PTOLEMY (PTOLEMAEUS), CLAUDIUS. - Astronomer, geographer and mathematician, b. perhaps at Ptolemais in the Thebaid around 100 A.D., lived in Alexandria in Egypt and d. at Canopus around 178 A.D.
His principal work is the great treatise in 13 books, Μεγάλη Σύνταξις ("Great Compendium"). The Arab translators, perhaps out of admiration, changed μεγάλη (= great) into the superlative μεγέστη (= greatest), whence in Arabic Al Magisti, in Latin Almagestum, in Italian Almagesto. The work acquired great fame in the East and in the West, and remained for about 14 centuries the principal treatise on astronomy. It had many editions and was translated from Greek into Arabic around the year 827 A.D. by order of the caliph of Baghdad, al-Ma'mun; and into Latin around 1230, whence Dante learned of it and perhaps from it drew his knowledge of the Southern Cross, of which he speaks in Purg., I, 23 ff. The treatise is for the most part a work of compilation. After a trigonometric introduction, it proceeds to describe the order of the Universe; the motions of the sun and moon, eclipses, the motions of the planets, with a catalogue of 1028 stars.
The Ptolemaic system is based on the theory of Hipparchus, with the spherical Earth placed at the center of the celestial sphere. The Earth has dimensions so insignificant that any point on its surface may be considered the center of the Universe. The Sun and the Moon move along circles of which the Earth occupies a point very close to the center. In this connection T. completed the explanation given by Hipparchus of the motion of the Moon, introducing a new irregularity, the prosneusis, which has been confirmed by celestial mechanics; he also determined approximately the mean distance between the Moon and the Earth. To account for the complicated apparent motion of the planets, which seem to shift among the stars now in one direction, now in the opposite, and sometimes seem to stop, T. imagines that each of them moves describing a circle called an epicycle, of which a point very close to the center moves in turn, always in the same direction, upon another circle, called the deferent, which has the Earth almost at its center.
Notable is the contribution made by T. to geometry and trigonometry: to be remembered are the rules he suggested for the construction of a table of chords, T.'s theorem on the quadrilateral inscribed in a circle, and the applications of the stereographic projection devised by Hipparchus.
Gabriella Armellini Conti
The geographical work of T. had less resonance than the astronomical, although he shared with Marinus of Tyre the merit of having scientifically fixed the geographical heritage of classical antiquity; moreover, while the work of Marinus (who lived at Alexandria and flourished not later than 120 A.D.) was lost, that of T. survived throughout the Middle Ages down to our own day.
As may be gathered from T. himself, Marinus, author of a Τοῦ γεωγραφικῶ πύκλους διόροσις, taking up again the theories of Hipparchus (one of the greatest astronomers of antiquity, born at Nicaea in Bithynia and who carried on a great part of his activity at Rhodes between 161 and 126 B.C.), had departed from the Eratosthenic model, maintaining that the calculation of the position of places could be made by the same method used to establish the position of the stars on the celestial sphere, thus employing only latitude and longitude: he, however, imagined that the ecumene extended from the Canaries to China for 225°, an excessively exaggerated extent, and his map, constructed on the basis of a grid of straight parallels and meridians, according to an equidistant cylindrical projection and without taking into account the convergence of the meridians toward the pole, presented the greatest deformations in the peripheral zones. T. intended to reduce the errors of Marinus of Tyre, but if he remained essentially bound to the system of his predecessor both in the Almagest and in the Μαθηματικὴ σύνταξις πετράβιβλος, a complement to the former, and in the minor astronomical works, he nevertheless demonstrated original conceptions in the more properly geographical field and particularly in the cartographic one, compiling the Γεωγραφικὴ ὑφήγησις. He reduced the extent of the ecumene to 180°, approaching the real one of 125°; he accepted the cylindrical projection only for representing small portions of the earth's surface (chorographic maps); he proposed a simple conic projection in which the meridians are straight lines converging toward the poles and the parallels are arcs of concentric circles, and on these bases he constructed, or at least offered the bases for the construction of, a general map of the known world comprising the whole territory from N. to S. between Thule and Agisymba, and from E. to W. from the Canary (Fortunate) Islands and Sera Metropolis, capital of the country of the Seres (China). Scholars indeed debate whether the 27 maps (one general and the others regional) inserted in the medieval manuscripts of his work (fundamental among all the Greek Urbinate 82, in the Vatican Apostolic Library, of the 12th century) go back directly to T., albeit with inevitable deformations in the development of the tradition, or were instead constructed later on the text of T., which, moreover, consisting essentially in a long list of localities with their respective latitude and longitude, was particularly subject to interpolations.
The (erroneous) value in the measurement of the greatest circumference of the earth, at 180,000 stades (about 33,000 km., notably less than the true one, of 40,070 km.), had an influence, perpetuating itself in the Middle Ages, also on the calculations of Christopher Columbus and on the discovery of the New World. Other errors of T. had repercussions on later cartography, especially Arab, such as the exaggerated stretching in an E.-W. direction of the Mediterranean Sea, the orientation of the Italian peninsula, and his having closed the Indian Ocean to the S. of Zanzibar, where the African coast is made to turn decisively toward the E.
While the astronomical work of T., diffused especially through the Arabs, remained at the basis of all medieval cosmography and scholastic philosophy, the geographical work had little fortune, especially in the West, where only in the 15th century did a first Latin translation of 1409 mark the beginning of a new epoch in the history of cartography.
T. also wrote on chronology, mechanics, optics, music, etc.
Bibliography8 works
Gastone Imbrighi