Astronomical Curiosities: Facts and Fallacies
TekstThe appearance of this temporary star in the Andromeda nebula seems to afford further evidence against the hypothesis of the nebula being an external universe. For, as I have shown above, our sun, if placed at a distance of 150,000 light years, would shine only as a star of the 23rd magnitude, or over 15 magnitudes fainter than the temporary star. This would imply that the star shone with a brightness of over a million times that of the sun, and would therefore indicate a body of enormous size. But the rapid fading of its light would, on the contrary, imply a body of comparatively small dimensions. We must, therefore, conclude that the nebula, whatever it may be, is not an external universe, but forms a member of our own sidereal system.
In Sir John Herschel’s catalogue of Nebulæ and Clusters of Stars, published in 1833, in the Philosophical Transactions of the Royal Society, there are many curious objects mentioned. Of these I have selected the following: —
No. 496 is described as “a superb cluster which fills the whole field; stars 9, 10 … 13 magnitude and none below, but the whole ground of the sky on which it stands is singularly dotted over with infinitely minute points.” This is No. 22 of Sir William Herschel’s 6th class, and will be found about 3 degrees south and a little east of the triple star 29 Monocerotis.
No. 650. This object lies about 3 degrees north of the star μ Leonis, the most northern of the bright stars in the well-known “Sickle,” and is thus described by Sir John Herschel: “A star 12th magnitude with an extremely faint nebulous atmosphere about 10″ to 12″. It is between a star 8-9 magnitude north preceding, and one 10th magnitude south following, neither of which are so affected. A curious object.”
No. 1558. Messier 53. A little north-east of the star α Comæ Berenices. Described as “a most beautiful highly compressed cluster. Stars very small, 12th … 20th magnitude, with scattered stars to a considerable distance; irregularly round, but not globular. Comes up to a blaze in the centre; indicating a round mass of pretty equable density. Extremely compressed. A most beautiful object. A mass of close-wedged stars 5′ in diameter; a few 12th magnitude, the rest of the smallest size and innumerable.” Webb says, “Not very bright with 37⁄10 inches; beautiful with 9 inches.” This should be a magnificent object with a very large telescope, like the Lick or Yerkes.
No. 2018. “A more than usually condensed portion of the enormous cluster of the Milky Way. The field has 200 or 300 stars in it at once.” This lies about 2° south-west of the star 6 Aquilæ, which is near the northern edge of the bright spot of Milky Way light in “Sobieski’s Shield” – one of the brightest spots in the sky.
No. 2093. “A most wonderful phenomenon. A very large space 20′ or 30′ broad in Polar Distance, and 1m or 2m in Right Ascension, full of nebula and stars mixed. The nebula is decidedly attached to the stars, and is as decidedly not stellar. It forms irregular lace-work marked out by stars, but some parts are decidedly nebulous, wherein no star can be seen.” Sir John Herschel gives a figure of this curious spot, which he says represents its “general character, but not the minute details of this object, which would be extremely difficult to give with any degree of fidelity.” It lies about 3 degrees west of the bright star ζ Cygni.
Among the numerous curious objects observed by Sir John Herschel during his visit to the Cape of Good Hope, the following may be mentioned: —
h 2534 (H iv. 77). Near τ4 Eridani. Sir John Herschel says, “Attached cometically to a 9th magnitude star which forms its head. It is an exact resemblance to Halley’s comet as seen in a night glass.”… “A complete telescopic comet; a perfect miniature of Halley’s comet, only the tail is rather broader in proportion.”366
h 3075. Between γ Monocerotis and γ Canis Majoris. “A very singular nebula, and much like the profile of a bust (head, neck, and shoulders) or a silhouette portrait, very large, pretty well defined, light nearly uniform, about 12′ diameter. In a crowded field of Milky Way stars, many of which are projected on it.”367
h 3315 (Dunlop 323). In the Milky Way; about 3° east of the Eta Argûs nebula. Sir John Herschel says, “A glorious cluster of immense magnitude, being at least 2 fields in extent every way. The stars are 8, 9, 10, and 11th magnitudes, but chiefly 10th magnitude, of which there must be at least 200. It is the most brilliant object of the kind I have ever seen” … “has several elegant double stars, and many orange-coloured stars.”368 This should form a fine object in even a comparatively small telescope, and may be recommended to observers in the southern hemisphere. A telescope of 3-inches aperture should show it well.
Among astronomical curiosities may be counted “clusters within clusters.” A cluster in Gemini (N.G.C. 2331) has a small group of “six or seven stars close together and well isolated from the rest.”
Lord Rosse describes No. 4511 of Sir John Herschel’s General Catalogue of Nebulæ and Clusters (Phil. Trans., 1864) as “a most gorgeous cluster, stars 12-15 magnitude, full of holes.”369 His sketch of this cluster shows 3 rings of stars in a line, each ring touching the next on the outside. Sir John Herschel described it as “Cluster; very large; very rich; stars 11-15 magnitude (Harding, 1827),” but says nothing about the rings. This cluster lies about 5 degrees south of δ Cygni.
Dr. See, observing with the large telescope of the Lowell Observatory, found that when the sky is clear, the moon absent, and the seeing perfect, “the sky appeared in patches to be of a brownish colour,” and suggests that this colour owes its existence to immense cosmical clouds, which are shining by excessively feeble light! Dr. See found that these brown patches seem to cluster in certain regions of the Milky Way.370
From a comparison of Trouvelot’s drawing of the small elongated nebula near the great nebula in Andromeda with recent photographs, Mr. Easton infers that this small nebula has probably rotated through an angle of about 15° in 25 years. An examination I have made of photographs taken in different years seems to me to confirm this suspicion, which, if true, is evidently a most interesting phenomenon.
Dr. Max Wolf of Heidelberg finds, by spectrum photography, that the well-known “ring nebula” in Lyra consists of four rings composed of four different gases. Calling the inner ring A, the next B, the next C, and the outer D, he finds that A is the smallest ring, and is composed of an unknown gas; the next largest, B, is composed of hydrogen gas; the next, C, consists of helium gas; and the outer and largest ring, D, is composed – like A – of an unknown gas. As the molecular weight of hydrogen is 2·016, and that of helium is 3·96, Prof. Bohuslav Brauner suggests that the molecular weight of the gas composing the inner ring A is smaller than that of hydrogen, and the molecular weight of the gas forming the outer ring D is greater than that of helium. He also suggests that the gas of ring A may possibly be identical with the “coronium” of the solar corona, for which Mendelief found a hypothetical atomic and molecular weight of 0·4.371
With reference to the nebular hypothesis of Laplace, Dr. A. R. Wallace argues that “if there exists a sun in a state of expansion in which our sun was when it extended to the orbit of Neptune, it would, even with a parallax of 1⁄60th of a second, show a disc of half a second, which could be seen with the Lick telescope.” My reply to this objection is, that with such an expansion there would probably be very little “intrinsic brightness,” and if luminous enough to be visible the spectrum would be that of a gaseous nebula, and no known star gives such a spectrum. But some planetary nebulæ look like small stars, and with high powers on large telescopes would probably show a disc. On these considerations, Dr. Wallace’s objection does not seem to be valid.
It is usually stated in popular works on astronomy that the spectra of gaseous nebulæ show only three or four bright lines on a faint continuous background. But this is quite incorrect. No less than forty bright lines have been seen and measured in the spectra of gaseous nebulæ.372 This includes 2 lines of “nebulium,” 11 of hydrogen, 5 of helium, 1 of oxygen (?), 3 of nitrogen (?), 1 of silicon (?), and 17 of an unknown substance. In the great nebulæ in Orion 30 bright lines have been photographed.373
D’Arrest found that “gaseous nebulæ are rarely met with outside the Milky Way, and never at a considerable distance from it.”374
Mr. A. E. Fath thinks that “no spiral nebula investigated has a truly continuous spectrum.” He finds that so feeble is the intensity of the light of the spiral nebulæ that, while a spectrogram of Arcturus can be secured with the Mills spectrograph “in less than two minutes,” “an exposure of about 500 hours would be required for the great nebula in Andromeda, which is of the same spectral type.”375 Mr. Fath thinks that in the case of the Andromeda nebula, the “star cluster” theory “seems to be the only one that can at all adequately explain the spectrum obtained.”376
Prof. Barnard finds that the great cluster in Hercules (Messier 13) is “composed of stars of different spectral types.” This result was confirmed by Mr. Fath.377
From observations with the great 40-inch telescope of the Yerkes Observatory (U.S.A.), Prof. Barnard finds that the nucleus of the planetary nebula H. iv. 18 in Andromeda is variable to the extent of at least 3 magnitudes. At its brightest it is about the 12th magnitude; and the period seems to be about 28 days. Barnard says, “I think this is the first case in which the nucleus of a planetary or other nebula has been shown to be certainly variable.” “The normal condition seems to be faint – the nucleus remaining bright for a few days only. In an ordinary telescope it looks like a small round disc of a bluish green colour.” He estimated the brightness of the nebula as that of a star of 8·2 magnitude.378 Even in a telescope of 4 inches aperture, this would be a fairly bright object. It lies about 3½ degrees south-west of the star ι Andromedæ.
The so-called “globular clusters” usually include stars of different brightness; comparatively bright telescopic stars of the 10th to 13th magnitude with faint stars of the 15th to 17th magnitude. Prof. Perrine of the Lick Observatory finds that (a) “the division of the stars in globular clusters into groups, differing widely in brightness, is characteristic of these objects”; (b) “the globular clusters are devoid of true nebulosity”; and (c) “stars fainter than 15th magnitude predominate in the Milky Way and globular clusters, but elsewhere are relatively scarce.” He found that “exposures of one hour or thereabouts showed as many stars as exposures four to six times as long; the only effect of the longer exposures being in the matter of density.” This last result confirms the late Dr. Roberts’ conclusions. Perrine finds that for clusters in the Milky Way, the faint stars (15th to 17th magnitude) “are about as numerous in proportion to the bright stars (10th to 13th magnitude) as in the globular clusters themselves.” This is, however, not the case with globular clusters at a distance from the Milky Way. In these latter clusters he found that “in the regions outside the limits of the cluster there are usually very few faint stars, hardly more than one-fourth or one-tenth as many as there are bright stars”; and he thinks that “this paucity of faint stars” in the vicinity of these clusters “gives rise to the suspicion that all regions at a distance from the Galaxy may be almost devoid of these very faint stars.” The late Prof. Keeler’s series of nebular photographs “in or near the Milky Way” tend to confirm the above conclusions. Perrine finds the northernmost region of the Milky Way “to be almost, if not entirely, devoid of globular clusters.”379
According to Sir John Herschel, “the sublimity of the spectacle afforded” by Lord Rosse’s great telescope of 6 feet in diameter of some of the “larger globular and other clusters” “is declared by all who have witnessed it, to be such that no words can express.”380
In his address to the British Association at Leicester in 1907, Sir David Gill said —
“Evidence upon evidence has accumulated to show that nebulæ consist of the matter out of which stars have been and are being evolved… The fact of such an evolution with the evidence before us, can hardly be doubted. I most fully believe that, when the modifications of terrestrial spectra under sufficiently varied conditions of temperature, pressure, and environment, have been further studied, this connection will be greatly strengthened.”
CHAPTER XVIII
Historical
The grouping of the stars into constellations is of great antiquity. The exact date of their formation is not exactly known, but an approximate result may be arrived at from the following considerations. On the celestial spheres, or “globes,” used by the ancient astronomers, a portion of the southern heavens of a roughly circular form surrounding the South Pole was left blank. This space presumably contained the stars in the southern hemisphere which they could not see from their northern stations. Now, the centre of this circular blank space most probably coincided with the South Pole of the heavens at the time when the constellations were first formed. Owing to the “Precession of the Equinoxes” this centre has now moved away from the South Pole to a considerable distance. It can be easily computed at what period this centre coincided with the South Pole, and calculations show that this was the case about 2700 B.C. The position of this circle also indicates that the constellations were formed at a place between 36° and 40° north latitude, and therefore probably somewhere in Asia Minor north of Mesopotamia. Again, the most ancient observations refer to Taurus as the equinoxial constellation. Virgil says —
“Candidus auratis aperit cum cornibus annum Taurus.”381
This would indicate a date about 3000 B.C. There is no tradition, however, that the constellation Gemini was ever seen to occupy this position, so that 3000 B.C. seems to be the earliest date admissible.382
Prof. Sayce thinks that the “signs of the Zodiac” had their origin in the plains of Mesopotamia in the twentieth or twenty-third century B.C., and Brown gives the probable date as 2084 B.C.383
According to Seneca, the study of astronomy among the Greeks dates back to about 1400 B.C.; and the ancient constellations were already classical in the time of Eudoxus in the fourth century B.C. Eudoxus (408-355 B.C.) observed the positions of forty-seven stars visible in Greece, thus forming the most ancient star catalogue which has been preserved. He was a son of Eschinus, and a pupil of Archytas and probably Plato.
The work of Eudoxus was put into verse by the poet Aratus (third century B.C.). This poem describes all the old constellations now known, except Libra, the Balance, which was at that time included in the Claws of the Scorpion. About B.C. 50, the Romans changed the Claws, or Chelæ, into Libra. Curious to say, Aratus states that the constellation Lyra contained no bright star!384 Whereas its principal star, Vega, is now one of the brightest stars in the heavens!
With reference to the origin of the constellations, Aratus says —
“Some men of yore
A nomenclature thought of and devised
And forms sufficient found.”
This shows that even in the time of Aratus the constellations were of great antiquity.
Brown says —
“Writers have often told us, speaking only from the depths of their ignorance, how ‘Chaldean’ shepherds were wont to gaze at the brilliant nocturnal sky, and to imagine that such and such stars resemble this or that figure. But all this is merely the old effort to make capital out of nescience, and the stars are before our eyes to prove the contrary. Having already certain fixed ideas and figures in his mind, the constellation-former, when he came to his task, applied his figures to the stars and the stars to his figures as harmoniously as possible.”385 “Thus e. g. he arranged the stars of Andromeda into the representation of a chained lady, not because they naturally reminded him (or anybody else) of such a figure, but because he desired to express that idea.”
A coin of Manius Aquillus, B.C. 94, shows four stars in Aquila, and seems to be the oldest representation extant of a star group. On a coin of B.C. 43, Dr. Vencontre found five stars, one of which was much larger than the others, and concludes that it represents the Hyades (in Taurus). He attributes the coin to P. Clodius Turrinus, who probably used the constellation Taurus or Taurinus as a phonetic reference to his surname. A coin struck by L. Lucretius Trio in 74 B.C., shows the seven stars of the Plough, or as the ancients called them Septem Triones. Here we have an allusion to the name of the magistrate Trio.386
In a work published in Berne in 1760, Schmidt contends that the ancient Egyptians gave to the constellations of the Zodiac the names of their divinities, and expressed them by the signs which were used in their hieroglyphics.387
Hesiod mentions Orion, the Pleiades, Sirius, Aldebaran, and Arcturus; and Homer refers to Orion, Arcturus, the Pleiades, the Hyades, the Great Bear (under the name of Amaxa, the Chariot), and the tail of the Little Bear, or “Cynosura.”
Hipparchus called the constellations Asterisms (ἀστερίσμος), Aristotle and Hyginus Σομάτα (bodies), and Ptolemy Σχημάτα (figures). By some they were called Μορφώσεις (configurations), and by others Μετεώρε. Proclus called those near the ecliptic Ζωδία (animals). Hence our modern name Zodiac.
Hipparchus, Ptolemy, and Al-Sufi referred the positions of the stars to the ecliptic. They are now referred to the equator. Aboul Hassan in the thirteenth century (1282) was the first to use Right Ascensions and Declinations instead of Longitudes and Latitudes. The ancient writers described the stars by their positions in the ancient figures. Thus they spoke of “the star in the head of Hercules,” “the bright star in the left foot of Orion” (Rigel); but Bayer in 1603 introduced the Greek letters to designate the brighter stars, and these are now universally used by astronomers. These letters being sometimes insufficient, Hevelius added numbers, but the numbers in Flamsteed’s Catalogue are now generally used.
Ptolemy and all the ancient writers described the constellation figures as they are seen on globes, that is from the outside. Bayer in his Atlas, published in 1603, reversed the figures to show them as they would be seen from the interior of a hollow globe and as, of course, they are seen in the sky. Hevelius again reversed Bayer’s figures to make them correspond with those of Ptolemy. According to Bayer’s arrangement, Betelgeuse (α Orionis) would be on the left shoulder of Orion, instead of the right shoulder according to Ptolemy and Al-Sufi, and Rigel (β Orionis) on the right foot (Bayer) instead of the left foot (Ptolemy). This change of position has led to some confusion; but at present the positions of the stars are indicated by their Right Ascensions and Declinations, without any reference to their positions in the ancient figures.
The classical constellations of Hipparchus and Ptolemy number forty-eight, and this is the number described by Al-Sufi in his “Description of the Fixed Stars” written in the tenth century A.D.
Firminicus gives the names of several constellations not mentioned by Ptolemy. M. Fréret thought that these were derived from the Egyptian sphere of Petosiris. Of these a Fox was placed north of the Scorpion; a constellation called Cynocephalus near the southern constellation of the Altar (Ara); and to the north of Pisces was placed a Stag. But all these have long since been discarded. Curious to say neither the Dragon nor Cepheus appears on the old Egyptian sphere.388
Other small constellations have also been formed by various astronomers from time to time, but these have disappeared from our modern star maps. The total number of constellations now recognized in both hemispheres amounts to eighty-four.
The first catalogue formed was nominally that of Eudoxus in the fourth century B.C. (about 370 B.C.). But this can hardly be dignified by the name of catalogue, as it contained only forty-seven stars, and it omits several of the brighter stars, notably Sirius! The first complete (or nearly complete) catalogue of stars visible to the naked eye was that of Hipparchus about 129 B.C. Ptolemy informs us that it was the sudden appearance of a bright new or “temporary star” in the year 134 B.C. in the constellation Scorpio which led Hipparchus to form his catalogue, and there seems to be no reason to doubt the accuracy of this statement, as the appearance of this star is recorded in the Chinese Annals. The Catalogue of Hipparchus contains only 1080 stars; but as many more are visible to the naked eye, Hipparchus must have omitted those which are not immediately connected with the old constellation figures of men and animals.
Hipparchus’ Catalogue was revised by Ptolemy in his famous work the Almagest. Ptolemy reduced the positions of the stars given by Hipparchus to the year 137 A.D.; but used a wrong value of the precession which only corresponded to about 50 A.D.; and he probably adopted the star magnitudes of Hipparchus without any revision. Indeed, it seems somewhat doubtful whether Ptolemy made any observations of the brightness of the stars himself. Ptolemy’s catalogue contains 1022 stars.
Prof. De Morgan speaks of Ptolemy as “a splendid mathematician and an indifferent observer”; and from my own examination of Al-Sufi’s work on the Fixed Stars, which was based on Ptolemy’s work, I think that De Morgan’s criticism is quite justified.
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.389
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.”390
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.