It was in the fourteenth century that Chaucer lived and wrote, and his interest in astronomical lore is, therefore, not surprising. Although the theories of astronomy current in Chaucer's century have been made untenable by the _De Revolutionibus Orbium_ of Copernicus, and by Kepler's discovery of the laws of planetary motion; although the inaccurate and unsatisfactory methods of astronomical investigation then in use have been supplanted by the better methods made possible through Galileo's invention of the telescope and through the modern use of spectrum analysis; yet, of all scientific subjects, the astronomy of that period could most nearly lay claim to the name of science according to the present acceptation of the term. For, as we have seen, the interest in astrology during the Middle Ages had fostered the study of observational astronomy, and this in turn had furnished the science a basis of fact and observation far surpassing in detail and accuracy that of any other subject. Practically all of Chaucer's writings contain some reference to the movements and relative positions of the heavenly bodies, and to their influence on human and mundane affairs, and in some of his works, especially the treatise on _The Astrolabe_, a very technical and detailed knowledge of astronomical and astrological lore is displayed. There is every reason to suppose that, so far as it satisfied his purposes, Chaucer had made himself familiar with the whole literature of astronomical science. His familiarity with Ptolemaic astronomy is shown in his writings both by specific mention[1] of the name of Ptolemy and his _Syntaxis_, commonly known as the 'Almagest,' and by many more general astronomical references. Even more convincing evidence of Chaucer's knowledge of the scientific literature of his time is given in his _Treatise on the Astrolabe_. According to Skeat, Part I and at least two-thirds of Part II are taken, with some expansion and alteration, from a work on the Astrolabe by Messahala[2], called, in the Latin translation which Chaucer used, "Compositio et Operatio Astrolabie." This work may have been ultimately derived from a Sanskrit copy, but from Chaucer's own words in the _Prologue to the Astrolabe_[3] it is clear that he made use of the Latin work. The rest of Part II may have been derived from some general compendium of astronomical and astrological knowledge, or from some other of the treatises on the Astrolabe which Chaucer says were common in his time.[4]