The Caliph al-Mam[ = u]n (c. A.D. 815), the son and successor of H[ = a]r[ = u]n al-Rash[ = i]d, caused an Arabic version of Ptolemy's great astronomical work ([Greek: Suntaxis megiste]) to be made, which is known as the _Almagest_, the word being nothing more than the Gr. [Greek: megiste] with the Arabic article _al_ prefixed. The geography of Ptolemy was also known and is constantly referred to by Arab writers. The Arab astronomers measured a degree on the plains of Mesopotamia, thereby deducing a fair approximation to the size of the earth. The caliph's librarian, Abu Jafar Muhammad Ben Musa, wrote a geographical work, now unfortunately lost, entitled _Rasm el Arsi_ ("A Description of the World"), which is often referred to by subsequent writers as having been composed on the model of that of Ptolemy. Geography in the middle ages. The middle ages saw geographical knowledge die out in Christendom, although it retained, through the Arabic translations of Ptolemy, a certain vitality in Islam. The verbal interpretation of Scripture led Lactantius (c. A.D. 320) and other ecclesiastics to denounce the spherical theory of the earth as heretical. The wretched subterfuge of Cosmas (c. A.D. 550) to explain the phenomena of the apparent movements of the sun by means of an earth modelled on the plan of the Jewish Tabernacle gave place ultimately to the wheel-maps--the T in an O--which reverted to the primitive ignorance of the times of Homer and Hecataeus.[6] The journey of Marco Polo, the increasing trade to the East and the voyages of the Arabs in the Indian Ocean prepared the way for the reacceptance of Ptolemy's ideas when the sealed books of the Greek original were translated into Latin by Angelus in 1410. Revival of geography. The old arguments of Aristotle and the old measurements of Ptolemy were used by Toscanelli and Columbus in urging a westward voyage to India; and mainly on this account did the crossing of the Atlantic rank higher in the history of scientific geography than the laborious feeling out of the coast-line of Africa. But not until the voyage of Magellan shook the scales from the eyes of Europe did modern geography begin to advance. Discovery had outrun theory; the rush of new facts made Ptolemy practically obsolete in a generation, after having been the fount and origin of all geography for a millennium. Apianus.