Copernican heliocentrism

Copernican heliocentrism is the astronomical model developed by Nicolaus Copernicus and published in 1543. This model positioned the Sun at 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 displaced the geocentric model of Ptolemy that had prevailed for centuries, which had placed Earth at the center of the Universe.

Although he had circulated an outline of his own heliocentric 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. Copernicus's challenge was to present a practical alternative to the Ptolemaic model by more elegantly and accurately determining the length of a solar year while preserving the metaphysical implications of a mathematically ordered cosmos. Thus, his heliocentric model retained several of the Ptolemaic elements, causing inaccuracies, such as the planets' circular orbits, epicycles, and uniform speeds, while at the same time using 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.

Background

Antiquity

Philolaus (4th century BCE) was one of the first to hypothesize ''movement of the Earth'', probably inspired by Pythagoras' 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' theories (speaking of a "revolution of the Earth on its axis" every 24 hours). Though his original text has been lost, a reference in Archimedes' book '' The Sand Reckoner'' (''Archimedis Syracusani Arenarius & Dimensio Circuli'') 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 (a contemporary of Aristarchus and head of the Stoics) 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 (identifying the object of the verb) with a nominative (the subject of the sentence), 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.

In 499 CE, the Indian astronomer and mathematician Aryabhata 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 Abu Sa'id 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 r engineers(''muhandisīn''), 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 (although not heliocentric). 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 Mo'ayyeduddin 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

= Ptolemaic system

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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 the Roman citizen Claudius Ptolemy in his ''Almagest,'' dating from about 150 CE. 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 (the epicycle) about a center, which itself revolved in a larger circle (the deferent) 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 (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. 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