August 3, 1900.] 



SCIENCE. 



185 



place in the coming autumn, when its mini- 

 mum distance from the earth will be about 

 0.3, or twice its absolutely shortest dis- 

 tance. It is very desirable that astrono- 

 mers who devote their attention to the 

 determination of the solar parallax, should 

 take advantage of this opportunity for we 

 shall have a parallactic displacement of 

 Eros exceeding three times and possibly 

 five times the solar parallax. But there 

 are many drawbacks, the planet is faint ; 

 it will probably be, when nearest to us, but 

 little above the ninth magnitude, and it 

 will therefore be scarcely possible from its 

 rapid motion to photograph it, or to ascer- 

 its true position upon the plate at any given 

 time. It is difficult to ascertain just how 

 the observations upon it should be made. 

 With the view of getting the ideas of those 

 who are interested in the matter, and of as- 

 certaining how the stations can best be 

 planned, I have published in the Astronomical 

 Journal a paper upon this subject, accom- 

 panied by diagrams, to aid in the selection 

 of the observing stations. These diagrams 

 give four projections showing the position 

 of the earth as seen from the direction of 

 Eros, at different periods during the op- 

 position. (A copy of one of the diagrams 

 was placed upon the blackboard.) If we 

 imagine an observer looking down upon the 

 earth from Eros, the relative parallax be- 

 tween any two points upon the earth's 

 surface, will be the distance of those two 

 points as seen by the observer upon the 

 planet. These diagrams therefore show us 

 the advantages and disadvantages of any 

 two stations we may choose. The line of 

 sunset, showing when Eros will be visible 

 to us, will be nearly the same for several 

 nights in succession. Taking for example 

 December 16th ; in that part of the earth 

 upon the left of the diagram, it will be day, 

 and observers there cannot see the planet at 

 all. It is only in the remaining region that 

 an observer can see the planet. I will mark 



upon this portion degrees of latitude, show- 

 ing the position of different observatories. 

 On the circle of 60° north latitude, Pulkowa 

 and Helsingfors are situated, their position 

 upon the circle depending upon the time of 

 observation. It is evident from the diagram 

 that the parallax at those points will be 

 very great ; and those two places have the 

 advantage of being near one end of the 

 base line giving the greatest observable 

 amount of parallax. The asteroid be- 

 ing so far north on December 16th, does 

 not set at all at those places. It is evi- 

 dent that the best corresponding point, 

 giving the longest base line, will be in the 

 portion of the diagram south of the equator. 

 The planet at that time is only visible about 

 16° south of the equator and the only ob- 

 servatory in that region is at Arequipa. If 

 then observations are made at the Arequipa 

 observatory, for an hour or two, and if ob- 

 servations are also made at Pulkowa or 

 Helsingfors for an hour or two, it will give 

 us the longest possible base line for these 

 observations. Observations may be made 

 at other points, but they will be less avail- 

 able in the determination of the parallax 

 than those made at Pulkowa or Helsingfors 

 and at Arequipa. Another point for con- 

 sideration is that if the two sets of observa- 

 tions are made at different times, the planet 

 will be moving in the meanwhile. Its mo- 

 tion is very rapid, aud an element of uncer- 

 tainty is introduced by the necessity of de- 

 termining the amount of this motion. If 

 the observations at Pulkowa or at Helsing- 

 fors and at Arequipa are not simultaneous 

 this uncertainty affects the result. The 

 question arises therefore whether simul- 

 taneous observations can be obtained at the 

 ends of this base line. To determine that 

 question we have only to imagine a map of 

 the world upon this diagram. As the ob- 

 server at Eros comes in sight of the oceans 

 and continents of the earth, passing before 

 him, he will see the two stations, at Helsing- 



