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 latitude. Many of these differences were, however, corrected by the help of new manuscripts. 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 latitudes 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 (dzirâ), or a span (schibr), units of length which were used at that time to measure apparent celestial distances. The term dzirâ 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 dzirâ (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 God 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 classes. One followed the method of astronomers and trust to spheres designed by artists, who not knowing, the stars themselves, take only the longitudes and latitudes 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, Atârid 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 Atârid, Al-Sufi goes on to say that the other class 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-Dînavari. 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 latitudes, 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 71½ 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 circumference 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-scharataïn, “the two marks” (α and β 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 space described by the moon in a day and a night. In this way they found al-butaïn (ε, δ, and ρ 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 (λ 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 (α 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 β, η, γ, δ, and ε Virginis, which form with Spica (α Virginis) a Y-shaped figure. Spica was called simak al-azal, the unarmed simak; the “armed simak” being Arcturus, simak al-ramih. These old Arabic names seem very fanciful.
Al-Sufi relates that in the year 337 of the Hegira (about A.D. 948) he went to Ispahan with Prince Abul-fadhl, who introduced him to an inhabitant of that city, named Varvadjah, well known in that country, and famous for his astronomical acquirements. Al-Sufi asked him the names of the stars on an astrolabe which he had, and he named Aldebaran, the two bright stars in the Twins (Castor and Pollux), Regulus, Sirius, and Procyon, the two Simaks, etc. Al-Sufi also asked him in what part of the sky Al-fard (α Hydræ) was, but he did not know! Afterwards, in the year 349, this same man was at the court of Prince Adhad-al-Davlat, and in the presence of the Prince, Al-Sufi asked him the name of a bright star—it was al-nasr al-vaki, the falling Vulture (Vega), and he replied, “That is al-aijuk” (Capella)! thus showing that he only knew the names of the stars, but did not know them when he saw them in the sky. Al-Sufi adds that all the women “who spin in their houses” knew this star (Vega) by the name of al-atsafi, the Tripod. But this could not be said even of “educated women” at the present day.