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"Ever since antiquity astronomy has consisted of both theory and observation, but these two components have often received different treatments in the original sources. In the medieval period we find many texts that present theories (even new theories) for the motions of the Sun, the Moon, and the planets; and other texts that describe instruments (some newly invented) for making observations. Moreover, medieval scholars carefully read various works that survived from antiquity, notably Ptolemy's Almagest, and these treatises served as a guide for the scientific study of astronomy. In particular, Ptolemy described methods of determining the planetary models (or parts of them) from sets of dated observations, and he gave numerous examples (including many based on observations he himself made) which take up a major portion of his magnus opus. In this respect, however, the vast majority of his successors did not follow him, for we find surprisingly few planetary observations in the medieval astronomical corpus. (A similar paucity of observations of the Sun, the Moon, and eclipses has also been noted.) Indeed, in most astronomical tables compiled in the Middle Ages observations play no role, and it can be demonstrated that the tabular entries are largely based on earlier astronomical theories."