6 M. Arago on Double Stars. 



Galileo had proposed this method. Dr Long put it into prac- 

 tice. A little later Dr Herschel applied it to those binary groups 

 that were catalogued in his time, and which seemed to promise 

 the greatest prospect of a successful result. But, as often hap- 

 pens to every one, though all have not the candour to avow it, 

 in seeking for one thing, the celebrated astronomer of Slough 

 found another. He discovered that usually the stars of unequal 

 sizes forming groups are not, as had been previously imagined, 

 independent stars, accidentally placed in two closely approxi- 

 mated visual lines ;— that their proximity within a very limited 

 space is not a simple effect of projection or perspective ; he dis- 

 covered that these stars are associated with each other ; — that 

 they form true systems ; — that their relative position is ever 

 changing ; — that^ in shorty ihe smaller stars revolve round the 

 greater, precisely as the planets Mars, Jupiter, Saturn, he. re- 

 volve round the sun.* 



In virtue of these circulatory movements, the smaller star is 

 sometimes precisely to the east, and sometimes exactly to the 

 west, of the greater. At certain epochs this moving star will 

 be found exactly to the north of the more brilliant one, which 

 appears to be its centre of motion. And at other epochs, it will 

 be seen in the opposite position, or in the south. 



These simple remarks will be sufficient to verify the relative 

 displacement of the two stars. But after that we have seen the 

 motion, we have naturally a desire to know by what law it is 

 effected. For the acquisition of this knowledge, we must multi- 

 ply observations, and give them a precise accuracy, by the help 

 of a plan which we shall now endeavour to explain. 



Two very fine threads must be stretched through the focus of a 



• Mathematically speaking, the two stars, the one as well as the other, 

 move round the common centre of gravity. And yet, the usual astronomical 

 observations exhibit only the successive positions of the smaller star in rela- 

 tion to the greater. But if we reflect on it, we shall perceive that, practically, 

 the elements of a relative motion — the orbit to which the discussion of these 

 elements will conduct — cannot also be other than a relative orbit. In a word, 

 it will be the curve along which an observer situated in the greater star, and 

 who imagines himself motionless, will perceive the lesser to move. Besides, 

 we do nothing more when we wish to determine the orbits of Jupiter, Saturn, 

 &c. In truth, we every day report the position of these planets in reference 

 to the sun, without considering if this luminary has or has not an especial 

 motion of progress in space. 



