THE GREAT STAR MAP 553 



It should perhaps be remarked here that the relative distances 

 of all the planets from the sun and from one another are known 

 with great precision from the times which they take to describe 

 their revolutions round the sun ; so that when any one of 

 them has been determined, we can obtain any other we please 

 by a simple rule-of-three sum. Or, to put the matter in another 

 way, we can make an accurate map of our solar system, being 

 in doubt only about the scale of miles which usually accompanies 

 such a map. Any single distance on the map being known, 

 we could construct this scale and so find all the others. 

 Hence it did not much matter whether we determined the 

 distance of Mars or of Venus or of any other planet which might 

 offer greater advantages than they ; the new discovery of Eros 

 ofifered just such greatly increased advantages. The opportu- 

 nity is however not open always but only at certain times 

 and seasons. One particularly tempting opportunity had been 

 unfortunately lost in 1894 owing to our ignorance of the 

 planet's existence. But it was seen that another opportunity 

 was coming in 1901, not so favourable as that of 1894 but 

 still well worthy our attention. It may be added that the 

 next good chance will not come till 193 1, so that it is easy to 

 understand the anxiety of astronomers to take advantage of 

 the occasion of January 1901. They were, however, taken at a 

 disadvantage by the comparatively short notice. There was 

 no time to think of preparing special instruments ; prudence 

 suggested utilising such instruments as were already in good 

 working order, and especially the battery of photographic 

 telescopes engaged in making the Great Star Map. It needed 

 a good reason to justify this diversion of their activities from 

 the great work, which was alone sufficient to occupy their 

 undivided attention, but the reason which had presented itself 

 so suddenly was felt to be good enough. At the meeting at 

 Paris, in 1900, of the Committee charged with the work on 

 the map, the President (M. Loewy) proposed that this digression 

 should be made ; and the proposal was unanimously adopted. 



Accordingly for some months during the winter 1 900-1, 

 most of the telescopes were withdrawn from the work on the 

 map and were turned on the little planet Eros. The chief 

 aim of the programme was to take photographs as soon as 

 possible after sunset and as late as possible before sunrise ; 

 for on these occasions the telescope would be as nearly as 



