184 ON AN OCCTJLTATION OF SPICA VIEGINIS BY THE MOON. 



ral confidence, have occasionally witnessed a phenomenon in occupa- 

 tions, which has hitherto heen unexplained. According to them it 

 sometimes happens that after the occulted star has passed behind 

 the limb of the moon, it contiuues to be seen even for a considerable 

 time, notwithstanding the actual interposition of the body of the 

 moon. If this be not an optical illusion, and if the vertical rays come 

 straight to the observer, they must pass through a deep fissure in 

 the moon. 



Mr. Hind observes : some authorities adduce an argument in favor 

 of the presence of a lunar atmosphere, from a curious appearance 

 occasionally noticed, when the moon passes before a star — a 

 phenomenon technically known as an occultation — it most frequently 

 happens that the star disappears instantaneously in coming in con- 

 tact with the moon's limb, and reappears as suddenly and com- 

 pletely, when emerging from behind her disc. But this is not in- 

 variably the case ; it has been remarked that instead of vanishing 

 entirely at the moment of contact, the star is sometimes seen pro- 

 jected on the moon's disc, for several seconds of time, and a similar 

 appearance takes place, (though more rarely,) before the final 

 emerging from the other limb. About twenty years ago, a good 

 deal of interest was excited amongst astronomers in reference to 

 this matter, and some occultations of the bright star Aldebaran, were 

 closely watched at the principal European observatories. The result 

 proved far from conclusive — at the royal observatory, Greenwich, 

 some observers saw nothing unusual either at the immersion or 

 emersion of Aldebaran, the star disappeared and reappeared instan- 

 taneously ; others on the contrary, saw it distinctly projected on the 

 moon's disc for a second or two, before being occulted, and these 

 persons even observed with similar instruments, and from the same 

 station. 



Instances are on record where a star instead of disappearing finally, 

 when first in contact with the moon's limb, has run along it and re- 

 appeared several times, evidently between the mountains on the edge 

 of'her disc. On the 7th March, 1794, Professor Koch, saw Aldebaran 

 disappear and reappear three times, about thirty seconds or so 

 intervening between immersion and emersion. Another observa- 

 tion of a similar kind was made by Mr. Eumker, at Hamburg, on the 

 19th February, 1820 ; a star of 7th magnitude appeared to run with 

 extreme rapidity along the summits of the mountains in the moon's 

 edge, by which it was eclipsed from time to time. This " magnificient 

 spectacle" continued nearly ten minutes, when the star finally 

 vanished. 



