Other theories of the tides entered the West from Arabic sources. Al-Biṭrūjī’s _On the Sphere_ ascribed their origin not to the moon but to the general circulation of the heavens.[354] Averroës, in a commentary on the _Meteorology_ of Aristotle, devoted a confused chapter[355] to an attempt at showing that ebb and flood are the results partly of currents produced by differences in level between the ocean and certain seas and partly of the moon’s attraction of the waters. The possibility of differences in level between seas and ocean had probably become known to the Spanish scholar through some garbled rendering of Eratosthenes’ observations on the currents and levels of the Mediterranean.[356] MEASUREMENT OF A TERRESTRIAL DEGREE That the Saracens also were interested in the more strictly mathematical aspects of astronomical geography is emphatically proved by the fact that they undertook actually to measure the length of a terrestrial degree[357] and thereby to determine the circumference of the earth. Some knowledge of this great work came to the Western world in our period through translations of the _Astronomy_ of Al-Farghānī.[358] GEOGRAPHICAL POSITIONS The Arabic investigations, however, which most profoundly interested the men of the West were those concerned with the determination of the location of places on the earth’s surface rather than those whose aim was to find the size of the globe. Stimulated by their interest in Ptolemy, the Moslems felt a special need for the accurate knowledge of positions, for upon such knowledge depended the construction of mosques, which, according to religious law, must face in the direction of Mecca. Astrology also necessitated this type of investigation. In order to cast a horoscope one must know what stars are overhead at a particular moment; and, to ascertain this, one must know latitude and longitude. In the Arabic astronomic works there occur rules for determining positions and tables of the latitudes and longitudes of places throughout the world.[359] One of the most practical results of Arabic investigations in this field was a reduction of Ptolemy’s exaggerated estimate of the length of the Mediterranean Sea. The Greek geographer gave the length as 62° or about half again too long. Al-Khwārizmī cut this figure down to about 52°, and, if we are right in our interpretation of the available data, Az-Zarqalī still further reduced it to approximately the correct figure, 42°. As we shall see in a later chapter, the results of these corrections became known in the medieval West.[360]