An Enquiry into the Life and Legend of Michael Scot

by J. Wood Brown

Favourable, however, as Scot’s situation in Toledo undoubtedly was, much of what we are considering lay beyond his reach, being yet in the womb of the future. The Moorish astronomers, and he doubtless with them, felt far from satisfied with the Ptolemaic system as expounded in the Almagest. While no one as yet ventured to interfere with its fundamental conception of the earth as the centre of the universe, every fresh observation, by bringing into view more of the delicacy and subtlety of the heavenly movements, made additions and modifications of that theory constantly necessary. Hence arose a series of Arabian works on the sphere, each superseding that which had preceded it, and reflecting the last results obtained with the astrolabe. Such a line of progress could not but lead to the time when the Ptolemaic theory no longer lent itself by any modification to the full explanation of ascertained facts. Then and then only arose the new astronomy of the sixteenth and seventeenth centuries, which is thus seen to be vitally connected, even in its highest reach and most splendid developments with the now forgotten theories of the Moorish schools.

Considering then the epoch at which he lived, and the incomplete material which existed in his days for a true science of the heavens, Michael Scot did all that could be reasonably expected of him. He sat at the feet of those who were then the best authorities on this subject. He used his opportunities at Toledo to make the last and most subtle theories of the Moors intelligible to those less fortunate scholars whose attention these must otherwise have escaped.

His services to astronomy appeared in the Latin version which he made from a treatise on the Sphere lately composed by Alpetrongi. This author’s name is said to have been, in its Arabic form, Nured-din el Patrugi. Munk, in his Mélanges, tells us that the latter designation was derived from a village called Petroches lying a little to the north of Cordova.[144] The Latins corrupted the name in different ways, so that among them it became Avenalpetrandi, Alpetrongi, or Alpetragius. The astronomer who bore it flourished about the year 1190, and is said to have been a renegade, and a scholar of the celebrated Ibn Tofail, the author of the curious Sufic romance called Hay Ibn Yokhdan.

In the preface to his book on the Sphere Alpetrongi begs to be excused if he has ventured to differ from the tradition of the ancients in his theory of the heavenly movements, and especially from Ptolemy the great master of this science. His apology reminds us that it may be well to examine more exactly than we have yet done the various advances which had been made up to this time by the Arabian astronomy.

As early as the ninth century the mathematicians of that nation had simplified the problems of the circle by discovering the way of measurement by sine and tangent instead of by the chord. This improvement is ascribed to Albategni who lived between the years 877 and 929. Calculation was soon made still easier by the invention of algebra. The year 820 is given as the age of Mohammed ben Moussa, surnamed Al Khowaresmi, who had the honour of this important discovery. From the surname of this mathematician the Latins afterwards formed by corruption their common noun Algorisma or Algorithmus, from which our word arithmetic is derived.

These improved methods of calculation were soon applied to astronomy. Al Mamun, whose reign commenced in the year 813, summoned an assembly of scholars learned in that science. They met in the great Babylonian plain, having chosen that place as suitable for their observations, and measured the declination of the ecliptic, which they determined to be 23° 33ʺ. About the same time the secular motion of the heavens began to attract attention. Albategni corrected the observations of Ptolemy here, and showed that the retrograde movement amounted to one degree, not in a century as the Greek philosopher had said, but in a shorter period which is variously stated as sixty-six or seventy years. Alfargan repeated this calculation, and amended that relating to the declination of the ecliptic, which he computed at 23° 35ʺ.

This was the progress and these the data which led the Moorish astronomers to abandon the earlier and simpler theories of the sphere as inconsistent with ascertained facts. They were aware of motions among the heavenly bodies not to be explained by the mere supposition that round the earth as a centre moved the concentric spheres on the axes of their poles. It is true that even Ptolemy himself had felt something of this difficulty and had endeavoured to meet it by a theory of eccentrics and epicycles. As knowledge increased, however, this primitive explanation was felt to be cumbrous and unsatisfactory. Aboasar[145] and Azarchel gained fame by boldly striking out in new paths, and later Moorish astronomers eagerly followed the lead thus given them, each adding some modification of his own.

Thus then we return to the preface of Alpetrongi prepared to understand his position when he declares himself obliged to depart from previous traditions. He proceeds to avow himself a scholar of Azarchel, but when we examine his work we find that the theory he proposes differs considerably even from that taught by his immediate master. It was one which, through the labours of Michael Scot, as translator of Alpetrongi, exercised no small influence on the study of astronomy among the Latins, and we may well spend a moment in considering the chief features which it presents.

One of the most important problems which called for solution at the hands of the Moorish astronomers was that of the recession of the heavenly bodies, by which, when observed at sufficient intervals of time, they were seen to fall short of the positions they might have been expected to reach. This recession, as we have remarked already, had been very accurately studied, and computed as exactly as the methods of the time allowed; but a reason for so remarkable a phenomenon was yet to seek. Alpetrongi boldly declared that the eastward motion was apparent only and not real. He explained that the source of power lay in the primum mobile or ninth sphere; that lying outside the sphere of the fixed stars. From hence the force producing circular motion was derived to the eighth, and so to the inferior spheres; each handing on a part of the impulse to that which lay beneath it. In the course of transmission, however, the prime force became gradually exhausted. Thus, said Alpetrongi, it happens that each sphere moves rather more slowly than the one above it, and so the apparent recession is accounted for in a way which shows it to be relative only and not absolute.

Another matter which exercised the minds of those who studied the heavens was the difference of elevation which the heavenly bodies showed according to the seasons of summer and winter. The sun, for example, at noonday of the summer solstice stood, they saw, at his highest point in the heavens, while he sank to his lowest on the shortest day of winter. Between these extremes he held gradually every intermediate position, and as he was meanwhile supposed to be moving in a circular path round the earth, his course came to be conceived of as a spiral alternately rising and declining. How was this spiral motion to be explained?