Copernican heliocentrism

• 1. Ancient Heliocentric Roots

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  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. He developed some theories from Heraclides Ponticus, speaking of a revolution of the Earth on its axis every 24 hours. Archimedes described this work in his book The Sand Reckoner, though Aristarchus's original text has been lost. A common misconception suggests that contemporaries like Cleanthes charged him with impiety for moving the Earth. Plutarch reported that Cleanthes jokingly told Aristarchus he should be charged with impiety, but later translations by Gilles Ménage altered the grammar to make it seem like an accusation against the heliocentric sustainer rather than a joke about worshiping the Sun. This resulting misunderstanding persists today despite evidence that the charge was not actually brought. Philolaus in the 4th century BC had also hypothesized movement of the Earth, probably inspired by Pythagoras's theories about a spherical, moving globe.

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• 2. The Ptolemaic Dominance

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  Claudius Ptolemy created the geocentric model in his Almagest around 150 AD, which became the prevailing astronomical system in Europe for over 1,400 years leading up to the 16th century. Throughout the Middle Ages, scholars spoke of it as the authoritative text on astronomy, although Ptolemy remained a little understood figure frequently mistaken as one of the Ptolemaic rulers of Egypt. Stars were embedded in a large outer sphere rotating relatively rapidly while planets dwelt in smaller spheres between, a separate one for each planet. To account for anomalies like apparent retrograde motion, astronomers used deferents and epicycles where a planet revolved in a small circle about a center that itself revolved in a larger circle about a point near Earth. The unique contribution of Ptolemy was the equant, a point about which the center of a planet's epicycle moved with uniform angular velocity but was offset from the center of its deferent. This violated Aristotelian cosmology principles that planetary motions should be explained through uniform circular motion and was considered a serious defect by many medieval astronomers.

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• 3. Islamic Mathematical Innovations

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  Several Islamic astronomers questioned the Earth's apparent immobility and centrality within the universe during the medieval period. Al-Sijzi invented an astrolabe based on beliefs held by some contemporaries that the motion we see is due to the Earth's movement and not that of the sky. In the 12th century, Nur ad-Din al-Bitruji proposed a complete alternative to the Ptolemaic system declaring it an imaginary model successful at predicting positions but not real or physical. His alternative spread through most of Europe during the 13th century. Mathematical techniques developed between the 13th and 14th centuries by Arab and Persian astronomers Mu'ayyad al-Din al-Urdi, Nasir al-Din al-Tusi, and Ibn al-Shatir for geocentric models closely resemble those used later by Copernicus in his heliocentric models. Otto E. Neugebauer argued in 1957 that debates in 15th-century Latin scholarship must have been informed by criticism produced after Averroes by the Ilkhanid-era Persian school associated with the Maragheh observatory. Copernicus cited several Islamic astronomers including al-Battani, Thabit ibn Qurra, al-Zarqali, Averroes, and al-Bitruji in De Revolutionibus.

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• 4. De Revolutionibus Publication

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  Nicolaus Copernicus published his major work De revolutionibus orbium coelestium in Nuremberg during 1543, containing six books released in the year of his death though he had arrived at his theory decades earlier. The first edition appeared in 1543 followed by a second edition in Basel in 1566. When the compendium was published it contained an unauthorized anonymous preface by Andreas Osiander stating that Copernicus wrote his account as a mathematical hypothesis rather than truth or probability to soften religious backlash against contradicting Old Testament passages like Joshua 10:12-13. There is no evidence that Copernicus himself considered the model merely mathematically convenient separate from reality. A letter from his friend Nikolaus von Schönberg Cardinal Archbishop of Capua urged Copernicus to publish before dedicating the book to Pope Paul III explaining motives related to calendar reform needed for the Church. The work divided into six books covering general vision theoretical principles apparent motions of the Sun description of the Moon concrete exposition including planetary longitude and latitude.

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• 5. Early Scientific Resistance

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  From publication until about 1700 few astronomers were convinced by the Copernican system despite relatively wide circulation with around 500 copies of the first and second editions surviving today. Even forty-five years after De Revolutionibus publication astronomer Tycho Brahe constructed a cosmology precisely equivalent to that of Copernicus but with Earth held fixed in center instead of Sun. For contemporaries ideas presented by Copernicus were not markedly easier to use than geocentric theory nor did they produce more accurate predictions of planetary positions. Tycho Brahe appreciated elegance of Copernican system yet objected to moving Earth on basis of physics astronomy and religion since Aristotelian physics offered no explanation for motion of massive body like Earth while easily explaining heavenly bodies as made of substance called aether. Tycho stated that Copernican system expertly circumvents all superfluous or discordant elements in Ptolemy's system yet ascribes to Earth that hulking lazy body unfit for motion a quick triple motion. Many astronomers accepted some aspects of Copernicus's theory at expense of others due to these physical contradictions.

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• 6. The Keplerian Transition

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  Using detailed observations by Tycho Brahe Johannes Kepler discovered Mars orbit was an ellipse with Sun at one focus and speed varied with distance from Sun. This discovery appeared in his 1609 book Astronomia nova along with claim that all planets had elliptical orbits and non-uniform motion stating sun itself would melt all Ptolemaic apparatus like butter. In years following publication of de Revolutionibus leading astronomers such as Erasmus Reinhold found key attraction was reinstatement of uniform circular motion for planets rather than actual heliocentrism acceptance. During 17th century several further discoveries eventually led to wider acceptance including Galileo observing four large moons of Jupiter evidence that Solar System contained bodies not orbiting Earth phases of Venus more observational evidence not properly explained by Ptolemaic theory and rotation of Sun about fixed axis. Giovanni Zupi saw phases of Mercury using telescope in 1639 providing additional confirmation beyond what earlier models could explain through complex precession of Sun's axis.

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• 7. Newtonian Synthesis

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  Isaac Newton proposed universal gravity and inverse-square law of gravitational attraction in 1687 to explain Kepler's elliptical planetary orbits completing the physical proof for Copernican system. The Copernican Revolution spanned over a century beginning with De revolutionibus orbium coelestium publication and ending with Newton's work establishing final validation. From modern point of view Copernican model has number of advantages giving clear account of cause of seasons where Earth's axis is not perpendicular to plane of its orbit. Theory provided strikingly simple explanation for apparent retrograde motions as parallactic displacements resulting from Earth's motion around Sun important consideration in Johannes Kepler conviction that theory was substantially correct. In geocentric model these are explained by ad hoc use of epicycles whose revolutions mysteriously tied to that of Sun whereas heliocentric model shows natural consequence of their orbits occurring at opposition to Sun.

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