The Evolution of the Sciences 



magnitude in April and May, the third in July 

 and August, the fourth in November, the sixth 

 in February 1574. The following month the star 

 disappeared without leaving any trace visible 

 to the naked eye, after having shined for seven- 

 teen months." 



No one has seen it since, and the phenomenon 

 observed by Tycho-Brahe would have remained 

 inexplicable if later observations had not put 

 us on the track again. In 1638 Hal war da 

 discovered in the Whale a star of the third 

 magnitude, which disappeared at the end of a 

 few months; in 1639 he saw it again in the same 

 place and of the same brightness. It again 

 disappeared, to reappear a third time, and 

 since Halwarda's time it has followed the same 

 course. This marvellous star, Mira Ceti, is 

 periodic, visible during four months, invisible 

 during seven. It has therefore a period of 

 eleven months. 



Since then many periodical stars have been 

 discovered about two hundred, with periods 

 varying from two years to a few hours. Many 

 stars have also been discovered with irregular 



or unknown periods. All these observations 



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