Page:Popular Science Monthly Volume 58.djvu/655

Rh planet was in perihelion on January 22, only a few hours later than the earth arrived at the same longitude; so that the opposition at that time was nearly as favorable as can ever occur. Since the period of Eros is 643d.10, it will be easy to compute when the planet will again come to perihelion near the date January 21. The relation between the periods of Eros and the earth is such that a close approach will always be followed in seven years by one not so good, but yet favorable. This is illustrated by the near approach of 1894 and the less favorable opposition of 1901. Seven revolutions of the earth take 2556d.8, and four revolutions of Eros, 2572d.4. Hence every seventh year the position of Eros will be repeated, with respect to the earth, within 156d. So that if Eros arrived at perihelion one day later than the earth reached the same longitude in 1894, it would arrive there about seventeen days later in 1901, thirty-two days later in 1908, etc. It is evident that by following this series no close approach would come again till far into the next century. This series includes one-fourth of the perihelion returns of Eros. Three other series will include the remainder. They may be reckoned from that of 1895, when it occurred eighty-seven days earlier than the earth reached the same longitude, that of 1897, 175 days earlier, and that of 1899, 262 days earlier. Beginning with the difference of eighty-seven days in 1895, the number decreases by 15d.6 every seven years, so that in 1931 Eros will arrive at perihelion about ten days ahead of the earth, and in 1938 about six days later. This pair of oppositions appear to be the best which will occur during the next half century. The series which begins in 1897 with a difference of 175 days would apparently give a close approach after about three quarters of a century, and the remaining series much later still. It seems, therefore, that not till the latter part of the present century can so favorable an opposition recur, as that of 1894, which was lost except for the Harvard photographs. These conclusions may, however, be modified by a study of the perturbations of Eros by the other planets, which have not been considered in the above computations.

During the last few months great attention has been given to Eros at fifty of the leading observatories of the world. Professor Campbell, Director of the Lick Observatory, says that for two or three months fully half the resources of that institution have been devoted to this object. The positions of 700 fundamental stars have been determined by the meridian circle, and photographs made, which will be measured at the Observatory of Columbia University under the direction of Dr. Rees. At the Harvard Observatory several hundred photographs have been taken, and very extended photometric observations made. Owing to the exceptional conditions which prevail at the Arequipa branch of the observatory and the power of the Bruce