With reference to the number of stars which can be seen with the naked eye, Al-Sufi says, “Many people believe that the total number of fixed stars is 1025, but this is an evident error. The ancients only observed this number of stars, which they divided into six classes according to magnitude. They placed the brightest in the 1st magnitude; those which are a little smaller in the 2nd; those which are a little smaller again in the 3rd; and so on to the 6th. As to those which are below the 6th magnitude, they found that their number was too great to count; and this is why they have omitted them. It is easy to convince one’s self of this. If we attentively fix our gaze on a constellation of which the stars are well known and registered, we find in the spaces between them many other stars which have not been counted. Take, for example, the Hen [Cygnus]; it is composed of seventeen internal stars, the first on the beak, the brightest on the tail, the others on the wings, the neck and the breast; and below the left wing are two stars which do not come into the figure. Between these different stars, if you examine with attention, you will perceive a multitude of stars, so small and so crowded that we cannot determine their number. It is the same with all the other constellations.” These remarks are so correct that they might have been written by a modern astronomer. It should be added, however, that all the faint stars referred to by Al-Sufi—and thousands of others still fainter—have now been mapped down and their positions accurately determined.
About the year 1437, Ulugh Beigh, son of Shah Rokh, and grandson of the Mogul Emperor Tamerlane, published a catalogue of stars in which he corrected Ptolemy’s positions. But he seems to have accepted Al-Sufi’s star magnitudes without any attempt at revision. This is unfortunate, for an independent estimate of star magnitudes made in the fifteenth century would now be very valuable for comparison with Al-Sufi’s work and with modern measures. Ulugh Beigh’s catalogue contains 1018 stars, nearly the same number as given by Ali-Sufi.[393]
CHAPTER XIX
The Constellations[394]
Curious to say, Al-Sufi rated the Pole Star as 3rd magnitude; for it is now only slightly less than the 2nd. At present it is about the same brightness as β of the same constellation (Ursa Minor) which Al-Sufi rated 2nd magnitude. It was, however, also rated 3rd magnitude by Ptolemy (or Hipparchus), and it may possibly have varied in brightness since ancient times. Admiral Smyth says that in his time (1830) it was “not even a very bright third size” (!)[395] Spectroscopic measures show that it is approaching the earth at the rate of 16 miles a second; but this would have no perceptible effect on its brightness in historical times. This may seem difficult to understand, and to some perhaps incredible; but the simple explanation is that its distance from the earth is so great that a journey of even 2000 years with the above velocity would make no appreciable difference in its distance! This is undoubtedly true, as a simple calculation will show, and the fact will give some idea of the vast distance of the stars. The well-known 9th magnitude companion to the Pole Star was seen by day in the Dorpat telescope by Struve and Wrangel; and “on one occasion by Encke and Argelander.”[396]
The star β Ursæ Minoris was called by the Arabians Kaukab al-shamáli, the North Star, as it was—owing to the precession of the Equinoxes—nearer to the Pole in ancient times than our present Pole Star was then.
The “Plough” (or Great Bear) is supposed to represent a waggon and horses. “Charles’ Wain” is a corruption of “churl’s wain,” or peasant’s cart. The Arabians thought that the four stars in the quadrilateral represented a bier, and the three in the “tail” the children of the deceased following as mourners! In the Greek mythology, Ursa Major represented the nymph Callisto, a daughter of Lycaon, who was loved by Jupiter, and turned into a bear by the jealous Juno. Among the old Hindoos the seven stars represented the seven Rishis. It is the Otawa of the great Finnish epic, the “Kalevala.” It was also called “David’s Chariot,” and in America it is known as “The Dipper.”
Closely north of the star θ in Ursa Major is a small star known as Flamsteed 26. This is not mentioned by Al-Sufi, but is now, I find from personal observation, very visible, and indeed conspicuous, to the naked eye. I find, however, that owing to the large “proper motion” of the bright star (1″·1 per annum) the two stars were much closer together in Al-Sufi’s time than they are at present, and this probably accounts for Al-Sufi’s omission. This is an interesting and curious fact, and shows the small changes which occur in the heavens during the course of ages.