Page:Popular Science Monthly Volume 58.djvu/653

Rh 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 photographed 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

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 provided 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