NUMBER OF THH FIXED STARS. 107 



1st Mag. SdMag. 3d Mag. 4th Mag. 5th Mag. 



20 65 190 425 1100 



6th Mag. 7th Mag. 8th Mag. 9th Mag. 



3200 13,000 40,000 142,000 

 The number of stars distinctly visible to the naked eye 

 (amounting in the horizon of Berlin to 4022, and in that of 

 Alexandria to 4638) appears at first sight strikingly small.* 

 If we assume the moon's mean semi-diameter at 15' 33"' 5, 

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

 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 +90 observed by Lalande. As this space is 723 1 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 than 5600 stars from the first to the sixth magnitude inclusive, 

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

 of the G-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 3960 and 5900, and 

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

 perceive 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 very 

 careful 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 the number of stare, from the first to the sixth 

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

 to the calculation made by myself in Cosmos, vol. i., p. 150), and up- 

 ward of 5000 for the horizon of Paris. He gives 70,000 for the whole 

 sphere, including stars of the ninth magnitude. (Astronomic, th. in., s. 

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

 58,000 from the first to the eighth magnitude. 



