58. The careful observations of the Arabs soon shewed the defects in the Greek astronomical tables, and new tables were from time to time issued, based on much the same principles as those in the Almagest, but with changes in such numerical data as the relative sizes of the various circles, the positions of the apogees, and the inclinations of the planes, etc.
To Tabit ben Korra, mentioned above as the translator of the Almagest, belongs the doubtful honour of the discovery of a supposed variation in the amount of the precession (chapter II., [§§ 42], 50). To account for this he devised a complicated mechanism which produced a certain alteration in the position of the ecliptic, thus introducing a purely imaginary complication, known as the trepidation, which confused and obscured most of the astronomical tables issued during the next five or six centuries.
59. A far greater astronomer than any of those mentioned in the preceding articles was the Arab prince called from his birthplace Al Battani, and better known by the Latinised name Albategnius, who carried on observations from 878 to 918 and died in 929. He tested many of Ptolemy’s results by fresh observations, and obtained more accurate values of the obliquity of the ecliptic (chapter I., [§ 11]) and of precession. He wrote also a treatise on astronomy which contained improved tables of the sun and moon, and included his most notable discovery—namely, that the direction of the point in the sun’s orbit at which it is farthest from the earth (the apogee), or, in other words, the direction of the centre of the eccentric representing the sun’s motion (chapter II., [§ 39]), was not the same as that given in the Almagest; from which change, too great to be attributed to mere errors of observation or calculation, it might fairly be inferred that the apogee was slowly moving, a result which, however, he did not explicitly state. Albategnius was also a good mathematician, and the author of some notable improvements in methods of calculation.[38]
60. The last of the Bagdad astronomers was Abul Wafa (939 or 940-998), the author of a voluminous treatise on astronomy also known as the Almagest, which contained some new ideas and was written on a different plan from Ptolemy’s book, of which it has sometimes been supposed to be a translation. In discussing the theory of the moon Abul Wafa found that, after allowing for the equation of the centre and for the evection, there remained a further irregularity in the moon’s motion which was imperceptible at conjunction, opposition, and quadrature, but appreciable at the intermediate points. It is possible that Abul Wafa here detected an inequality rediscovered by Tycho Brahe (chapter V., [§ 111]) and known as the variation, but it is equally likely that he was merely restating Ptolemy’s prosneusis (chapter II., [§ 48]).[39] In either case Abul Wafa’s discovery appears to have been entirely ignored by his successors and to have borne no fruit. He also carried further some of the mathematical improvements of his predecessors.
Another nearly contemporary astronomer, commonly known as Ibn Yunos (?-1008), worked at Cairo under the patronage of the Mahometan rulers of Egypt. He published a set of astronomical and mathematical tables, the Hakemite Tables, which remained the standard ones for about two centuries, and he embodied in the same book a number of his own observations as well as an extensive series by earlier Arabian astronomers.
61. About this time astronomy, in common with other branches of knowledge, had made some progress in the Mahometan dominions in Spain and the opposite coast of Africa. A great library and an academy were founded at Cordova about 970, and centres of education and learning were established in rapid succession at Cordova, Toledo, Seville, and Morocco.
The most important work produced by the astronomers of these places was the volume of astronomical tables published under the direction of Arzachel in 1080, and known as the Toletan Tables, because calculated for an observer at Toledo, where Arzachel probably lived. To the same school are due some improvements in instruments and in methods of calculation, and several writings were published in criticism of Ptolemy, without, however, suggesting any improvements on his ideas.
Gradually, however, the Spanish Christians began to drive back their Mahometan neighbours. Cordova and Seville were captured in 1236 and 1248 respectively, and with their fall Arab astronomy disappeared from history.
62. Before we pass on to consider the progress of astronomy in Europe, two more astronomical schools of the East deserve mention, both of which illustrate an extraordinarily rapid growth of scientific interests among barbarous peoples. Hulagu Khan, a grandson of the Mongol conqueror Genghis Khan, captured Bagdad in 1258 and ended the rule of the Caliphs there. Some years before this he had received into favour, partly as a political adviser, the astronomer Nassir Eddin (born in 1201 at Tus in Khorassan), and subsequently provided funds for the establishment of a magnificent observatory at Meraga, near the north-west frontier of modern Persia. Here a number of astronomers worked under the general superintendence of Nassir Eddin. The instruments they used were remarkable for their size and careful construction, and were probably better than any used in Europe in the time of Coppernicus, being surpassed first by those of Tycho Brahe (chapter V.).
Nassir Eddin and his assistants translated or commented on nearly all the more important available Greek writings on astronomy and allied subjects, including Euclid’s Elements, several books by Archimedes, and the Almagest. Nassir Eddin also wrote an abstract of astronomy, marked by some little originality, and a treatise on geometry. He does not appear to have accepted the authority of Ptolemy without question, and objected in particular to the use of the equant (chapter II., [§ 51]), which he replaced by a new combination of spheres. Many of these treatises had for a long time a great reputation in the East, and became in their turn the subject-matter of commentary.