THE PLANET EROS. 



645 



Prof. Edward C. Pickering, began photographing the heavens, and 

 at the present time there are in the Observatory more than 100,000 

 photographs of the sky made during those years. Some of these are 

 on a large scale, and are of special objects, but many thousands of 

 them are charts on so small a scale that the entire sky has been pho- 

 tographed many times. On nearly all these plates stars are shown to 

 the tenth magnitude, and in many cases stars as faint as the fifteenth 

 or sixteenth magnitude appear. The early elements of Eros showed 

 that the planet made a close approach to the earth in 1894, and a 

 search was promptly instituted on the Harvard photographs. At first 

 the available observations were insufficient to give the elements with 

 the accuracy which was necessary in order to determine the planet's 



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 XO' 

 10' 



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Fig. 1. Path of Eros in 1893 and 1894. The Circular Dots represent the Positions 

 which were determined from the harvard photographs. 



position in 1894. An error of 1" in the mean daily motion would 

 change the right ascension in 1894 by about half an hour. On this 

 account no image of the planet was found on the photographs first 

 examined. By an examination, however, of plates made in 1896 Mrs. 

 Fleming found several images of Eros, and Mr. Chandler then pro- 

 vided a corrected ephemeris, by means of which the planet was readily 

 found on plates made in 1893 and 1894. Thus several years' history 

 of this remarkable object was at once presented to the astronomical 

 world. 



While the mean distance of Eros is 135 million miles, its aphelion 

 distance is 166 millions and its perihelion distance 105 millions. 

 Since this planet is sometimes within and sometimes without the orbit 

 of Mars, it might be expected that at favorable times it would approach 



