Ptolemy and Al-Sûfi

In a careful examination of Al-Sûfi’s description of the stars written in the Tenth Century, and a comparison with modern estimates and measures, I have found several very interesting cases of apparent change in the brightness of the lucid stars. Al-Sûfi was an excellent and careful observer, and as a rule his estimates agree well with modern observations. We can therefore place considerable reliance on his estimates of star magnitudes. The Story of Theta Eridani has been well told by Dr. Anderson, and there seems to be no doubt that this southern star, which is now only of the third magnitude, was a bright star of the first magnitude in Al-Sûfi’s time! The following are other interesting cases of apparent change which I have met with in my examination of Al-Sûfi’s work. The Pole Star was rated third magnitude by both Ptolemy and Al-Sûfi, but it is now of the second magnitude, or a little less. The star γ Geminorum was rated third magnitude by Ptolemy and Al-Sûfi, or equal to δ Geminorum, but γ is now of the second magnitude, and its great superiority in brightness over δ is noticeable at a glance. Another interesting case is that of ζ and ο Persei, two stars which lie near each other, about seven degrees north of the Pleiades. Al-Sûfi distinctly describes these stars as both of the 3—4 magnitude; but Argelander,[69] Heis, and the photometric measures at Harvard agree in making ζ about one magnitude brighter than ο. The stars being close are easily compared, and their present great difference in brightness is very noticeable. This is one of the most remarkable cases I have met with in Al-Sûfi’s work, and strongly suggests variation in ο, as ζ is still about the same brightness as Al-Sûfi made it. The identity of the stars is beyond all doubt, as Al-Sûfi describes their positions very clearly, and says there is no star between them and the Pleiades, a remark which is quite correct for the naked eye. The remarkable decrease in brightness of β Leonis (Denebola) since Al-Sûfi’s time has been considered in my paper on Some Suspected Variable Stars. That it was a bright star of the first magnitude is fully proved by the observations of Al-Sûfi and Tycho Brahe. These were careful and accurate observers, and they could not have been mistaken about a star of the first magnitude. β Leonis is now fainter than an average star of the second magnitude, and there can be no reasonable doubt that it has faded considerably since the Tenth Century.