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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" and kentron = "Center"). 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.
To anyone who stands and looks at the sky, it seems clear 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 counter-intuitive idea of heliocentrism, that it is the Earth, not the heavens, that is actually moving, 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.
The (possibly semi-mythical) Egyptian philosopher Hermes Trismegistus proposed a heliocentric worldview, and it is notable that Copernicus actually states in De revolutionibus orbium coelestium his own debt to Trismegistus' ideas. Accurate dating is impossible, but Trismegistus probably lived in pharaonic Egypt, although actual texts we have that constitute the hermetic tradition probably date from about the time of Christ.
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." In developing his theories of planetary motion, Copernicus was probably indebted to the earlier work of the Arabic astronomer Ibn al-Shatir and the Persian Nasir al-Din al-Tusi.
The Galileo affair
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.
However, historians have argued:
- Contrary to legend, Galileo and the Copernican system were well regarded by church officials. Galileo was the victim of his own arrogance, the envy of his colleagues, and the politics of Pope Urban VIII. He was not accused of criticising the Bible, but disobeying a papal decree.
Catholic scientists also:
- appreciated that the reference to heresy in connection with Galileo or Copernicus had no general or theological significance, (Heilbron 1999).
Cardinal Robert Bellarmine himself considered that Galileo's model made "excellent good sense" on the ground of mathematical simplicity. And he said:
- If there were a real proof that the Sun is in the centre of the universe, that the Earth is in the third sphere, and that the Sun does not go round the Earth but the Earth round the Sun, then we should have to proceed with great circumspection in explaining passages of Scripture which appear to teach the contrary, and we should rather have to say that we did not understand them than declare an opinion false which has been proved to be true. But I do not think there is any such proof since none has been shown to me. (Koestler 1959)
And indeed the Church even allowed the cathedrals themselves to be used as solar observatories called meridiane. I.e. they were turned into "reverse sundials", or gigantic pinhole cameras, where the sunís image was projected from a hole in a window in the cathedralís lantern onto a meridian line. Although this analysis of the sunís motion further weakened the geocentric model, this research was well supported. And only 50 years after Galileo, astronomers of the Jesuit Order, "the intellectual spearhead of the Catholic Church", taught geokinetic astronomy in China.
Archimedes had objected to Heliocentrism on the grounds that, if it were true, parallax should be observed in the apparent positions of the fixed stars. So Galileo's opponents were not without their scientific reasons. This objection 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.) Therefore an astronomer on a hypothetical extrasolar planet would observe a "wobble".
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.
Fred Hoyle wrote:
- The relation of the two pictures [geocentricity and heliocentricity] is reduced to a mere coordinate transformation and it is the main tenet of the Einstein theory that any two ways of looking at the world which are related to each other by a coordinate transformation are entirely equivalent from a physical point of view (Hoyle, 1973)
- Heilbron, J.L., The Sun in the Church: Cathedrals as Solar Observatories, Harvard University Press, Cambridge, MA, 1999.
- Sir Fred Hoyle, Nicolaus Copernicus, Heinemann Educational Books Ltd., London, p. 78, 1973.
- Arthur Koestler, The Sleepwalkers: a history of man's changing vision of the universe, Hutchinson, London, pp. 447Ė8, 1959.
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