M. Arago on Double Stars. f& 



they are observed, will indicate the place which the radiant body 

 occupied at the moment of their departure. 



One of these rays will arrive at the earth. Suppose it has 

 taken a considerable time in coming, — say a month. During 

 this time the star will not have remained immoveable ; it will 

 have quitted its former station. Thus we shall see it in this first 

 position, when it is there no longer. 



Let us now admit, that our ideas may be specific, that the star 

 has moved, at the same time withdrawing itself from the earthy 

 along the arch of a curve of a certain extent, — an arc, we shall 

 say of a circle, which, placed obliquely in space, is nearer to us 

 at one of its extremities than at the other. 



We perceive the star moving, on this arc, at the extremity 

 nearest to the earth, thirty days, we have supposed, after that 

 it has quitted this spot. It follows from this, that it would re- 

 quire more than thirty days for the rays of light to reach us from 

 that extremity which is most distant from us. The star will have 

 reached this more distant extremity — it will have left this position 

 Jbr a longer period than thirty days at the time when, from 

 the earth, we see it as placed in that spot. When, then, from 

 the date of this latter observation, which is thus found posterior 

 by more than thirty days to the date of the real arrival of the 

 star at the extremity of the arc, we subtract the date of the ob- 

 servation of its departure, the error of which, by the hypothesis, 

 was only, and exactly, thirty days, the difference will be greater 

 than that which we find by subtracting the one from the other, 

 if the BEAL dates of the transits of the same star through the 

 observed points were known. 



If, instead of making the moving star to start from the point 

 nearest to us, and so tracing it to the most distant, we had given 

 it the reversed course : if the point of the first observation had 

 been the more distant, it is evident, that the difference between 

 the observed transits, that is to say the transits as influenced by 

 the propagation of the light, instead of being greater, would be 

 smaller than the difference between the real transits. 



In general terms, if, in its curvilinear course, a star is gradually 

 removed farther from the earth, the luminous rays which ema- 

 nate from it come more and more tardily, to shew us in what 

 positions it is successively placed. To go from one of these po- 



