At the present time the study of regional geography is largely dependent on a precise knowledge of the geographical coördinates of places. The foremost duty of the explorer is to know where he is from day to day and to find this out by astronomical means, if possible. In classical times and among the Moslems the importance of such observations was not only well understood, but several methods of carrying them through were described by astronomers and geographers, and the latitudes of a great many stations had been determined astronomically. Longitude, on the other hand, long remained a stumbling block, and before the twelfth century, certainly, no systematic attempts to ascertain the longitudes of any large number of places had ever met with success. A few relics of classical and Moslem study in this field became familiar in the West as a result of the intense interest in Arabic astronomy prevailing in Europe between the tenth and thirteenth centuries.[1116] Various figures representing the results of Arabic corrections of and additions to the data given in Ptolemy’s _Geography_ found their way into Western astrological tables. The most interesting of these occur in a list of the latitudes and longitudes of some sixty odd cities appended to the Paris manuscript of the _Marseilles Tables_ of Raymond of Marseilles[1117] and also to most of the Latin versions of the _Toledo Tables_.[1118] This list and certain figures scattered through the astrological tables and canons[1119] reveal the results of the reductions made by Al-Khwārizmī and by Az-Zarqalī of Ptolemy’s gross overestimate of the length of the Mediterranean, to which we have referred in a preceding chapter.[1120] The European student of these astrological works might have drawn a by no means contemptible map from the figures to be found in them had he been interested in what these figures could teach him of geography. Figure 6 is a map compiled from the coöordinates given in the Paris manuscript of the _Marseilles Tables_. At the end of this list of geographical coördinates in many manuscripts additional figures not derived from Moslem sources are given. These show the positions of such points in Europe as London, Hereford, Paris, Toulouse, Barcelona, Marseilles, Novara, Cremona, Florence, and Naples[1121] (see Fig. 12, in Notes to Ch. X). They were undoubtedly determined by observations made during our period or shortly after. [Illustration: