Dante and the early astronomers . 1° in a century, according to Ptolemy, sothat it completes a revolution in 36,000 years. Coming now^ to the fixed stars, their number, andbrightness, Alfraganus does not copy Ptolemys greatcatalogue, but informs us that learned men (sapientes)did number all the fixed stars as far south as theycould see in the 3rd climate, and divided them accord-ing to magnitude into six classes. To the first classthey assigned the bright and shining stars such asCanis (Sirius) and Procyon, Vultur Cadens (Vega) andCor Leonis (Regulus). Stars a little less bright theycalled sec
Dante and the early astronomers . 1° in a century, according to Ptolemy, sothat it completes a revolution in 36,000 years. Coming now^ to the fixed stars, their number, andbrightness, Alfraganus does not copy Ptolemys greatcatalogue, but informs us that learned men (sapientes)did number all the fixed stars as far south as theycould see in the 3rd climate, and divided them accord-ing to magnitude into six classes. To the first classthey assigned the bright and shining stars such asCanis (Sirius) and Procyon, Vultur Cadens (Vega) andCor Leonis (Regulus). Stars a little less bright theycalled second magnitude: such are Alf arcatein andBenet Naax, Arab constellations which the Latinversion describes as the two bright stars of Ursa Minor, 188 ARAB ASTRONOMY. aud those brilliant ones in the tail of Ursa they proceeded with the other magnitudes, thesmallest measured being of the sixth magnitude. Thenumber of stars in each class is given, and the total of1022;^ then a list of the 15 first magnitude stars, MercurVj. Fig. 36. Method (erroneous) of estimating planetary distances,described by Alfraganus. The distance EA being known, and Up being assumed equal to it, the distance Eacan be calculated from the known ratio Ea : p a. which are the same as Ptolemys (See p. 155). This isfollowed by a list of the Arab Mansions of the far (with the exception of the mansions of themoon) the Arab writer has followed the Greek, but wehave now reached a point where he diverges. Ptolemy,he says, only tells us the distances and sizes of the sun ^ The Catalogue of Hipparchus is said to have contained 1080 stars,but Ptolemys has only 1022. ARAB ASTRONOMY. 189 and moon, and said nothing about the other heavenlybodies; but if we suppose the greatest distance of themoon to be the same as the least distance of Mercury, andfrom this calulate his greatest distance (for the ratiois known), and if we proceed in the same way withMercury and Venus, we shall find that the greatestdista
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