Masterpieces of Science 



point in the sky for two or three hours. Because 

 the stars are virtually motionless in a time so 

 short, they register themselves as tiny round 

 dots. The asteroids, on the other hand, have an 

 appreciable motion across the field of view, some- 

 what as the moon has, and so they betray them- 

 selves as minute but measurable streaks. On 

 August 13, 1898, a streak of this kind disclosed 

 to Herr Witt at the Observatory of Urania, at 

 Berlin, that most interesting and important of all 

 asteroids, Eros, about ten miles in diameter, 

 which approaches the earth more closely than 

 any heavenly body but the moon. It is expected 

 that observations of Eros will enable astrono- 

 mers to revise with new precision their com- 

 putations of the distance of the sun and the 

 planets. A faint streak similar to that ob- 

 served by Herr Witt once told Professor Barnard 

 that a comet had passed in front of his telescope 

 a comet so small and flimsy that only a photo- 

 graphic plate could see it.. Early in 1899 Pro- 

 fessor William Pickering thus discovered a new 



FIG. 87. 



Satellites of Saturn. Phoebe, the ninth, discovered by 

 Professor William Pickering. 



satellite of Saturn, making its known retinue 

 nine in number. This new moon made its ap- 

 pearance on four plates exposed with the Bruce 



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