118 astronomy: Europe devastating The chief intellectual language of the Roman Empire had been Greek, and in the Western world all knowledge of the Greek language as well as of the information in Greek books was lost (except for the little that had been translated into Latin or summarized or excerpted in Latin encyclopedic works) Detailed knowledge of ancient astronomy was often replaced by opinion offered by such Christian Latin authors as Lactantius (ca 240–ca 320 c.e.), who in his Divinae institutiones (Divine Institutions) reverted to a prescientific astronomy based on the Old Testament, according to which the earth was a flat circle surrounded by a single dome containing the heavenly bodies Practically the only concern of the Catholic Church (the most sophisticated institution in western Europe) with astronomy was to regulate the calendar and especially to reckon the date of Easter each year In the Eastern Roman Empire, Greek astronomical learning was never lost, but it was not advanced either Nor did much communication between the Latin and Byzantine spheres exist on intellectual matters until the early Renaissance (after 1400 in Italy) As such, Western Europe’s reacquaintance with ancient astronomy came instead from contact with the Islamic world In 11th-century Spain, numerous books of Greek philosophy and science as well as treatises in which Arab scholars had synthesized and advanced Greek ideas, began to be translated from Arabic into Latin The most important Greek work on astronomy was the Almagest (Great Treatise), by Ptolemy (ca 90–ca 168 c.e.) Western languages still refer to this book by its Arabic title, Almagest This encyclopedic work was translated into Arabic only around 900 c.e., and this translation was further rendered into Latin by Gerard of Cremona (ca 1114–87) at Toledo, in Spain, after 1144 The intellectual framework of all Arabic work in astronomy, however, was the science of the Greek philosopher Aristotle (384–322 b.c.e.), and this held true for European medieval culture as well, which received Greco-Arab astronomy as part of the general body of Aristotelian science in the 12th century Science at this level was the concern solely of university professors, who in medieval Europe were necessarily members of the Catholic clergy The Roman Catholic Church, however, had some inclination to view the philosophical learning received form Arab culture as a threat, not only because it was foreign and not Christian but also because it seemed to set up a new body of knowledge based on human reason that stood apart from the divine wisdom privileged by the church as an institution These problems were soon solved by Thomas Aquinas (ca 1225–74 c.e.), who in his Summa theologica (Summary of Theology) and other works synthesized a new philosophical system incorporating Aristotelian science that withstood theological scrutiny In the system of astronomy that he worked out based on his limited knowledge of the work of Aristotle and Ptolemy, Aquinas conceived of the earth as a sphere The earth sat motionless at the center of the universe and was surrounded by celestial spheres, to which were affixed the seven planets visible to the naked eye (from closest to farthest): the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn These celestial spheres were commonly referred to as crystal, but in fact they were held to be made of a perfect form of matter that could not exist in the fallen and imperfect physical world of the earth Each planet was mounted on its sphere by an armature consisting of several smaller spheres whose movements were called epicycles This geometrically complex arrangement was necessary because only by presupposing such a mechanism could an accurate mathematical model be worked out for predicting the motions of the planets, allowing for phenomena such as pausation and retrogradation, when planets seem to suddenly stop or reverse their direction of movement Zodiac sign of Aries the Ram (figure decorated with stars, suggesting constellation, against blue sky background); Italy, 1473 (Courtesy the Morgan Library and Museum)