At the beginning of his Epitome astronomiae Copernicanae, Kepler lists the following components of astronomy, all of which he considers necessary to the science of celestial phenomena (Kepler 1953:23). The astronomer’s task, he says, consists of five main parts: historical, to do with the recording and classification of observations; optical, to do with the shaping of the hypotheses; physical, dealing with the causes underlying hypotheses; arithmetical, concerned with tables and computation; and mechanical, relating to instruments. The first three areas, adds Kepler, involve mainly theory; the last two are more concerned with practical aspects.
In each of the areas identified by Kepler, the contribution of Arabic science was essential to the birth and subsequent development of astronomy in the Latin West. Prior to this contribution, there was indeed no astronomy of any advanced level in those countries. 1 What was understood by astronomy was scarcely more than a collection of imprecise cosmological ideas concerning the shape and size of the world, and some basic notions about the movements of celestial bodies, principally concerning synodical phenomena, such as heliacal risings and settings. The needs of the Church with regard to the regulation of the calendar had nourished a tradition of chronological calculation following the De temporum ratione of Bede (d. 735). But this literature of computation, with which the names of Raban Maur, Dicuil or Garlande are associated, was not based on any mathematical treatment of the phenomena. A single example will suffice: in Bede, the planetary movements are represented by simple eccentrics, and the second planetary anomaly thus remains unexplained. In short, the science of the heavens in the early Middle Ages lacked observations, geometrical analysis