In astronomy, heliocentrism is the theory that the Sun is at the center of the Universe and/or the Solar System. The word is derived from the Greek Helios ("Sun"). Historically, heliocentrism is opposed to geocentrism and currently to modern geocentrism, which places the earth at the center. (The distinction between the Solar System and the Universe was not clear until modern times, but extremely important relative to the controversy over cosmology and religion.) In the 16th and 17th centuries, when the theory was revived and defended by Copernicus, Galileo, and Kepler, it became the center of a major dispute. Since at the time of the controversy, Galileo, Kepler and Copernicus stated that the sun was the center of the universe, and the Church held the earth was, the three astronomers could be accused of ipso facto rejecting the providence of God.
To anyone who stands and looks at the sky, it is apparent that the earth stays in one place while everything in the sky goes around once every day. Observing over a longer time, one sees more complicated movements. The Sun makes a slower circle over the course of a year; the planets have similar motions, but they sometimes turn around and move in the reverse direction for a while (retrograde motion). As these motions became better understood, they required more and more elaborate descriptions, the most famous of which was the Ptolemaic system, formulated in the 2nd century.
The strange idea that it is the other way around—a heliocentric theory— was suggested at least as early as the 4th century BC- In chapter 13 of book two of his On the Heavens , Aristotle wrote that "At the centre, they [the Pythagoreans] say, is fire, and the earth is one of the stars, creating night and day by its circular motion about the centre." The reasons for this placement were philosophic based on the classical elements rather than scientific- fire was more precious than earth in the opinion of the Pythagoreans, and for this reason the sun (representing fire) should be central. Aristotle dismissed this argument and advocated geocentrism.
Later, heliocentrism was again proposed by Aristarchus (c. 270 BC). By the time he was writing, the size of the Earth had been calculated accurately, and he himself measured the size and distance of the Moon and Sun; his figures were not accurate by modern standards, but a serious start. Perhaps, as many people have suggested, paying attention to these numbers led him to think that it made more sense for the Earth to be moving than for the huge Sun to be moving around it. Some people in his own time, though, considered the idea to be against religion.
Aristarchus' heliocentric model was countered by Archimedes, who argued in The Sand Reckoner that the world (universe) was of large but finite size, and based on his set of initial assumptions he calculated an upper limit to the diameter of the universe to be 10,000,000,000 stadia. Given this number for the size of the universe, it followed that there was maximum distance that stars could be from the center. In a heliocentric system, the earth would move twice the earth-sun distance each year, and given this distance to stars should see visible parallax as it got closer or farther to various stars. As no parallax was observed, heliocentrism was dismissed because it would require the stars to be an implausible (to Archimedes) distance away.
In the 5th century AD (apparently independently of Aristarchus) the Indian astronomer Aryabhata also proposed a heliocentric Universe. As his work was not translated into Latin until after Copernicus had written De revolutionibus orbium coelestium, his theories were generally ignored in the West.
For many centuries, Heliocentrism was countered with the apparent common sense view that, if the Earth were spinning and moving around the Sun, people and objects would tend to fall off.
In the 16th century the theory was revived by Nicolaus Copernicus, in a form consistent with then-current observations. This theory resolved the issue of planetary retrograde motion by arguing that such motion was only perceived and apparent, rather than real: it was a parallax effect, as a car that one is passing seems to move backwards against the horizon. This issue was also resolved in the geocentric Tychonian system; the latter, however, while eliminating the major epicycles, retained as a physical reality the irregular back-and-forth motion of the planets, which Kepler characterized as a "pretzel."
Heliocentrism was notably advanced by Galileo, Kepler, and Newton. It was vigorously resisted, though, by elements in the Roman Catholic Church, who prevailed in showdowns in 1616 and 1633 and officially suppressed heliocentrism. The favored system had been that of Ptolemy, in which the Earth was the center of the universe and all celestial bodies orbited it. (The Catholic support for geocentricism should not be confused with the idea of a flat earth, which the Church never supported.) When prominent Catholic astronomers, including Clavius, became dissatisfied with the Ptolemaic system, many moved to the rival Tychonian system, a geocentric compromise; after 1633, the use of this system was almost mandatory. For advancing heliocentric theory Galileo was put under house arrest for the last several years of his life.
Archimedes' objection to Heliocentrism that, if it were true, parallax should be observed in the apparent positions of the fixed stars was finally put to rest when stellar parallax was observed with the greatly improved instruments of the 19th century, by which time the model of a heliocentric solar system in a very big universe was accepted by almost everyone.
The realization that the heliocentric view was also not true in a strict sense was achieved in steps. That the Sun was not the center of the universe, but one of innumerable stars, was strongly advocated by the mystic Giordano Bruno; Galileo made the same point, but said very little on the matter, perhaps not wishing to incur the church's wrath. Over the course of the 18th and 19th centuries, the status of the Sun as merely one star among many became increasingly obvious. By the 20th century, even before the discovery that there are many galaxies, it was not an issue. Even if the discussion is limited to the Solar system, the sun is not at the geometric center of any planet's orbit, but rather at one focus of the elliptical orbit. Furthermore, to the extent that a planet's mass cannot be neglected in comparison to the Sun's mass, the center of gravity of the Solar system is displaced slightly away from the center of the Sun. (The masses of the planets, mostly Jupiter, amount to 0.14% of that of the Sun.)
Giving up the whole concept of a center of the universe and even of being "at rest" is related to the principle of relativity. While it was clear there is no privileged position in space, until postulation of the special theory of relativity by Albert Einstein, at least the existence of a priviliged class of inertial systems absolutely at rest was assumed, in particular in the form of the hypothesis of the luminiferous aether. Some forms of Mach's principle consider the frame at rest with respect to the masses in the universe to have special properties.
Modern use of geocentric and heliocentric in science
In practical calculations, the origin and orientation of a coordinate system often has to be selected. For practical reasons, systems with their origin in the center of Earth's mass, solar mass or in the center of mass of solar system are frequently selected. The adjectives geocentric or heliocentric may be used in this context. However, such selection of coordinates has no philosophical or physical implications.