THE ASTRONOMY OF THE INVISIBLE 377 



perhaps not always so numerously, perhaps sometimes more 

 so. But with two central masses, each of the proportions, let 

 us say, of our sun to the rest of the planets, it is difficult to 

 conceive how such a system could long subsist. If they were 

 spaced as closely as the two components of the Pole-star appear 

 to be, for example, the problem would be relatively simple. 

 Then their satellites might revolve around the common centre 

 of gravity ; but obviously with a somewhat perturbed motion. 



But if they were as widely separated as Neptune and the 

 sun, as some of the binaries appear to be, probably each of the 

 pair would have its satellites. If these latter were at any con- 

 siderable distance, like Jupiter, they would be most curiously 

 pulled about in their orbits when they came in conjunction 

 between the pair of suns. Life on such planets, if they 

 exist, would be a topsy-turvy affair. When in conjunction, day 

 would be of course continuous, ocean tides would probably 

 be enormous, the course of the seasons would doubtless be quite 

 broken up certainly if the axes of the planets were inclined 

 to the plane of the ecliptic like our own. Their inhabitants 

 would experience the most violent changes of temperature, with 

 consequent storms and a general derangement of physical 

 conditions. 



Altogether, when we try to project our own little scheme of 

 things into the general order of the universe, we are hopelessly 

 estopped by the very beginnings of our knowledge of stellar 

 arrangements. In another two or three centuries, doubtless, the 

 case will be different and our knowledge considerable. 



In a general way, our representation of our immediate 

 planetary system is complete. We cannot conceive it as very 

 materially altered by any subsequent discovery. We know the 

 distance and size of the sun, the planets, their satellites, and 

 this so accurately and surely that " astronomical knowledge " 

 has become the synonym for fixity and certitude. 



Our knowledge of the solar system represents an effort of 

 the human mind extending through several thousand years. 

 The larger and more definite part of this knowledge has all 

 come within three centuries, the last details within fifty or 

 sixty years. 



Accurate star-knowledge dates from Bradley and Herschel 

 that is to say, from a little more than a century and a half 

 and it is concerned with motions for the most part less per- 



