The view of the universe which we find reflected in Chaucer's poetry is chiefly based on the Ptolemaic system of astronomy, though it shows traces of very much more primitive cosmological ideas. The Ptolemaic system owes its name to the famous Alexandrian astronomer of the second century A. D., Claudius Ptolemy, but is based largely on the works and discoveries of the earlier Greek philosophers and astronomers, especially Eudoxus, Hipparchus, and Aristarchus, whose investigations Ptolemy compiled and, along some lines, extended. Ptolemaic astronomy was a purely geometrical or mathematical system which represented the observed movements and relative positions of the heavenly bodies so accurately that calculations as to their positions at any given time could be based upon it. Ptolemy agreed with his contemporaries in the opinion that to assign causes for the celestial movements was outside the sphere of the astronomer. This was a proper field of philosophy; and the decisions of philosophers, especially those of Aristotle, were regarded as final, and their teaching as the basis upon which observed phenomena should be described. According to the Ptolemaic system the earth is a motionless sphere fixed at the center of the universe. It can have no motion, for there must be some fixed point in the universe to which all the motions of the heavenly bodies may be referred; if the earth had motion, it was argued, this would be proportionate to the great mass of the earth and would cause objects and animals to fly off into the air and be left behind. Ptolemy believed this reason sufficient to make untenable the idea of a rotatory motion of the earth, although he was fully aware that to suppose such a motion of the earth would simplify exceedingly the representations of the celestial movements. It did not occur to him that to suppose the earth's atmosphere to participate in its motion would obviate this difficulty. The earth was but a point in comparison with the immense sphere to which the stars were attached and which revolved about the earth once in every twenty-four hours, imparting its motion to sun, moon, and planets, thus causing day and night and the rising and setting of the heavenly bodies. The irregular motions of the planets were accounted for by supposing them to move on circles of small spheres called 'epicycles', the centres of which moved around the 'deferents', or circles of large spheres which carried the planets in courses concentric to the star sphere. By giving each of the planets an epicycle and deferent of the proper relative size and velocity the varied oscillations of the planets, as far as they could be followed by means of the simple instruments then in use, were almost perfectly accounted for.