$ 294] The Minor Planets 377 



too difficult even for his unrivalled powers of observation. 

 The minor planets were also found to be remarkable for 

 the great inclination and eccentricity of some of the orbits ; 

 the path of Pallas, for example, makes an angle of 35 with 

 the ecliptic, and its eccentricity is |, so that its least dis- 

 tance from the sun is not much more than half its greatest: 

 distance. These characteristics suggested to Olbers that 

 the minor planets were in reality fragments of a primeval 

 planet of moderate dimensions which had been blown 

 to pieces, and the theory, which fitted most of the facts 

 then known, was received with great favour in an age 

 when "catastrophes" were still in fashion as scientific 

 explanations. 



The four minor planets named were for nearly 40 years 

 the only ones known ; then a fifth was discovered in 

 1845 by Karl Ludivig Hencke (1793-1866) after 15 years 

 of search. Two more were found in 1847, another in 

 1848, and the number has gone on steadily increasing 

 ever since. The process of discovery has been very much 

 facilitated by improvements in star maps, and latterly by 

 the introduction of photography. In this last method, 

 first used by Dr. Max Wolf of Heidelberg in 1891, a 

 photographic plate is exposed for some hours ; any planet 

 present in the region of the sky photographed, having 

 moved sensibly relatively to the stars in this period, is thus 

 detected by the trail which its image leaves on the plate. 

 The annexed figure shews (near the centre) the trail of the 

 minor planet Svea, discovered by Dr. Wolf on March 

 2ist, 1892. 



At the end of 1897 no less than 432 minor planets were 

 known, of which 92 had been discovered by a single 

 observer, M. Chariots of Nice, and only nine less by 

 Professor Palisa of Vienna. 



The paths of the minor planets practically occupy the 

 whole region between the paths of Mars and Jupiter, 

 though few are near the boundaries ; no orbit is more 

 inclined to the ecliptic than that of Pallas, and the 

 eccentricities range from almost zero up to about . 



Fig. 89 shews the orbits of the first two minor planets 

 discovered, as well as of No. 323 (Brucid), which comes 

 nearest to the sun, and of No. 361 (not yet named), 



