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.

Though it was a purely mathematical system which only attempted to give a basis for computing celestial motions, Ptolemaic astronomy is of great importance historically as it remained the foundation of theoretical astronomy for more than 1400 years. Throughout the long dark centuries of the Middle Ages it survived in the studies of the retired students of the monasteries and of the few exceptionally enlightened men who still had some regard for pagan learning in the days when many of the Church Fathers denounced it as heretical.

Ptolemy was the last of the great original Greek astronomers. The Alexandrian school produced, after him, only copyists and commentators, and the theoretical astronomy of the Greeks, so highly perfected in Ptolemy’s Almagest, was for many centuries almost entirely neglected. The Roman State gave no encouragement to the study of theoretical astronomy and produced no new school of astronomy. Although it was the fashion for a Roman to have a smattering of Greek astronomy, and famous Latin authors like Cicero, Seneca, Strabo and Pliny wrote on astronomy, yet the Romans cared little for original investigations and contributed nothing new to the science. The Romans, however, appreciated the value of astronomy in measuring time, and applied to the Alexandrian school to satisfy their practical need for a calendar. What Julius Caesar obtained from the Alexandrian Sosigenes, he greatly improved and gave to the Empire, as the calendar which, with the exception of the slight change made by Gregory XIII, we still use.

The pseudo-astronomical science of astrology, or the so-called ‘judicial astronomy’ was pursued during the Roman Empire and throughout the Middle Ages with much greater zeal than theoretical astronomy. The interest in astrology, to be sure, encouraged the study of observational astronomy to a certain extent; for the casting of horoscopes to foretell destinies required that the heavenly bodies be observed and methods of calculating their positions at any time or place be known. But there was no desire to inquire into the underlying laws of the celestial motions or to investigate the real nature of the heavenly phenomena.

If the Roman State did not encourage astronomy, the Roman Church positively discouraged it. The Bible became and long remained the sole authority recognized by the Church Fathers as to the constitution of the universe. By many of the Patristics Ptolemaic astronomy was despised; not because it did not describe accurately the observed phenomena of the heavens, for it did this in a way that could scarcely have been improved upon with the facilities for observation then available; and not because it was founded upon the false assumption that the earth is the motionless center of the universe about which all heavenly bodies revolve; but because there was no authority in Scripture for such a system, and it could not possibly be made consistent with the cosmology of Genesis. Allegorical descriptions of the universe based on the Scriptures held almost complete sway over the mediaeval mind. The whole universe was represented allegorically by the tabernacle and its furniture. The earth was flat and rectangular like the table of shew bread, and surrounded on all four sides by the ocean. The walls of heaven beyond this supported the firmament shaped like a half-cylinder. Angels moved the sun, moon, and stars across the firmament and let down rain through its windows from the expanse of water above.

By no means all of the early Church Fathers were wholly without appreciation of the fruits of Greek astronomical science. Origen and Clement of Alexandria, while believing in the scriptural allegories, tried to reconcile them with the results of pagan learning. In the West, Ambrose of Milan and later Augustine, were at least not opposed to the idea of the earth’s sphericity, and of the existence of antipodes, although they could not get away from the queer notion of the waters above the firmament. A few enlightened students like Philoponus of Alexandria, Isidore of Seville, the Venerable Bede, and Irish scholars like Fergil and Dicuil, studied the Greek philosophers and accepted some of the pagan scientific teachings.

Fortunately the study of those ancient Latin writers whose works had preserved some of the astronomy of the Greeks had taken firm root among the patient scholars of the monasteries, and slowly but steadily the geocentric system of cosmology was making its way back into the realm of generally accepted fact, so that by the ninth century it was the system adopted by nearly all scholars.

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