HOW EROS WAS DISCOVERED 71 



for something else; he found what he was looking 

 for, and a new planet besides. What he was looking 

 for was one of the so-called nuisances of the heavens, 

 an asteroid, one of about 450, named 433 d . To search 

 for it as Herschel had to do, even though its where- 

 abouts was known, called for labour and time. The 

 astronomer, who was on the lookout for it, lessened 

 both by exposing a photographic plate to the starry 

 sky. He was spreading a net to catch planets and 

 comets. A fixed star does not change its place during 

 the exposure of the plate, or, rather, the plate moves 

 as the star moves: a moving body, be it planet or 

 comet, does change its place. A point will thus repre- 

 sent a fixed star ; a line, however short, and however 

 faint the trace, represents a moving body. When 

 Herr Witt examined the exposed plate, he saw at 

 once the trace left by the asteroid he was in search 

 of; but another, a fainter and a longer trace of a 

 moving body, was also seen on the plate. It was the 

 trace of a planet hitherto unknown. An examination 

 of the stranger resulted in the discovery that he was 

 a ball twenty miles in diameter, and, excepting our 

 moon, the nearest of the planets to us, so near that 

 he may be made to tell us the exact distance we are 

 from the sun. His discoverer called him Eros, Love or 

 Cupid, evidently from his childish size. 1 Herschel had 

 no such short-cuts to discovery in his day. 



An immense impulse was given to the study of the 

 stars by Herschel's discovery. It was not merely 

 what he achieved by being on the spot and on the 

 lookout. It was also by the lesson he taught astro- 

 nomers to do as he did. A band of twenty-four 



1 Nineteenth Century, April 1899, p. 612. 



