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CaptionThe first person known to have proposed a heliocentric system however was Aristarchus of Samos c. 270 BC. Like Eratosthenes Aristarchus calculated the size of the Earth and measured the size and distance of the Moon and Sun in a treatise which has survived. From his estimates he concluded that the Sun was six to seven times wider than the Earth and thus hundreds of times more voluminous. His writings on the heliocentric system are lost but some information is known from surviving descriptions and critical commentary by his contemporaries such as Archimedes. Some have suggested that his calculation of the relative size of the Earth and Sun led Aristarchus to conclude that it made more sense for the Earth to be moving than for the huge Sun to be moving around it. Though the original text has been lost a reference in Archimedes' book The Sand Reckoner describes another work by Aristarchus in which he advanced an alternative hypothesis of the heliocentric model. Archimedes wrote: You King Gelon are aware the 'universe' is the name given by most astronomers to the sphere the center of which is the center of the Earth while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses wherein it appears as a consequence of the assumptions made that the universe is many times greater than the 'universe' just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved that the Earth revolves about the Sun on the circumference of a circle the Sun lying in the middle of the orbit and that the sphere of fixed stars situated about the same center as the Sun is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.[10] Aristarchus thus believed the stars to be very far away and saw this as the reason why there was no visible parallax that is an observed movement of the stars relative to each other as the Earth moved around the Sun. The stars are in fact much farther away than the distance that was generally assumed in ancient times which is why stellar parallax is only detectable with telescopes. Archimedes says that Aristarchus made the stars' distance larger suggesting that he was answering the natural objection that heliocentrism requires stellar parallactic oscillations. He apparently agreed to the point but placed the stars so distant as to make the parallactic motion invisibly minuscule. Thus heliocentrism opened the way for realization that the universe was larger than the geocentrists taught.
HeadlineHeliocentrism vs. Geocentrism was a key debate in Alexandria we know very little about but Aristarchus of Samos looms large in this continuing story of the Earth revolving around the Sun
KeywordsAristarchus Heliocentrism

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