Stockholm around 1880, and another in Vienna a few years later. The major features of Copernican theory are: Heavenly motions are uniform, eternal, and circular or compounded of several circles (epicycles). The center of the universe is near the Sun. Around the Sun, in order, are Mercury, Venus, the Earth and Moon, Mars, Jupiter, Saturn, and the fixed stars. 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, which in short was also influenced by planets and other celestial bodies around Earth. 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 (although possibly out of reluctance to quote pre-Christian sources), 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. De revolutionibus orbium coelestium When Copernicus' compendium was published, it contained an unauthorized, anonymous preface by a friend of Copernicus, the Lutheran theologian Andreas