Heliocentrism

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.

Development of the idea

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.

Related Topics:
Planet - Retrograde motion - Ptolemaic system - 2nd century

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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 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.

Related Topics:
Counter-intuitive - 4th century BC - Aristotle - Pythagoreans - Fire - Philosophic - Classical element

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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 by Eratosthenes, and he himself measured the size and distance of the Moon and Sun; while his figures for the Moon were fairly decent, those he obtained for the Sun were quite far off 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.

Related Topics:
Aristarchus - 270 BC - Earth - Eratosthenes - Moon

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Aristarchus' original work on heliocentrism has not survived and is known only from others' accounts; hence the uncertainty as to his arguments on its behalf. It appears, though, that he understood the problem of stellar parallax: if the Earth moves over huge distances in circling the Sun, then the nearer of the fixed stars should be seen to move relative to the farther ones, as nearby hills move relative to distant mountains when one is traveling. Aristarchus explained the lack of any such visible effect by saying that the stars were at extremely large distances: the sphere of the fixed stars was to the Earth's orbit as the surface of a sphere is to its center. That would make the stars infinitely distant; whether he meant that literally, or just meant to convey an extremely large ratio, is not possible to determine now. (In the event his explanation turned out to be right, though the distances are finite; stellar parallax was observed in the 19th century.)

Related Topics:
Parallax - 19th century

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Aristarchus' heliocentric model was considered by Archimedes in

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The Sand Reckoner. The purpose of this work was to prove that extremely large numbers, even the number of grains of sand that it would take to fill the universe, could be expressed mathematically and did not have to treated vaguely

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as "infinite". To this end, he took the largest existing model of the universe, which was that of Aristarchus, to calculate the amount of sand that would fill even that universe. Pointing out that mathematically it made no sense to talk of a ratio between the surface of a sphere and its center, which has no magnitude, Archimedes made the working assumption that the distance of the fixed stars was in the same relation to the radius of the Earth's orbit as that orbit was in relation to the Earth itself. Under these conditions stellar parallax would be beyond current observers' ability to detect, as it was in fact. http://www.lix.polytechnique.fr/Labo/Ilan.Vardi/sand_reckoner.ps This treatment shows that the problem of stellar parallax was well understood, though it

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produces no information on whether the Earth's motion was a reality.

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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 apparently unknown in the West.

Related Topics:
5th century - India - Astronomer - Aryabhata - Latin

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Copernicus also drew on the ideas of the (possibly semi-mythical) Egyptian philosopher Hermes Trismegistus in De revolutionibus orbium coelestium but it is not clear to what extent Trismegistus was actually proposing a heliocentric world view. 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.

Related Topics:
Hermes Trismegistus - Hermetic - Christ

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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 or spin out into space; an object dropped from a tower would fall behind the tower as the latter rotated with the Earth and would land to the West; and so on. A response to these objections required much better understanding of physics.

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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; they had resolved significant problems in the Ptolemaic system, though retaining an essentially geocentric arrangement.

Related Topics:
16th century - Nicolaus Copernicus - Real - Parallax - Tychonian system - Epicycles - Pretzel - Ibn al-Shatir - Nasir al-Din al-Tusi

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