THE PLANET EROS. 647 



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 643 d . 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 op- 

 position of 1901. Seven revolutions of the earth take 2556 d .8, 

 and four revolutions of Eros, 2572 d .4. Hence every seventh year 

 the position of Eros will be repeated, with respect to the earth, within 

 156 d . 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 oc- 

 curred eighty-seven days earlier than the earth reached the same lon- 

 gitude, 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 15 d .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 re- 

 maining 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 per- 

 turbations 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 meas- 

 ured at the Observatory of Columbia University under the direction 

 of Dr. Rees. At the Harvard Observatory several hundred photo- 

 graphs 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 pho- 



