NUMliER OF TUB FIXED STARS. 101 



Tst Mag. 2d Mag. 3d Ma^. 4th Mag. 5tli Mag 



20 Go 190 425 1100 



Gth Mag. 7th Mag. 8th Mas. 9tli Mas. 



3200 13,000 40,000 142,000 



The number of stars distinctly visible to the naked e}'e 

 [amounting in the horizon of Berhn to 4022, and in that of 

 Alexandria to 4G38) appears at first sight strikingly small. =^ 

 If we assum.e the moon's mean semi-diam.eter at 15' 33'''5, 

 it would require 195,291 surfaces of the full moon to covci 

 the whole heavens. If we further assume that the stars are 

 uniformly distributed, and reckon in round numbers 200,000 

 stars from the first to the ninth magnitude, we shall have 

 nearly a single star for each full-moon surface. This result 

 explains why, also, at any given latitude, the moon does not 

 more frequently conceal stars visible to the naked eye. If the 

 calculation of occultations of the stars were extended to those 

 of the ninth magnitude, a stellar eclipse would, according to 

 Galle, occur on an average every 44' 30", for in this period 

 the moon traverses a portion of the heavens equal in extent 

 to its own surface. It is singular that Pliny, who was un- 

 doubtedly acquainted with Hipparchus's catalogue of stars, 



and -f-90° observed by Lalande. As this space is 0-7231 of the whole 

 heavens, we should again have for this zone 5255 stars visible to the 

 naked eye. An examination of Bode's Uranography (containing 17,240 

 stars), which is composed of the most heterogeneous elements, does not 

 give more tiian 5G00 stars from the first to the sixth magnitude inclusive, 

 after deducting the nebulous spots and smaller stars, as well as those 

 of the 6-7th magnitude, which have been raised to the sixth. A simi- 

 lar estimate of the stars registered by La Caille between the south pole 

 and the tropic of Capricorn, and varying from the first to the sixth mag 

 nitude, presents for the whole heavens two limits of 39G0 and 5900, and 

 thus confirms the mean result already given by yourself. You will 

 })erceive that I have endeavored to fulfill your wish for a more thor- 

 ough investigation of these numbers, and I may further observe that M. 

 Heis, of Aix-la-Chapelle, has for many years been engaged in a veiy 

 carefid revision of my Uranometrie. From the portions of this work 

 already complete, and from the great additions made to it by an observ 

 er gifted with keener sight than myself, I find 2836 stars from the first 

 to the sixth magnitude inclusive for the northern hemisphere, and there- 

 fore, on the presupposition of equal distribution, 5672 as the number 

 of stars visible, throughout the whole firmament to the keenest unaided 

 vision." {From the Manuscripts of Professor Argelander, March, 1850.) 

 * Schubert reckons Ihe number of stai-s, from the first to the sixth 

 magnitude, at 7000 for the whole heavens (which closely approximates 

 to the calculation made by mys»df in Cosmos, vol. i.. p. 150), and ui> 

 ward of 5000 for the horizon ol Taris. He gives 70,000 for tne whole 

 spher3, including stars of the ninth magnitude. (Asfronomi6, th. iii., s, 

 54.) These numbers are aii much too high. Argelander finds only 

 08,000 from lhe first to the eighth magnitude. 



