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Heliocentrism, or heliocentricism,〔The Shorter Oxford English Dictionary (6th ed., 2007)〕 is the astronomical model in which the Earth and planets revolve around the Sun at the center of the Solar System. The word comes from the Greek ( ''helios'' "sun" and ''kentron'' "center"). Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. The notion that the Earth revolves around the Sun had been proposed as early as the 3rd century BC by Aristarchus of Samos,〔Dreyer (1953), (pp.135–48 ); Linton (2004), (pp.38–9) ). The work of Aristarchus's in which he proposed his heliocentric system has not survived. We only know of it now from a brief passage in Archimedes's ''The Sand Reckoner''.〕 but at least in the post-ancient world Aristarchus's heliocentrism attracted little attention—possibly because of the loss of scientific works of the Hellenistic Era.〔according to Lucio Russo, the heliocentric view was expounded in Hipparchus's work on gravity. (source: Lucio Rosso, ''The Forgotten Revolution, How Science was Born in 300BC and Why it had to be Reborn'', pp 293-296)〕 It was not until the 16th century that a geometric mathematical model of a 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 elaborated upon and expanded this model to include elliptical orbits, and Galileo Galilei presented supporting observations made using a telescope. With the observations of William Herschel, Friedrich Bessel, and others, astronomers realized that the sun was not the center of the universe as heliocentrists at the time of Copernicus had supposed. Modern thinking is that there is no specific location that is the center of the universe, per Albert Einstein's principle of relativity. ==Early developments== To anyone who stands and looks up at the sky, it seems that the Earth stays in one place, while everything in the sky rises in the east and sets in the west once a day. However, with more scrutiny one will observe more complicated movements. The positions at which the Sun and moon rise change over the course of a year, some planets and stars do not appear at all for many months, and planets sometimes appear to have moved in the reverse direction for a while, relative to the background stars. As these motions became better understood, more elaborate descriptions were required, the most famous of which was the geocentric Ptolemaic system, which achieved its full expression in the 2nd century. The Ptolemaic system was a sophisticated astronomical system that managed to calculate the positions for the planets to a fair degree of accuracy.〔, (Chapter V, page 76 ) 〕 Ptolemy himself, in his ''Almagest'', points out that any model for describing the motions of the planets is merely a mathematical device, and since there is no actual way to know which is true, the simplest model that gets the right numbers should be used.〔 In Book 1 section 7 he admits that a model in which the earth revolves with respect to the stars would be simpler but doesn't go as far as considering a heliocentric system.〕 However, he rejected the idea of a spinning earth as absurd as he believed it would create huge winds. His planetary hypotheses were sufficiently real that the distances of moon, sun, planets and stars could be determined by treating orbits' celestial spheres as contiguous realities. This made the stars' distance less than 20 Astronomical Units,〔Dennis Duke, (Ptolemy's Universe )〕 a regression, since Aristarchus of Samos's heliocentric scheme had centuries earlier necessarily placed the stars at least two orders of magnitude more distant. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heliocentrism」の詳細全文を読む スポンサード リンク
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