<P> Adherence to the geocentric model stemmed largely from several important observations . First of all, if the Earth did move, then one ought to be able to observe the shifting of the fixed stars due to stellar parallax . In short, if the Earth was moving, the shapes of the constellations should change considerably over the course of a year . If they did not appear to move, the stars are either much farther away than the Sun and the planets than previously conceived, making their motion undetectable, or in reality they are not moving at all . Because the stars were actually much further away than Greek astronomers postulated (making movement extremely subtle), stellar parallax was not detected until the 19th century . Therefore, the Greeks chose the simpler of the two explanations . Another observation used in favor of the geocentric model at the time was the apparent consistency of Venus' luminosity, which implies that it is usually about the same distance from Earth, which in turn is more consistent with geocentrism than heliocentrism . In reality, that is because the loss of light caused by Venus' phases compensates for the increase in apparent size caused by its varying distance from Earth . Objectors to heliocentrism noted that terrestrial bodies naturally tend to come to rest as near as possible to the center of the Earth . Further barring the opportunity to fall closer the center, terrestrial bodies tend not to move unless forced by an outside object, or transformed to a different element by heat or moisture . </P> <P> Atmospheric explanations for many phenomena were preferred because the Eudoxan--Aristotelian model based on perfectly concentric spheres was not intended to explain changes in the brightness of the planets due to a change in distance . Eventually, perfectly concentric spheres were abandoned as it was impossible to develop a sufficiently accurate model under that ideal . However, while providing for similar explanations, the later deferent and epicycle model was flexible enough to accommodate observations for many centuries . </P> <P> Although the basic tenets of Greek geocentrism were established by the time of Aristotle, the details of his system did not become standard . The Ptolemaic system, developed by the Hellenistic astronomer Claudius Ptolemaeus in the 2nd century AD finally standardised geocentrism . His main astronomical work, the Almagest, was the culmination of centuries of work by Hellenic, Hellenistic and Babylonian astronomers . For over a millennium European and Islamic astronomers assumed it was the correct cosmological model . Because of its influence, people sometimes wrongly think the Ptolemaic system is identical with the geocentric model . </P> <P> Ptolemy argued that the Earth was a sphere in the center of the universe, from the simple observation that half the stars were above the horizon and half were below the horizon at any time (stars on rotating stellar sphere), and the assumption that the stars were all at some modest distance from the center of the universe . If the Earth was substantially displaced from the center, this division into visible and invisible stars would not be equal . </P>

Who did 30 years of very accurate observations of the planets