http://www.perseus.tufts.edu/GreekScience/Students/Tom/AristotleAstro.html
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Four elements made up the central part of the universe: earth,
air, fire, and water.
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These elements each have their own place in the universe
determined by their heaviness or 'specific gravity.'
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They each move in a straight line towards that place, making
terrestrial motion linear, and stopping once reaching their respective
proper places.
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Contrary to the central part of the universe, he believed
the heavens to move continually in a circle. He determined that it must
be of a different element, which he termed aither.
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Aither was defined as incapable of change other than change
in location.
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