90 COSMOS. 



century and a half before the time of Hipparchus), we pos- 

 sess in the astronomy of the Greeks a limit for the period 

 when the fixed stars had not yet been arranged according 

 to their relative magnitudes. In the enumeration of the 

 stars belonging to each constellation, as given in the Catas- 

 terisms, frequent reference is made to the number of the 

 largest and most luminous, or of the dark and less easily rec- 

 ognized stars ;* but we find no relative comparison of the 

 stars contained in the different constellations. The Catas- 

 terisms are, according to Bernhardy, Baehr, and Letronne, 

 more than two hundred years less ancient than the catalogue 

 of Hipparchus, and are, besides, a careless compilation and 

 a mere extract from the Poeticum Astronomicum (ascribed 

 to Julius Hyginus), if not from the poem 'Epju^f of the older 

 Eratosthenes. The catalogue of Hipparchus, which we pos- 

 sess in the form given to it in the Almagest, contains the ear- 

 liest and most important determination of classes of magni- 

 tude (gradations of brightness) of 1022 stars, and therefore 

 of about one fifth of all the stars in the firmament visible to 

 the naked eye, and ranging from the first to the sixth mag- 

 nitude inclusive. It remains undetermined whether these 

 estimates are all due to Hipparchus, or whether they do not 

 rather appertain in part to the observations of Timocharis 

 or Aristyllus, which Hipparchus frequently used. 



This work constituted the important basis on which was 

 established the science of the Arabs and of the astronomers 

 of the Middle Ages : the practice, transmitted to the nine- 

 teenth century, of limiting the number of stars of the first 

 magnitude to 15 (although Madler counts 18, and Riimker, 

 after a more careful observation of the southern celestial hem- 

 isphere, upward of 20), takes its origin from the classifica- 

 tion of the Almagest, as given at the close of the table of 

 stars in the eighth book. Ptolemy, referring to natural vi- 

 sion, called all stars dark which were fainter than those of 

 his sixth class ; and of this class he singularly enough only 

 instances 49 stars distributed almost equally over both hem- 

 ispheres. Considering that the catalogue enumerates about 

 one fifth of all the fixed stars visible to the naked eye, it 

 should, according to Argelander's investigations, have given 



* Eratosthenes, Catasterismi, ed. Schaubach, 1795, and Eratotthenica, 

 ed. G. Bernhardy, 1822, p. 110-116. A distinction is made between 

 stars hafinpovc (jieyuTiOvf) and afjtavpovf (cap. 2, 11, 41). Ptolemy also 

 limits ol {ipopfyuToi to those stars which do not regularly belong to a con- 

 stellation. 



