The Moslems, as a general rule, measured longitudes from the prime meridian which Ptolemy had used, that of the Fortunate Islands (now the Canaries), situated in the Western Ocean at the westernmost limit of the habitable earth; but individual writers came to make use of another meridian farther west, a meridian destined to become known to the Christian world as that of the True West as distinguished from the supposed border of the habitable West.[361] Abū Maʿshar, on the other hand, referred his prime meridian to a fabulous castle of Kang-Diz, far to the east in the China Sea.[362] ARIN The western prime meridian was commonly supposed to be 90° from a mythical city called Arin (or Arim) situated on the equator, halfway between the farthest east and the farthest west. This city was said to have neither latitude nor longitude, and its meridian came arbitrarily to be placed at 10° east of that of Baghdad. The idea of Arin probably originated among the Hindus,[363] who believed that the city of Langka in Ceylon (or perhaps Sumatra)—the abode of devils—lies on the equator. They traced their prime meridian from Langka through Odjein, a place in India, to Mount Meru at the north pole—the abode of angels. Odjein was transliterated into Arabic as “Arin” or “Arim” and was shifted by the Moslems to the equator. It was made known to the Christian world through such works as Adelard of Bath’s translation of the _Khorazmian Tables_ (which, as we have already seen, was an Arabic redaction of a Hindu work) and Plato of Tivoli’s translation of Al-Battānī’s _Astronomy_. In the latter, Arin was represented as a cupola or tower; and on Christian maps and diagrams of the Middle Ages it was not infrequently so depicted. _ARABIC EXPLORATION AND TRAVEL_