CHAPTER V. The Astronomical Writings of Scot

THE ASTRONOMICAL WRITINGS OF SCOT

The alchemy of the thirteenth century, to the progress of which Michael Scot contributed not a little, bore a close relation to the opinions then entertained in another branch of science: that of astronomy. We have already noticed how chemistry, as practised in Egypt, was largely influenced by Eastern theories regarding the stars and their power over earthly elements. That this connection and sympathy was still a matter of common belief at the time Scot wrote is not only probable but can readily be established by direct evidence. The treatise ‘Cum studii solertis indagine,’ already referred to, has a curious passage which bears directly on the point in question. We find in the preface the following remarkable statement: ‘For the art of alchemy belongs to the deeper and more hidden physics, and in particular to that division thereof which … is called the lower astronomy,’ It is plain then that no chemist could in those days be considered fully competent for the task he undertook unless to a knowledge of the customary theories and processes of his art he added some acquaintance with the mysteries of the heavenly spheres as well.

To Michael Scot, even before he came to Toledo, the science of astronomy was already a beaten path. His progress in mathematical studies naturally led him to this, the highest sphere in which they could be exercised. At the court of Frederick he had made many an observation and cast many a horoscope. In the Liber Introductorius and Liber Particularis he had produced two manuals expounding in a popular way the twin sciences of astrology and astronomy; publications which no doubt reproduced pretty exactly the teaching he had given to the Emperor.

In Spain he not only kept up his interest in this subject but lost no opportunity of improving his past acquirements. He was constantly on the watch for new astronomical works. He read them, not only as a student eager to extend his knowledge, but as a translator anxious to find the opportunity of adding to the resources of other scholars by the production of some important book in a Latin dress.

As a resident in Toledo, Scot found himself very favourably situated for such studies. That city was now indeed to become what may be called the classic ground of Moorish astronomy. A Spanish author would have us believe that there presently assembled there an incredible number of astronomers drawn, not only from all parts of Spain, but from France as well, and especially from Paris. The king himself is said to have presided over this congress. The works of Ptolemy, with the commentaries of Montafan and Algazel, were translated into Latin for the use of those scholars who did not understand Arabic. Discussions were held in the Alcazar of Galiana upon the various theories of the heavenly bodies and their movements. These labours, which commenced in 1218, and are said to have lasted till 1262, resulted in a more exact series of observations than had hitherto been made. They were published, and became generally known as the Tables of Toledo.

It was in such a direction indeed that the line of true progress lay. As alchemy rose into a real chemistry rather by the practice of the laboratory than by the theory of the schools, so it was with regard to astronomy. The scheme of Ptolemy with its various modifications necessarily held the field, imperfect and erroneous as it was, till wider and more exact observations, such as those for which the wise king of Castile thus provided had, in the course of after ages, furnished adequate ground for the magical and illuminative speculations of Copernicus, Galileo, and Newton.

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. 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 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?

Each sphere, said Alpetrongi, has its own poles, which differ from those of the primum mobile, and thus each, while following the motion of the ninth sphere, accomplishes at the same time another revolution about its own proper poles. From the combination of these two movements arises one of the nature of a spiral which fully accounts for the seeming deviations of the heavenly bodies to north or south.

Such were the contributions of this philosopher to the astronomy of his time. They were the fruit, he assures us, of patient study of the ancients, and specially of Aristotle and his commentators. He offered them to his age as a distinct improvement on the cumbrous theories of Ptolemy, and as an advance even upon that of Azarchel, whom, in the main, he acknowledges as his master in science. Antiquated and childish as his explanations may seem to us, we cannot help feeling that he had at least grasped firmly some of the chief problems of the sky. He stood in the line of that inquiry and patient progress which have issued in the marvellous discoveries of later times.

Scot’s version of the Sphere of Alpetrongi has reached us accompanied by the date of its composition; a distinction which belongs to only one other among his translations, that of the Abbreviatio Avicennae. M. Jourdain had the merit of being the first who drew attention to this fortunate circumstance, and he did so by quoting the colophons of two manuscripts of the Sphere discovered by him in the Paris library. One of these closes thus: ‘Praised be Jesus Christ who liveth for ever throughout all time: on the eighteenth day of August, being Friday, at the third hour, cum aboleolente, in the year one thousand two hundred and fifty-five.’ The other gives the date thus: ‘The year of the Incarnation of Christ twelve hundred and seventeen.’ These two epochs coincide exactly, as the apparent difference arises from the date being expressed in the first manuscript according to the era of Spain. It is therefore doubly certain that Scot’s version of the Sphere of Alpetrongi was made in the year 1217.

In completing this translation Michael Scot anticipated by one year only the great astronomical congress which the King of Castile presently caused to assemble at Toledo. It may very possibly therefore have been one of the versions prepared with a view to this great occasion and designed for the use of the Latin astronomers who might come there. Certain it is that the author was not less fortunate in this than in his previous literary ventures. The text was well chosen, the time of publication opportune, and the Sphere of Alpetrongi as it came from Scot’s hand had a wide circulation and influenced profoundly the astronomical beliefs of the day.