Astronomical Curiosities Part 15

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 s.p.a.ce presumably contained the stars in the southern hemisphere which they could not see from their northern stations.

Now, the centre of this circular blank s.p.a.ce 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 lat.i.tude, 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 c.u.m cornibus annum Taurus."[382]

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.[383]

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.[384]

According to Seneca, the study of astronomy among the Greeks dates back to about 1400 B.C.; and the ancient constellations were already cla.s.sical 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 Chelae, into Libra. Curious to say, Aratus states that the constellation Lyra contained no bright star![385] Whereas its princ.i.p.al 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."[386] "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.[387]

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.[388]

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 (aste??s??), Aristotle and Hyginus S?ata (bodies), and Ptolemy S??ata (figures). By some they were called ???f?se?? (configurations), and by others ?ete??e. Proclus called those near the ecliptic ??d?a (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 Ha.s.san in the thirteenth century (1282) was the first to use Right Ascensions and Declinations instead of Longitudes and Lat.i.tudes. 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 (a Orionis) would be on the left shoulder of Orion, instead of the right shoulder according to Ptolemy and Al-Sufi, and Rigel ( 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 cla.s.sical 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. Freret 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.[389]

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-Rahman Bin Umar Bin Muhammad Bin Sahl Abu'l-husan 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 Ra 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 Budes--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-Rahman 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 ent.i.tled _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 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_.