About the year 1000 began the impetus to learning which culminated in the great revival of the Renaissance. One cause of this intellectual awakening was the contact of Europe with Arab culture through the crusades and through the Saracens in Sicily and the Moors in Spain. The Arabian influence resulted in an increased sense of the importance of astronomy and astrology; for, while the scholars of the Christian world had been devising allegorical representations of the world based on sacred literature, the Arabian scholars had been delving into Greek science, translating Ptolemy and Aristotle, and trying to make improvements upon Ptolemaic astronomy. The spheres of the planets, which Ptolemy had almost certainly regarded as purely symbolical, the Arabs conceived as having concrete existence. This made it necessary to add a ninth sphere to the eight mentioned by Ptolemy; for it was thought sufficient that the eighth sphere should carry the stars and give them their slow movement of precession from west to east. This ninth sphere was the outermost of all and it originated the "prime motion" by communicating to all the inner spheres its diurnal revolution from east to west. In mediaeval astronomy it came to be known as the _primum mobile_ or "first movable," while a tenth and motionless sphere was added as the abode of God and redeemed souls. The sun and moon were included among the planets, which revolved about the earth in the order Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn. At first the astronomy taught in the universities was based on Latin translations of Arabic commentaries and paraphrases of Aristotle, which had made their way into Europe through the Moors in Spain. For several centuries Aristotle represented in the eyes of most scholastics "the last possibility of wisdom and learning." But by the middle of the thirteenth century Ptolemy began to be rediscovered. The Ptolemaic system of planetary motions was briefly described in a handbook compiled by John Halifax of Holywood, better known as Sacrobosco. Roger Bacon wrote on the spheres, the use of the astrolabe, and astrology, following Ptolemy in his general ideas about the universe. The great mediaeval scholar and philosopher, Thomas Aquinas, was also familiar with the Ptolemaic system; but to most of the men of the thirteenth century Ptolemy's works remained quite unknown. The real revival of Greek astronomy took place in the fourteenth century when scholars began to realize that new work in astronomy must be preceded by a thorough knowledge of the astronomy of the Alexandrian school as exhibited in the _Syntaxis_ of Ptolemy. It was then that Greek and Latin manuscripts of works on astronomy began to be eagerly sought for and deciphered, and a firm foundation constructed for the revival of theoretical astronomy. II CHAUCER'S SCIENTIFIC KNOWLEDGE