section_path: Copernican system/Predecessors
section_name: Predecessors
paragraph_text: Predecessors
  
  Philolaus (c. 470 – c. 385 BCE) described an astronomical system in which a Central Fire (different from the Sun) occupied the centre of the universe, and a counter-Earth, the Earth, Moon, the Sun itself, planets, and stars all revolved around it, in that order outward from the centre. Heraclides Ponticus (387–312 BCE) proposed that the Earth rotates on its axis. Aristarchus of Samos (c. 310 BCE – c. 230 BCE) was the first to advance a theory that the Earth orbited the Sun. Further mathematical details of Aristarchus's heliocentric system were worked out around 150 BCE by the Hellenistic astronomer Seleucus of Seleucia. Though Aristarchus's original text has been lost, a reference in Archimedes ' book The Sand Reckoner (Archimedis Syracusani Arenarius & Dimensio Circuli) describes a work by Aristarchus in which he advanced the heliocentric model. Thomas Heath gives the following English translation of Archimedes's text:
  
  You are now aware that the "universe" is the name given by most astronomers to the sphere the centre of which is the centre of the earth, while its radius is equal to the straight line between the centre of the sun and the centre of the earth. This is the common account (τά γραφόμενα) as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the "universe" just mentioned. His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun on the circumference of a circle, the sun lying in the middle of the orbit, and that the sphere of the fixed stars, situated about the same centre as the sun, is so great that the circle in which he supposes the earth to revolve bears such a proportion to the distance of the fixed stars as the centre of the sphere bears to its surface.
  
  In an early unpublished manuscript of De Revolutionibus (which still survives), Copernicus mentioned the (non-heliocentric) 'moving Earth' theory of Philolaus and the possibility that Aristarchus also had a 'moving Earth' theory (though it is unlikely that he was aware that it was a heliocentric theory). He removed both references from his final published manuscript.
  
  Copernicus was probably aware that Pythagoras's system involved a moving Earth. The Pythagorean system was mentioned by Aristotle.
  
  Copernicus owned a copy of Giorgio Valla's De expetendis et fugiendis rebus, which included a translation of Plutarch's reference to Aristarchus's heliostaticism.
  
  In Copernicus's dedication of On the Revolutions to Pope Paul III —which Copernicus hoped would dampen criticism of his heliocentric theory by "babblers... completely ignorant of "—the book's author wrote that, in rereading all of philosophy, in the pages of Cicero and Plutarch he had found references to those few thinkers who dared to move the Earth "against the traditional opinion of astronomers and almost against common sense."
  
  The prevailing theory during Copernicus's lifetime was the one that Ptolemy published in his Almagest c. 150 CE; the Earth was the stationary center of the universe. Stars were embedded in a large outer sphere that rotated rapidly, approximately daily, while each of the planets, the Sun, and the Moon were embedded in their own, smaller spheres. Ptolemy's system employed devices, including epicycles, deferents and equants, to account for observations that the paths of these bodies differed from simple, circular orbits centered on the Earth.
  
  Beginning in the 10th century, a tradition criticizing Ptolemy developed within Islamic astronomy, which climaxed with Ibn al-Haytham of Basra 's Al-Shukūk 'alā Baṭalamiyūs ("Doubts Concerning Ptolemy"). 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 (d. c. 1020). According to al-Biruni, al-Sijzi 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 (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-Bitruji's alternative system spread through most of Europe during the 13th century, with debates and refutations of his ideas continued up to the 16th century.
  
  Mathematical techniques developed in the 13th to 14th centuries by Mo'ayyeduddin al-Urdi, Nasir al-Din al-Tusi, and Ibn al-Shatir for geocentric models of planetary motions closely resemble some of those used later by Copernicus in his heliocentric models. 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 Ibn al-Shatir (d. c. 1375) of Damascus. Ibn 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 yet 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.