ADVANCE OF ASTRONOMY. 291 



ment of a long exposure wliick led to tlie present important dis- 

 covery. If the stars had been bright enough to be photographed 

 by an exposure not longer than a few seconds or even than a few 

 minutes, then this new and wonderful planet Eros would not have 

 been revealed. 



Many points of light which were undoubtedly stars, and merely 

 stars, were shown on this picture taken by the German astronomer 

 at Urania. Among these points of light was, however, one object 

 which, though in appearance hardly distinguishable from a faint 

 star, was in truth a body of a very different character. No tele- 

 scope, however powerful, would show by mere inspection any ap- 

 preciable difference between the dot of light indicating a star and 

 the dot of light indicating the asteroid Eros. The fundamental 

 difference between the star and Eros was, however, revealed by 

 the long exposure. The stars in such a picture are, of course, at 

 rest. They have occupied for years and for centuries the places 

 where we now find them. If they are moving at all, their move- 

 ments are so slow that they need not now be considered. But this 

 starlike point, or, as we may at once call it, this asteroid, Eros, 

 is moving. ISTot that its movements seem very rapid from the dis- 

 tance at which alone we are compelled to view it. No casual glance 

 would indicate that Eros was flying along. The ordinary ob- 

 server would see no change in its place in a second — no change in 

 its place even in a minute. But when the exposure has lasted for 

 an hour this asteroid, in the course of the hour, has moved quite 

 appreciably. Hence arose a great difference between the repre- 

 sentation which the photograph has given of the stars, properly 

 so called, and of the asteroid. Each star is depicted as a sharp, 

 well-defined point. This little body which is not a star, this un- 

 steady sitter in the picture, could not be so represented; it merely 

 appeared as a streak. The completed photograph accordingly 

 shows a large number of well-marked dots for the stars, and among 

 them one faint line for the asteroid. 



Such a feature on a picture, though very unusual, does some- 

 times present itself. To detect such a streak on a photograph of 

 the stars is a moment of transcendent joy to the astronomer. It 

 is often for him the exciting occasion on which a discovery is made. 

 This little moving point is in actual fact as different from a star as 

 a pebble is different from a brilliant electric light. The resem- 

 blance of the asteroid to a star is merely casual; the resemblance 

 would wholly disappear if we were able to make a closer inspec- 

 tion. The star is a brilliant blazing orb like a sun, but so far away 

 that its luster is diminished to that of a point; the planet is com- 

 paratively near us; it is a dark body like our earth, and is like our 



