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History of gravitational theory
In physics, theories of gravitation postulate mechanisms of interaction governing the movements of bodies with mass. There have been numerous theories of gravitation since ancient times.
==Antiquity==
In the 4th century BC, the Greek philosopher Aristotle believed that there is no effect or motion without a cause. The cause of the downward motion of heavy bodies, such as the element earth, was related to their nature, which caused them to move downward toward the center of the universe, which was their natural place. Conversely, light bodies such as the element fire, move by their nature upward toward the inner surface of the sphere of the Moon. Thus in Aristotle's system heavy bodies are not attracted to the earth by an external force of gravity, but tend toward the center of the universe because of an inner ''gravitas'' or heaviness.〔Edward Grant, ''The Foundations of Modern Science in the Middle Ages'', (Cambridge: Cambridge Univ. Pr., 1996), pp. 60-1.〕〔Olaf Pedersen, ''Early Physics and Astronomy'', (Cambridge: Cambridge Univ. Pr., 1993), p. 130〕
In Book VII of his ''De Architectura'', the Roman engineer and architect Vitruvius contends that gravity is not dependent on a substance's "weight" but rather on its "nature" (cf. specific gravity).
If the quicksilver is poured into a vessel, and a stone weighing one hundred pounds is laid upon it, the stone swims on the surface, and cannot depress the liquid, nor break through, nor separate it. If we remove the hundred pound weight, and put on a scruple of gold, it will not swim, but will sink to the bottom of its own accord. Hence, it is undeniable that the gravity of a substance depends not on the amount of its weight, but on its nature.〔
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Brahmagupta, the ancient Indian astronomer and mathematician, held the view that the earth was spherical and that it attracts things. Al Hamdānī and Al Biruni quote Brahmagupta saying "Disregarding this, we say that the earth on all its sides is the same; all people on the earth stand upright, and all heavy things fall down to the earth by a law of nature, for it is the nature of the earth to attract and to keep things, as it is the nature of water to flow, that of fire to burn, and that of the wind to set in motion. If a thing wants to go deeper down than the earth, let it try. The earth is the only ''low'' thing, and seeds always return to it, in whatever direction you may throw them away, and never rise upwards from the earth."
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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