M. Arago ofi Double Stars* M 



moves from the earth in its passage Jrorn the point o/tJie orbit, 

 which is the nearest to the point which is the most distant. 



The position and the dimensiwis of the orbit of a stellary sa- 

 tellite, are connected in a necessary way with the total quantity 

 which the satellite removes from the earth, and again approaches, 

 during each of its revolutions. When the dimensions of the 

 orbit are known, we may thence easily infer, by calculation, the 

 value of the changes of the distance. And, reciprocally, from 

 the value of these changes, we can mount up to that of the di- 

 mensions of the orbit. But we have just demonstrated how, in 

 certain cases, the astronomer experimentally determines, in 

 leagues, the changes which the distance of the satellite star under- 

 goes from the earth. In these same cases, the greater axis of the 

 elliptical orbit which the star seems to describe, may also be ex- 

 pressed in leagues. The inclination under which this axis pre- 

 sents itself to us, is deduced from the position of the plane of 

 the orbit. The micrometer enables us also to know its apparent 

 extent, or how many seconds it subtends. But after this, there 

 is not a land-surveyor who does not know how to determine the 

 distance at which he stands from a certain base, as soon as he 

 has learnt the inclination of that base to the visual ray, its ab- 

 solute length, and the angle at which he examines it. The as- 

 tronomer has exactly the same calculations to make. He only 

 operates on numbers to a vast extent greater. His base is the 

 diameter of an orbit which is traced by a star ; but still, that 

 which he is seeking, and that which he will find, is the distance 

 of this star from the earth. 



M. Savary, to whom we are indebted as being the first to 

 point out the important part which the successive transmission of 

 light might one day play, as it respects the phenomena of the 

 double stars, fearing, without doubt, that it would be only with 

 great difficulty, on account of the slowness of the motion of 

 the satellite stars, that we could determine, with accuracy, the 

 difference of the duration of their ascending and descending se- 

 mi-revolutions, had contented himself with presenting the ob- 

 servations of these durations, as a mean of arriving, not at an 

 absolute distance, but only at a limit. We shall now show how 

 it is necessary to regard the method, if it be not wished to carry 

 it farther than this. 



