As an astronomer, Ptolemy was, of course, better qualified to explain the mathematical conditions of the earth and its relations to the celestial bodies than most preceding geographers. His general views had much in common with those of Eratosthenes and Strabo. He adopted from Hipparchus the division of the equatorial circle into 36o parts (our degrees), and supposed other circles (meridians) to be drawn through this, from the equator to the pole. Another set of circles was the series of parallels of latitude, and within the network of parallels of latitude and meridians of longitude it was Ptolemy's task to place the out line of the world, so far as known to him.
But at the very outset of his attempt he fell into an error vitiating all his conclusions. Eratosthenes (c. 276-196 B.c.) was the first who had attempted scientifically to determine the earth's circumference, and his result of 250,000 (or 252,000) stadia, i.e., 25,000 (25,200) geographical miles, was generally adopted by subsequent geographers, including Strabo. Posidonius, how ever (c. 135-5o B.c.), reduced this to 'Woo°, and the latter computation was inexplicably adopted by Marinus and Ptolemy. This error made every degree of latitude or longitude (measured at the equator) equal to only 500 stadia (5o geographical miles), instead of its true equivalent of 600 stadia. The mistake would have been somewhat neutralized had there existed a sufficient number of points of which the position was fixed by observation; but we learn from Ptolemy himself that such observations for latitude were very few, while the means of determining longi tudes were almost wholly wanting. Hence the positions laid down by him were, with few exceptions, the result of computations from itineraries and the statements of travellers, which, owing to the want of instruments, etc., were liable to much greater error in ancient times than at the present day. But, great as were the errors resulting from such imperfect means of calcu lation, they were increased by the permanent error arising from Ptolemy's system of graduation. Thus, if he concluded (from itineraries) that two places were 5,00o stadia distant, he would place them i o° apart, and thus in fact separate them by 6,000 stadia.
Ptolemy followed Marinus in taking, as his prime meridian from which to measure longitudes, a line drawn through the supposed position of the Fortunate islands (vaguely answering to the Canaries plus the Madeira group) which they placed 21° (instead of 9° 2o') west of the Sacred promontory (i.e., Cape
St. Vincent, regarded by Marinus and Ptolemy, as by previous geographers, as the westernmost point of Europe). Hence all Ptolemy's longitudes, reckoned eastwards, were about 7° less than they would have been if really measured from the meridian of Ferro, which continued so long in use.
The problem that had especially attracted the attention of geographers from Dicaearchus to Ptolemy was to determine the length and breadth of the inhabited world. This question had been fully discussed by Marinus, who had arrived at conclusions widely different from his predecessors. Towards the north, in deed, there was no great difference of opinion, the latitude of Thule being generally recognized as that of the highest northern land, and this was placed both by Marinus and Ptolemy in 63° N., not far beyond the true position of the Shetland islands, which had come to be generally identified with the mysterious Thule of Pytheas. The western extremity, as already mentioned, had been in like manner determined by the prime meridian drawn through the supposed position of the outermost of the Fortunate islands. But towards the south and east Marinus gave an enor mous extension to Africa and Asia, beyond what had been known to or suspected to exist by earlier geographers, and, though Ptolemy reduced Marinus' calculations, he retained an exaggerated estimate of their results.
Contents of the "Guide to Geography."—In Book i., after general explanations (cc. 1-6), Ptolemy applies himself to a correction of Marinus' results, taking account (I) of the ob served (astronomical) phenomena, (2) of the various recorded itineraries of travel (a) by land and (b) by sea. Marinus had assumed as the limit of knowledge to the south a region in Africa called Agisymba, inhabited by Ethiopians and abounding in rhinoceroses, which had been reached by Julius Maternus. Modi fying the data regarding this journey, Marinus placed Agisymba on the southern tropic. This made the "breadth" of the inhabited world from Thule to Agisymba =87°. Ptolemy, on the other hand, placed Agisymba and a certain promontory called Prason on a parallel as far south as Meroe is north of the equator; taking, then, the southern limit to be 16° 25' S., he finds the "breadth" of the inhabited earth to be 63°+16° 25'=79° 25', in round figures 8o°, or 40,000 stades.