Al-Sufi’s Description of the Fixed Stars was written in the tenth century and contains 1018 stars. He seems to have adopted the positions of the stars given by Ptolemy, merely correcting them for the effects of precession; but he made a very careful revision of the star magnitudes of Ptolemy (or Hipparchus) from his own observations, and this renders his work the most valuable, from this point of view, of all the ancient catalogues.
Very little is known about Al-Sufi’s life, and the few details we have are chiefly derived from the works of the historians Abu’-l-faradji and Casiri, and the Oriental writers Hyde, Caussin, Sedillot, etc. Al-Sufi’s complete name was Abd-al-Rahmän Bin Umar Bin Muhammad Bin Sahl Abu’l-husaïn al-Sufi al-Razi. The name Sufi indicates that he belonged to the sect of Sufis (Dervishes), and the name Razi that he lived in the town of Raï in Persia, to the east of Teheran. He was born on December 7, 903 A.D., and died on May 25, 986, so that, like many other astronomers, he lived to a good old age. According to ancient authorities, Al-Sufi—as he is usually called—was a very learned man, who lived at the courts of Schiraz and Baghdad under Adhad-al-Davlat—of the dynasty of the Buïdes—who was then the ruler of Persia. Al-Sufi was held in high esteem and great favour by this prince, who said of him, “Abd-al-Rahmän al-Sufi taught me to know the names and positions of the fixed stars, Scharif Ibn al-Aalam the use of astronomical tables, and Abu Ali al-Farisi instructed me in the principles of grammar.” Prince Adhad-al-Davlat died on March 26, 983. According to Caussin, Al-Sufi also wrote a book on astrology, and a work entitled Al-Ardjouze, which seems to have been written in verse, but its subject is unknown. He also seems to have determined the exact length of the year, and to have undertaken geodetic measurements. The al-Aalam mentioned above was also an able astronomer, and in addition to numerous observations made at Baghdad, he determined with great care the precession of the equinoxes. He found the annual constant of precession to be 51″·4, a value which differs but little from modern results.
In the year 1874, the late M. Schjellerup, the eminent Danish astronomer, published a French translation of two Arabic manuscripts written by Al-Sufi and entitled “A Description of the Fixed Stars.” One of these manuscripts is preserved in the Royal Library at Copenhagen, and the other in the Imperial Library at St. Petersburgh.[390]
Al-Sufi seems to have been a most careful and accurate observer, and although, as a rule, his estimates of the relative brightness of stars are in fairly good agreement with modern estimates and photometric measures, there are many remarkable and interesting differences. Al-Sufi’s observations have an important bearing on the supposed “secular variation” of the stars; that is, the slow variation in light which may have occurred in the course of ages in certain stars, apart from the periodical variation which is known to occur in the so-called variable stars. More than 900 years have now elapsed since the date of Al-Sufi’s observations (about A.D. 964) and over 2000 years in the case of Hipparchus, and although these periods are of course very short in the life-history of any star, still some changes may possibly have taken place in the brightness of some of them. There are several cases in which a star seems to have diminished in light since Al-Sufi’s time. This change seems to have certainly occurred in the case of θ Eridani, β Leonis, ζ Piscis Australis, and some others. On the other hand, some stars seem to have certainly increased in brightness, and the bearing of these changes on the question of “stellar evolution” will be obvious.
In most cases Al-Sufi merely mentions the magnitude which he estimated a star to be; such as “third magnitude,” “fourth,” “small third magnitude,” “large fourth,” etc. In some cases, however, he directly states that a certain star is a little brighter than another star near it. Such cases—unfortunately not numerous—are very valuable for comparison with modern estimates and measures, when variation is suspected in the light of a star. The estimates of Argelander, Heis, and Houzeau are based on the same scale as that used by Ptolemy and Al-Sufi. Al-Sufi’s estimates are given in thirds of a magnitude. Thus, “small third magnitude” means 3⅓, or 3·33 magnitude in modern measures; “large fourth,” 3⅔ or 3·66 magnitude. These correspond with the estimates of magnitude given by Argelander, Heis, and Houzeau in their catalogues of stars visible to the naked eye, and so the estimates can be directly compared.
I have made an independent identification of all the stars mentioned by Al-Sufi. In the majority of cases my identifications concur with those of Schjellerup; but in some cases I cannot agree with him. In a few cases I have found that Al-Sufi himself, although accurately describing the position of the stars observed by him, has apparently misidentified the star observed by Hipparchus and Ptolemy. This becomes evident when we plot Ptolemy’s positions (as given by Al-Sufi) and compare them with Al-Sufi’s descriptions of the stars observed by him. This I have done in all cases where there seemed to be any doubt; and in this way I have arrived at some interesting results which have escaped the notice of Schjellerup. This examination shows clearly, I think, that Al-Sufi did not himself measure the positions of the stars he observed, but merely adopted those of Ptolemy, corrected for the effect of precession. The great value of his work, however, consists in his estimates of star magnitudes, which seem to have been most carefully made, and from this point of view, his work is invaluable. Prof. Pierce says, “The work which the learning of M. Schjellerup has brought to light is so important that the smallest errors of detail become interesting.”[391]
Although Al-Sufi’s work is mentioned by the writers referred to above, no complete translation of his manuscript was made until the task was undertaken by Schjellerup, and even now Al-Sufi’s name is not mentioned in some popular works on astronomy! But he was certainly the best of all the old observers, and his work is deserving of the most careful consideration.
Al-Sufi’s descriptions of the stars were, it is true, based on Ptolemy’s catalogue, but his work is not a mere translation of that of his predecessor. It is, on the contrary, a careful and independent survey of the heavens, made from his own personal observations, each of Ptolemy’s stars having been carefully examined as to its position and magnitude, and Ptolemy’s mistakes corrected. In examining his descriptions, Schjellerup says, “We soon see the vast extent of his labours, his perseverance, and the minute accuracy and almost modern criticism with which he executed his work.” In fact, Al-Sufi has given us a careful description of the starry sky as it appeared in his time, and one which deserves the greatest confidence. It far surpasses the work of Ptolemy, which had been without a rival for eight centuries previously, and it has only been equalled in modern times by the surveys of Argelander, Gould, Heis, and Houzeau. Plato remarked with reference to the catalogue of Hipparchus, Cœlam posteris in hereditatem relictum, and the same may be said of Al-Sufi’s work. In addition to his own estimates of star magnitudes, Al-Sufi adds the magnitudes given by Ptolemy whenever Ptolemy’s estimate differs from his own; and this makes his work still more valuable, as Ptolemy’s magnitudes given in all the editions of the Almagest now extant are quite untrustworthy.
In the preface to his translation of Al-Sufi’s work, Schjellerup mentions some remarkable discrepancies between the magnitudes assigned to certain stars by Ptolemy and Argelander. This comparison is worthy of confidence as it is known that both Al-Sufi and Argelander adopted Ptolemy’s (or Hipparchus’) scale of magnitudes. For example, all these observers agree that β Ursæ Minoris (Ptolemy’s No. 6 of that constellation) is of the 2nd magnitude, while in the case of γ Ursæ Minoris (Ptolemy’s No. 7), Ptolemy called it 2nd, and Argelander rated it 3rd; Argelander thus making γ one magnitude fainter than Ptolemy’s estimate. Now, Al-Sufi, observing over 900 years ago, rated γ of the 3rd magnitude, thus correcting Ptolemy and agreeing with Argelander. Modern photometric measures confirm the estimates of Al-Sufi and Argelander. But it is, of course, possible that one or both stars may be variable in light, and β has actually been suspected of variation. Almost all the constellations afford examples of this sort. In the majority of cases, however, Al-Sufi agrees well with Argelander and Heis, but there are in some cases differences which suggest a change in relative brightness.
Among other remarkable things contained in Al-Sufi’s most interesting work may be mentioned the great nebula in Andromeda, which was first noticed in Europe as visible to the naked eye by Simon Marius in 1612. Al-Sufi, however, speaks of it as a familiar object in his time.