552 SCIENCE PROGRESS 



The orbits of these small planets lie in general between those 

 of Mars and Jupiter and up to 1898 none of them had been 

 suspected of approaching the earth nearer than the planet Mars, 

 But it was seen that the orbit of Eros lay within that of Mars 

 and that only a few years previous to its discovery (namely, in 

 1894) the earth and Eros had been simultaneously in the adjacent 

 portions of their orbits and had therefore been very close 

 together. 



Now such a close approach affords an opportunity of a 

 special kind for determining accurately the distance of the little 

 planet from the earth. Usually the planets are so far away 

 that their distances are many hundreds of million miles, ex- 

 ceeding the diameter of our tiny earth so vastly that it is 

 difficult to institute an exact comparison between the two, as 

 we must if we wish to express the former in units familiar to 

 us. The difficulty is precisely the same as that which we find 

 in realising the distances of remote objects by the use of our 

 eyes alone. There is no similar difficulty in perceiving the 

 distances of objects close to us — say those within an ordinary 

 room : they present different aspects to our two eyes and from 

 these differences in aspect we are able to judge of the distances. 

 But the change of aspect is smaller for objects more remote 

 and we know that it is entirely insensible for an object so 

 remote as the moon. Indeed, our power of perceiving distance 

 by means of the difference in aspect for our two eyes breaks 

 down long before we reach the moon, although we do not 

 always realise the breakdown, because other methods based on 

 general experience frequently come to our aid. 



In the same way, astronomers pointing telescopes from 

 opposite sides of the earth to the same object can perceive its 

 distance by a method similar to that we use unconsciously when 

 we look at anything with our two eyes. But the observations 

 become difficult when the object is too far away and are only 

 satisfactory for a comparatively close object. 



The heavenly body closest to ourselves is of course the 

 moon ; and we know its distance within twenty miles. Up till 

 1898 the next closest known were Mars and Venus, on favour- 

 able occasions ; and accordingly much time and trouble have 

 been spent in determining the distances of these two planets 

 when there has been a Transit of Venus or a favourable 

 Opposition of Mars. 



