BornRossasecca, (Lazio, Italy), circa1223

DiedFossanova, (Lazio, Italy), 5–7 March 1274

Thomas Aquinas’ importance to the history of astronomy lies in his reconciliation of Aristotelian cosmology and twelfth-century astrology with Christian theology.

Saint Thomas Aquinas was the foremost Catholic theologian of the medieval world. Born into an aristocratic south Italian family, he became a Dominican Friar at the age of 16. In 1245, he arrived in Paris, where he became a student of Albert the Great , the most prominent exponent of Aristotelian philosophy. Aquinas took his bachelor’s degree in 1248, returning to Paris in 1253 to prepare for his master’s degree, which he received in 1257. He was sent to Italy to teach in various Dominican houses in 1259, returned to Paris in 1269, and was sent to Naples in 1272 to set up a Dominican school. His reputation in the modern world was affirmed in 1879 when Pope Leo XIII named him “the chief and master among all the scholastic doctors” in his encyclical Aeterni patris.

Aristotle ’s work had become familiar to Western scholars in the twelfth century partly through original translations, notably the Meteorologica (translated by Henry Aristippus between 1150 and 1160) and partly through the work of Arabic scholars such as Avicenna ( Ibn Sina ) and Averroes ( Ibn Rushd ). The overall effect of this material was quite revolutionary. It introduced into Catholic learning the work of a philosopher who had accepted Plato ’s doctrine of a single God, and hence whose work seemed compatible with Christianity, but argued for the eternity of the Universe, thus denying both the reality of the Genesis creation myth and the possibility of the Last Judgment and inauguration of the Kingdom of God. The introduction of Aristotelian material was accompanied by the translation of major astrological texts, particularly Claudius Ptolemy ’s Tetrabiblos (1138), the pseudo-Ptolemaic Centiloquium (1136), and the Maius Introductorium (1140), the major introduction to astrology composed by the Persian astrologer Abu Ma’shar. Combined with Aristotle’s statement that “the celestial element, … [the] source of all motion, must be regarded as first cause” (Meteorologica I.ii), such work established astrology as a central feature of Western science and an integral part of medieval astronomy. If, for example, an understanding of the wider celestial environment was essential to the analysis of events on Earth, then astronomy now possessed directly practical applications in the treatment of disease, the prophecy of peace and war, the prediction of individual fortunes, and the selection of auspicious moments to inaugurate important enterprises.

The extent of Aquinas’ writing is immense, and his highest achievement was the Summa Theologica, a complete systematization of Christian theology. His writings on the stars are contained in the Summa contra Gentiles, a textbook for missionaries, which summarizes the arguments to be put in response to various non scriptural claims. All thirteenth-century Catholic theologians were obliged to take a position vis-à-vis Aristotelian teaching, its implications for astronomy, and the safe philosophical ground it provided for astrology. Many were hostile. Aquinas, following Albertus Magnus’ example, was openly sympathetic to both Aristotle and the associated astrological texts, and his contribution to the history of astronomy lies in the third way he established between astral determinism and the requirement, central to Christianity, that the individual must be able to make a free choice between good and evil and thus achieve salvation. Saint Augustine ’s solution, which was still prevalent in the thirteenth century, was that the stars had no influence at all and that all power lay with God. Aquinas’ alternative solution, set out in Summa contra Gentiles (Chaps. 83–88), allowed the stars, as secondary causes in an Aristotelian sense, to rule the physical world, while retaining the Augustinian doctrine that the human will, and hence the chance of salvation, was responsible to God alone. Thus, any form of astrology that dealt with the consequences of natural disorder or physical passion was permissible. Medical astrology was acceptable, as was the prediction of war and peace. The election of auspicious astronomical moments to inaugurate new enterprises was deemed unacceptable because it impinged on God’s providential right to dictate the outcome of events, as was the use of interrogations, the casting of horoscopes to answer precise questions about the future. Genetheliacal astrology, which dealt with individual lives, was acceptable in as much as it dealt with physical existence, but not if it denied moral choice.

Aquinas’ work was condemned at Paris in 1277, but in 1278, the Dominican General Chapter officially imposed his teachings upon the order. His moral cosmology remained an influential component within Catholic thinking on astronomy until the seventeenth century and provided a rationale for astrology that was unavailable within more conservative wings of the Church, which remained loyal to Augustine.