THE CONCEPTUAL STRUCTURE OF PTOLEMAIC ASTRONOMY

In astronomy before Kepler the path of a planet was not its orbit but the pattern of its motion seen by an observer on a stationary earth against the hypothetical sphere of the heavens. It was recognized in antiquity that this pattern was not a real motion, but a complex outcome of the observer's viewpoint and a variety of circular motions that acted together. The task of astronomy was to define this pattern – to specify the path of the planet in this original sense. The real motion of the planet – its track in what we would now call three-dimensional space – was unknown, and possibly irrelevant. All other considerations – the causes of celestial motion, the actual dimensions of the heavens – were the business of a separate science, cosmology. It was well known that the goal of astronomy could be achieved, that is, the path of a celestial object could be predicted, without making specific assumptions about its distance from the earth, once appropriate rates of rotation were introduced (Evans 1998; Pedersen 1993).

The basic data of astronomy from antiquity to the sixteenth century – the explananda or, if you prefer, the ‘phenomena’ that needed to be ‘saved’ – were recorded observations of planetary positions. Sixteenth-century astronomy texts devoted most of their attention to motion in longitude.