Part 15 (2/2)
In the year 1874, the late M. Schjellerup, the eminent Danish astronomer, published a French translation of two Arabic ma.n.u.scripts written by Al-Sufi and ent.i.tled ”A Description of the Fixed Stars.” One of these ma.n.u.scripts 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 ? Eridani, Leonis, ? 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 3?, or 333 magnitude in modern measures; ”large fourth,” 3? or 366 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 ma.n.u.script 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 surpa.s.ses 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, _Clam 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 a.s.signed 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 Ursae Minoris (Ptolemy's No. 6 of that constellation) is of the 2nd magnitude, while in the case of ? Ursae Minoris (Ptolemy's No. 7), Ptolemy called it 2nd, and Argelander rated it 3rd; Argelander thus making ? one magnitude fainter than Ptolemy's estimate. Now, Al-Sufi, observing over 900 years ago, rated ? 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 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.
Schjellerup says--
”For a long time many of the stars in Ptolemy's catalogue could not be identified in the sky. Most of these discordances were certainly due to mistakes in copying, either in longitude or lat.i.tude. Many of these differences were, however, corrected by the help of new ma.n.u.scripts.
For this purpose Al-Sufi's work is of great importance. By a direct examination of the sky he succeeded in finding nearly all the stars reported by Ptolemy (or Hipparchus). And even if his criticism may sometimes seem inconclusive, his descriptions are not subject to similar defects, his positions not depending solely on the places given in Ptolemy's catalogue. For, in addition to the longitudes and lat.i.tudes quoted from Ptolemy, he has described by alignment the positions of the stars referred to. In going from the brightest and best known stars of each constellation he indicates the others either by describing some peculiarity in their position, or by giving their mutual distance as so many cubits (_dzira_), or a span (_schibr_), units of length which were used at that time to measure apparent celestial distances. The term _dzira_ means literally the fore-arm from the bone of the elbow to the tip of the middle finger, or an ell.
We should not, however, conclude from this that the Arabians were so unscientific as to measure celestial distances by an ell, as this would be quite in contradiction to their well-known knowledge of Geometry and Trigonometry.”
With reference to the arc or angular distance indicated by the ”cubit,”
Al-Sufi states in his description of the constellation Auriga that the _dzira_ (or cubit) is equal to 2 20'. Three cubits, therefore, represent 7, and 4 cubits 9 20'.
In Al-Sufi's own preface to his work, after first giving glory to G.o.d and blessings on ”his elected messenger Muhammed and his family,” he proceeds to state that he had often ”met with many persons who wished to know the fixed stars, their positions on the celestial vault, and the constellations, and had found that these persons may be divided into two cla.s.ses. One followed the method of astronomers and trust to spheres designed by artists, who not knowing, the stars themselves, take only the longitudes and lat.i.tudes which they find in the books, and thus place the stars on the sphere, without being able to distinguish truth from error.
It then follows that those who really know the stars in the sky find on examining these spheres that many stars are otherwise than they are in the sky. Among these are Al-Battani, Atarid and others.”
Al-Sufi seems rather hard on Al-Battani (or Albategnius as he is usually called) for he is generally considered to have been the most distinguished of the Arabian astronomers. His real name was Mohammed Ibn Jaber Ibn Senan Abu Abdallah Al-Harrani. He was born about A.D. 850 at Battan, near Harran in Mesopotamia, and died about A.D. 929. He was the first to make use of sines instead of chords, and versed sines. The _Alphonsine Tables_ of the moon's motions were based on his observations.
After some severe criticisms on the work of Al-Battani and Atarid, Al-Sufi goes on to say that the other cla.s.s of amateurs who desire to know the fixed stars follow the method of the Arabians in the science of _Anva_[392] and the mansions of the moon and the books written on this subject. Al-Sufi found many books on the _anva_, the best being those of Abu Hanifa al-Dinavari. This work shows that the author knew the Arabic tradition better than any of the other writers on the subject. Al-Sufi, however, doubts that he had a good knowledge of the stars themselves, for if he had he would not have followed the errors of his predecessors.
According to Al-Sufi, those who know one of these methods do not know the other. Among these is Abu-Hanifa, who states in his book that the names of the twelve signs (of the Zodiac) did not originate from the arrangement or configuration of the stars resembling the figure from which the name is derived. The stars, Abu-Hanifa said, ”change their places, and although the names of the signs do not change, yet the arrangement of the stars ceases to be the same. This shows that he was not aware of the fact that the arrangement of the stars does not change, and their mutual distances and their lat.i.tudes, north and south of the ecliptic, are neither increased nor diminished.” ”The stars,” Al-Sufi says, ”do not change with regard to their configurations, because they are carried along together by a physical motion and by a motion round the poles of the ecliptic. This is why they are called fixed. Abu-Hanifa supposed that they are termed fixed because their motion is very slow in comparison with that of the planets.”
”These facts,” he says, ”can only be known to those who follow the method of the astronomers and are skilled in mathematics.”
Al-Sufi says that the stars of the Zodiac have a certain movement following the order of the signs, which according to Ptolemy and his predecessors is a degree in 100 years. But according to the authors of _al-mumtahan_ and those who have observed subsequently to Ptolemy, it is a degree in 66 years. According to modern measures, the precession is about 50”35 per annum, or one degree in 71 years.
Al-Sufi says that the Arabians did not make use of the figures of the Zodiac in their proper signification, because they divided the circ.u.mference of the sky by the number of days which the moon took to describe it--about 28 days--and they looked for conspicuous stars at intervals which, to the eye, the moon appeared to describe in a day and a night. They began with _al-scharatan_, ”the two marks” (a and Arietis) which were the first striking points following the point of the spring equinox. They then sought behind these two marks another point at a distance from them, equal to the s.p.a.ce described by the moon in a day and a night. In this way they found _al-butan_ (e, d, and ? Arietis); after that _al-tsuraija_, the Pleiades; then _al-dabaran_, the Hyades, and thus all the ”mansions” of the moon. They paid no attention to the signs of the Zodiac, nor to the extent of the figures which composed them. This is why they reckoned among the ”mansions” _al-haka_ (? Orionis) which forms no part of the signs of the Zodiac, since it belongs to the southern constellation of the Giant (Orion). And similarly for other stars near the Zodiac, of which Al-Sufi gives some details. He says that Regulus (a Leonis) was called by the Arabians _al-maliki_, the Royal Star, and that _al-anva_ consists of five stars situated in the two wings of the Virgin.
These stars seem to be , ?, ?, d, and e Virginis, which form with Spica (a Virginis) a Y-shaped figure. Spica was called _simak al-azal_, the unarmed _simak_; the ”armed _simak_” being Arcturus, _simak al-ramih_.
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