Heliocentrism (also known as the heliocentric model) is a superseded astronomical model that placed the Sun at the center of the universe, with the Earth and the planets in its orbit. It superseded geocentrism, which placed the Earth at the center of the universe. In modern astronomy, heliocentrism has been superseded by models based on relativity, in which the universe does not have an absolute center or preferred frames of reference.
Historically, heliocentrism was opposed to geocentrism, which placed Earth at the center. The idea that Earth revolves around the Sun was proposed as early as the 3rd century BC by Aristarchus of Samos,; . The work of Aristarchus in which he proposed his heliocentric system has not survived. We only know of it now from a brief passage in Archimedes' The Sand Reckoner. who had been influenced by Philolaus's 5th-century BC concept that Earth was spherical and revolving around a "mystical" central fire that regulated the universe. In medieval Europe, however, Aristarchus's heliocentrism attracted little attention—possibly because of the loss of scientific works of the Hellenistic period.
It was not until the 16th century that a mathematical model of a virtually heliocentric system was presented by the Renaissance mathematician, astronomer, and Catholic cleric Nicolaus Copernicus, leading to the Copernican Revolution. In the following century, Johannes Kepler introduced elliptical orbits for all planets discovered up to his time (genuinely heliocentric), and Galileo Galilei presented observations of the Solar System made using his telescope.
With the observations of William Herschel, Friedrich Bessel, and other astronomers, it was realized that the Sun, while near the barycenter of the Solar System, was not central in the universe. Modern astronomy does not distinguish any universal center. After these discoveries, informal language still holds to the remaining truth value of the term, narrowed to the scope of our planetary system.
Ancient and medieval astronomy While the sphericity of Earth was widely recognized in Greco-Roman astronomy from at least the 4th century BC, Earth's daily rotation and yearly orbit around the Sun was never universally accepted until the Copernican Revolution.
While a moving Earth was proposed at least from the 4th century BCE in Pythagoreanism, and a fully developed heliocentric model was developed by Aristarchus of Samos in the 3rd century BCE, these ideas were not successful in replacing the view of a stationary spherical Earth, and from the 2nd century CE the predominant model, which would be inherited by medieval astronomy, was the geocentric model described in Ptolemy's Almagest.
Classical antiquity
Pythagoreans The first non-geocentric model of the universe was proposed by the Pythagorean philosopher Philolaus (d. 390 BCE), who taught that at the center of the universe was a "central fire", around which Earth, the Sun, the Moon, and planets revolved in uniform circular motion. This system postulated the existence of a Counter-Earth collinear with Earth and central fire, with the same period of revolution around the central fire as Earth. The Sun revolved around the central fire once a year, and the stars were stationary. Earth maintained the same hidden face towards the central fire, rendering both it and the "Counter-Earth" invisible from Earth. The Pythagorean concept of uniform circular motion remained unchallenged for approximately the next 2000 years, and it was to the Pythagoreans that Copernicus referred to show that the notion of a moving Earth was neither new nor revolutionary.Boyer, C. A History of Mathematics. Wiley, p. 54. Kepler gave an alternative explanation of the Pythagoreans' "central fire" as the Sun, "as most sects purposely hid[e] their teachings".
Heraclides of Pontus (4th century BCE) said that the rotation of Earth explained the apparent daily motion of the celestial sphere. It used to be thought that he believed Mercury and Venus to revolve around the Sun, which in turn (along with the other planets) revolves around Earth. Macrobius (CE 395423) later described this as the "Egyptian System," stating that "it did not escape the skill of the Egyptians," though there is no other evidence it was known in ancient Egypt.
Aristarchus of Samos
The first person known to have proposed a heliocentric system was Aristarchus of Samos . Like his contemporary Eratosthenes, Aristarchus calculated the size of Earth and measured the sizes and distances of the Sun and Moon. From his estimates, he concluded that the Sun was six to seven times wider than Earth, and thought that the larger object would have the most attractive force.
His writings on the heliocentric system are lost, but some information about them is known from a brief description by his contemporary, Archimedes, and from scattered references by later writers. Archimedes' description of Aristarchus' theory is given in the former's book, The Sand Reckoner. The entire description comprises just three sentences, which Thomas Heath translates as follows:. The italics and parenthetical comments are as they appear in Heath's original.
Aristarchus presumably took the stars to be very far away since if Earth's orbit were significant relative to the size of the celestial sphere, the apparent angular separations between the stars would vary over the course of the year. The stars are in fact so far away that stellar parallax only became detectable when sufficiently powerful telescopes had been developed in the 1830s.
No references to Aristarchus' heliocentrism are known in any other writings from before the common era. The earliest of the handful of other ancient references occur in two passages from the writings of Plutarch. These mention one detail not stated explicitly in Archimedes' accountAlthough it could obviously be reasonably inferred therefrom.—namely, that Aristarchus' theory had Earth rotating on an axis. The first of these references occurs in :. Most modern scholars share Heath's opinion that it is Cleanthes in this passage who is being held as having accused Aristarchus of impiety (see ; ; ; Cherniss 1957, p. 55; for example). The manuscripts of Plutarch's Concerning the Face Which Appears in the Orb of the Moon that have come down to us are corrupted, however, and the traditional interpretation of the passage has been challenged by Lucio Russo, who insists that it should be interpreted as having Aristarchus rhetorically suggest that Cleanthes was being impious for wanting to shift the Sun from its proper place at the center of the universe (; ).
Only scattered fragments of Cleanthes' writings have survived in quotations by other writers, but in Lives and Opinions of Eminent Philosophers, Diogenes Laërtius lists A reply to Aristarchus (Πρὸς Ἀρίσταρχον) as one of Cleanthes' works,Diogenes Laërtius (1972, Bk 7, ch 5, p. 281) and some scholars have suggested that this might have been where Cleanthes had accused Aristarchus of impiety.
The second of the references by Plutarch is in his Platonic Questions:
The remaining references to Aristarchus' heliocentrism are extremely brief, and provide no more information beyond what can be gleaned from those already cited. Ones which mention Aristarchus explicitly by name occur in Aëtius' Opinions of the Philosophers, Sextus Empiricus' Against the Mathematicians, and an anonymous scholiast to Aristotle. Another passage in Aëtius' Opinions of the Philosophers reports that Seleucus the astronomer had affirmed Earth's motion, but does not mention Aristarchus.
Seleucus of Seleucia
Since Plutarch mentions the "followers of Aristarchus" in passing, it is likely that there were other astronomers in the Classical period who also espoused heliocentrism, but whose work was lost. The only other astronomer from antiquity known by name who is known to have supported Aristarchus' heliocentric model was Seleucus of Seleucia (b. 190 BCE ), a Hellenistic astronomer who flourished a century after Aristarchus in the Seleucid Empire. Seleucus was a proponent of the heliocentric system of Aristarchus. Seleucus may have proved the heliocentric theory by determining the constants of a geometric model for the heliocentric theory and developing methods to compute planetary positions using this model. He may have used early trigonometric methods that were available in his time, as he was a contemporary of Hipparchus. A fragment of a work by Seleucus has survived in Arabic translation, which was referred to by Rhazes (b. 865).
Alternatively, his explanation may have involved the phenomenon of tides,Lucio Russo, Flussi e riflussi, Feltrinelli, Milano, 2003, . which he supposedly theorized to be caused by the attraction to the Moon and by the revolution of Earth around Earth and the Moon's center of mass.
Late antiquity There were occasional speculations about heliocentrism in Europe before Copernicus. In Roman Carthage, the pagan Martianus Capella (5th century CE) expressed the opinion that the planets Venus and Mercury did not go about Earth but instead circled the Sun.William Stahl, trans., Martianus Capella and the Seven Liberal Arts, vol. 2, The Marriage of Philology and Mercury, 854, 857, New York: Columbia Univ. Pr, 1977, pp. 332–333 Capella's model was discussed in the Early Middle Ages by various anonymous 9th-century commentators and Copernicus mentions him as an influence on his own work. Also Macrobius (420 CE) described a heliocentric model.
Ancient India
Aryabhata (476–550), in his magnum opus Aryabhatiya (499), propounded a planetary model in which Earth was taken to be spinning on its axis and the periods of the planets were given with respect to the Sun. His immediate commentators, such as Lalla, and other later authors, rejected his innovative view about the turning Earth. It has been argued that Aryabhatta's calculations were based on an underlying heliocentric model, in which the planets orbit the Sun,The concept of Indian heliocentrism has been advocated by B. L. van der Waerden, Das heliozentrische System in der griechischen, persischen und indischen Astronomie. Naturforschenden Gesellschaft in Zürich. Zürich:Kommissionsverlag Leeman AG, 1970.B.L. van der Waerden, "The Heliocentric System in Greek, Persian and Hindu Astronomy", in David A. King and George Saliba, ed., From Deferent to Equant: A Volume of Studies in the History of Science in the Ancient and Medieval Near East in Honor of E. S. Kennedy, Annals of the New York Academy of Science, 500 (1987), pp. 529–534. although this has also been rebutted.Noel Swerdlow, "Review: A Lost Monument of Indian Astronomy," Isis, 64 (1973): 239–243. The general consensus is that a synodic anomaly (depending on the position of the Sun) does not imply a physically heliocentric orbit (such corrections being also present in late Babylonian astronomical texts), and that Aryabhata's system was not explicitly heliocentric. He also made many astronomical calculations, such as the times of the solar and lunar eclipses, and the instantaneous motion of the Moon. Early followers of Aryabhata's model included Varahamihira, Brahmagupta, and Bhaskara II.
Medieval Islamic world
For a time, Muslim astronomers accepted the Ptolemaic system and the geocentric model, which were used by to show that the distance between the Sun and Earth varies. In the 10th century, accepted that Earth rotates around its axis. According to later astronomer al-Biruni, al-Sijzi invented an astrolabe called al-zūraqī based on a belief held by some of his contemporaries that the apparent motion of the stars was due to Earth's movement, and not that of the firmament. Islamic astronomers began to criticize the Ptolemaic model, including Ibn al-Haytham in his 'alā Baṭalamiyūs ("Doubts Concerning Ptolemy", c. 1028), who found contradictions in Ptolemy's model, but al-Haytham remained committed to a geocentric model.Nicolaus Copernicus, Stanford Encyclopedia of Philosophy (2004).