THE PLANET EROS. 649 



possibility of determining the solar parallax by observations of the 

 transits of Venus, well knew when he developed the methods that he 

 himself could not live to see the experiment tried, since he was then 

 sixty-three years of age, and the next transit of Venus did not come for 

 forty-two years. Perhaps few of the observers who are so enthusias- 

 tically at work on Eros at this opposition will be alive to make ob- 

 servations at a really close approach of that interesting body. 



At the Paris meeting of the International Astrophotographic Con- 

 gress, in August, 1900, a committee was appointed to suggest the most 

 favorable course to be pursued. The committee later advised that work 

 be done by the micrometer, the heliometer and by photographs. The 

 observations in each case give the distance of Eros in seconds of arc 

 from adjacent stars. The simplest case is where simultaneous ob- 

 servations are made by observers at widely separated stations. Let A 

 and B (Fig. 3) be two stations on the earth. The observer at A will 

 see Eros projected on the celestial sphere at E 1 , and the observer at B, at 



Fig. 3. Parallax of Eros. 



E 2 . It is only necessary for each observer to measure the distance 

 in seconds of arc between Eros and some adjacent stars, as 1, 2, 3 and 4. 

 The positions of the stars must be known with the greatest precis- 

 ion, so that the observations give the value of the arc E*E 2 , which 

 equals the angle AEB. We have then the necessary material for 

 computing the distance of Eros from the earth in miles. Given this 

 and the orbit of Eros, the distances of the earth and all the other 

 planets from the sun in miles follow from the known laws of gravi- 

 tation. The distance AB may lie in a north and south direction, or 

 in an east and west direction, or more probably in a combination of 

 the two. In the first case there must be two observers, widely sep- 

 arated, as, for example, at Arequipa, Peru, latitude south 16°, and 

 Helsingfors, Finland, north 50°. In the second case there may be 

 two stations, as, one in Europe and the other in the United States, 

 or the whole work may be done at one station by allowing the earth's 

 diurnal motion to carry the observer to a new position. Suppose, for 

 example, that one observation is made when the planet is rising in the 



