun's many astronomical functions previously attributed to the earth." Other historians have since argued that Kuhn underestimated what was "revolutionary" about Copernicus' work, and emphasized the difficulty Copernicus would have had in putting forward a new astronomical theory relying alone on simplicity in geometry, given that he had no experimental evidence.

Arthur Koestler puts Copernicus in a different light to what many authors seem to suggest, portraying him as a coward who was reluctant to publish his work due to a crippling fear of ridicule.

[edit] De revolutionibus orbium coelestium

Main article: De revolutionibus orbium coelestium

Copernicus' major work, (Six books) On the Revolutions of the Heavenly Spheres (first edition 1543 in Nuremberg, second ed. 1566 in Basel), was the result of decades of labor. It opened with an originally anonymous preface by Andreas Osiander, a theologian friend of Copernicus, who urged that the theory, which was considered a tool that allows simpler and more accurate calculations, did not necessarily have implications outside the limited realm of astronomy.

Copernicus' actual book began with a letter from his (by then deceased) friend Nikolaus Cardinal von Schönberg, the Archbishop of Capua, urging Copernicus to publish his theory. Then, in a lengthy introduction, Copernicus dedicated the book to Pope Paul III, explaining his ostensible motive in writing the book as relating to the inability of earlier astronomers to agree on an adequate theory of the planets, and noting that if his system increased the accuracy of astronomical predictions it would allow the Church to develop a more accurate calendar. At that time, a reform of the Julian Calendar was considered necessary and was one of the major reasons for Church funding of astronomy.

The work itself was then divided into six books:

1. General vision of the heliocentric theory, and a summarized exposition of his idea of the World
2. Mainly theoretical, presents the principles of spherical astronomy and a list of stars (as a basis for the arguments developed in the subsequent books)
3. Mainly dedicated to the apparent motions of the Sun and to related phenomena
4. Description of the Moon and its orbital motions
5. Concrete exposition of the new system
6. Concrete exposition of the new system (continued)

[edit] Acceptance of Copernican Heliocentrism

The ideas presented by Copernicus were not easier to use than the geocentric theory and did not produce more accurate results. They appeared to be contrary to common sense and to contradict the bible. Why then were they taken up by other astronomers? Ironically, the key attraction of Copernican theory was actualy incorrect. The system proposed by Copernicus used uniform circular motion as opposed to planets moving at varying speeds in the geocentric system.

During the 17th century, three discoveries eventually lead to the acceptance of heliocentrism:

• Galileo used a telescope to observe the moons of Jupiter;
• Kepler introduced the idea of elipse shaped orbits for the planets;
• Newton proposed gravity as a mechanism for the operation of the solar system.

[edit] Notes

1. ^ Owen Gingerich, The Book Nobody Read