Heliocentrism

Heliocentrism (also known as the heliocentric model) is a superseded astronomical model in which Earth and planets orbit around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed Earth at the center. The notion that Earth revolves around the Sun had been proposed as early as the 3rd century BC by Aristarchus of Samos, who had been influenced by a concept presented by Philolaus of Croton (c. 470 – 385 BC). In the 5th century BC the Greek philosophers Philolaus and Hicetas had the thought on different occasions that Earth was spherical and revolving around a "mystical" central fire, and that this fire regulated the universe. In medieval Europe, however, Aristarchus' 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 heliocentric system was presented by the Renaissance mathematician, astronomer, and Catholic cleric, Nicolaus Copernicus, leading to the Copernican Revolution. In 1576, Thomas Digges published a modified Copernican system. His modifications are close to modern observations. In the following century, Johannes Kepler introduced elliptical orbits, and Galileo Galilei presented supporting observations made using a 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.

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 BC in Pythagoreanism, and a fully developed heliocentric model was developed by Aristarchus of Samos in the 3rd century BC, these ideas were not successful in replacing the view of a stationary spherical Earth, and from the 2nd century AD the predominant model, which would be inherited by medieval astronomy, was the geocentric model described in Ptolemy's Almagest.

Problems with Ptolemy's system were well recognized in medieval astronomy, and an increasing effort to criticize and improve it in the late medieval period eventually led to the Copernican heliocentrism developed in Renaissance astronomy.

The first non-geocentric model of the universe was proposed by the Pythagorean philosopher Philolaus (d. 390 BC), 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. 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 BC) 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 (AD 395—423) 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.