Turning now from the sources to the material substance of the contribution of the Moslems, we find that, except in so far as it brought a knowledge of Aristotle to Europe, it added little to Western notions either of physical or of regional geography. Though the Moslems entirely failed to share with the Western World their wide practical acquaintance with lands and seas, the Arabic writers did nevertheless introduce some new ideas in the fields of astronomical—or, better, astrological—geography and in the closely allied study of tidal phenomena. THE GREAT YEARS The theory of the Great Years was very popular among the Orientals, possibly because it appealed to their fatalistic spirit. Arabic astronomers adopted Ptolemy’s calculation of the length of the Great Year at 36,000 terrestrial years[346] and seem to have believed that after every complete revolution of the sphere of the fixed stars, the planets, as well as the fixed stars, will find themselves in the same relative positions that they held at the beginning of the revolution.[347] The Arabic work on this subject most read in the Latin West was Hermann the Dalmatian’s translation of Abū Maʿshar’s book, in which it was explained that astral influences—especially the perpetual circulation of the fixed stars—are the cause of everything which is born and dies and of everything which occurs between birth and death on this earth. COSMIC CYCLES AND GEOGRAPHICAL CHANGES In much the same way that the Chaldeans, Hindus, and Greeks had done, the Moslems worked out a theory of the supposed influence of these cosmic cycles on geography.[348] The most striking elaboration of the theory was made by the “Brothers of Piety and Sincerity,” who formed a philosophical school in the tenth century after Christ. In the great encyclopedic work[349] produced by this school (which, incidentally, contains many other interesting speculations on the subject of physical geography) gradual alterations in the relative position of land and sea are ascribed to almost imperceptible changes in the longitude of the fixed stars resulting from the precession of the equinoxes. Not only do lands and seas change places, but various types of terrain; in the course of time “cultivated land becomes desert, desert becomes cultivated land, steppes become seas, and seas become steppes or mountains.”[350]