TANISHED STARS. 221 



The stars whose luminosity was of the shortest duration, 

 were those of 389, 827, and 1012. In the first of the above- 

 named years, the luminosity continued only for three weeks ; 

 in the second, four months ; in the third, three. On the other 

 hand, Tycho Brahe's star in Cassiopeia continued to shine for 

 seventeen months; while Kepler's star in Cygnus (1600) was 

 visible fully twenty-one years before it totally disappeared. 

 It was again seen in 1655, and still of the 3rd magnitude, 

 as at its first appearance, and afterwards dwindled down to 

 the 6th magnitude, without, however (according to Arge- 

 lander's observations), being entitled to rank among pe- 

 riodically variable stars. 



STAKS THAT HAVE DISAPPEARED. The observation and 

 enumeration of stars that have disappeared is of importance for 

 discovering the great number of small planets which probably 

 belong to our solar system. Notwithstanding, however, the 

 great accuracy of the catalogued positions of telescopic fixed 

 stars and of modern star-maps, the certainty of conviction that 

 a star in the heavens has actually disappeared since a certain 

 epoch can only be arrived at with great caution. Errors of 

 actual observation, of reduction, and of the press,? often dis- 

 figure the very best catalogues. The disappearance of a 



7 On instances of stars which have- not disappeared, see 

 Argelander in Schumacher's Astronom. Nachr., no. 624, 

 s. 371. To adduce an example from antiquity, I may point 

 to the fact that the carelessness with which Aratus com- 

 piled his poetical .catalogue of the stars has led to the 

 often-renewed question, whether Vega Lyra? is a new star 

 or one which varies in long periods. For instance, Aratus 

 asserts that the constellation of Lyra consists wholly of small 

 stars. It is singular that Hipparchus, in his Commentary, 

 does not notice this mistake, especially as he censures Aratus 

 for his statements as to the relative intensity of light in 

 the stars of Cassiopeia, and Ophiuchus. All this, however, 

 is only accidental and not demonstrative; for when Ara- 



