24 M. Arago oti Double Stars. 



If the difference of the two angles was only two seconds, we 

 should find a distance one fourth greater than the preceding. 



And, finally, if any one succeeded in establishing a difference 

 in the two angular elevations of a single second, the star would 

 be (16,000,000,000,000) sixteen billions of leagues from the 

 earth * 



But some who have heard of the extreme accuracy of modern 

 observations, will be ready to exclaim against our admission of 



• According to the notion, in general very plausible, that the most bril- 

 liant stars ought to be the least distant from the earth, astronomers formerly 

 agreed to examine the parallaxes, especially of the stars of the first and se- 

 cond magnitude. Latterly there has been some reason to suppose that cer- 

 tain stars, which are little remarkable for their intensity, might probably be 

 found amongst the nearest. We shall here mention a few of the consider- 

 ations which indicate this. 



Formerly the stars were caXiedi fixed stars. But assuredly they do not merit 

 this appellation. All in fact progress, — all have an individual motion. Nor 

 are we here speaking of the revolution of a smaller star around a greater, 

 with the consideration of which we have been so long engaged ; but of a mo- 

 tion which, since it has been observed, has always been going forward in the 

 same direction ;— of a motion apparently destined, in the long run, to mingle 

 together the stars of the different constellations. It is natural to suppose that 

 the more rapid this individual movement is, the nearer will the star in which 

 it is observed be to ourselves. According to this principle, the 61st of the 

 Swan, which has a proper annual motion of more than 5 seconds, natu- 

 rally presents itself as probably offering a sensible parallax. With this in 

 view, we, along with M. Mathieu, have observed it with extreme care during 

 the month of August 1812, and during the following month of November. 

 The angular height of the star above the horizon of Paris at the second epoch, 

 did not exceed its angular height at the first, but by the small fraction of //o 

 part of a second. An absolute parallax of a single second would have neces- 

 sarily inferred a difference of \".2 between these two elevations. Our observa- 

 tions then indicated that the diameter of the terrestrial orbit,— that 39,000,000 

 of leagues could not be seen at the €lst of the Swan under an angle of more 

 than half a second. But a base, seen perpendicularly, subtends an angle of 

 half a second, when it is elongated 412,000 times its own length. The 61st of 

 the Swan, then, is at least at a distance from the earth equal to 412,000 mul- 

 tii)lied by 39,000,000 leagues. The number that results from this multi- 

 plication, indicates a distance which light could not penetrate in less than six 

 years, though it flies, as every one knows, §0,000 leagues in a second. 



Another word, and we have done. The 61st of the Swan moves every year 

 in a right line more than five seconds. At the distance we are removed, a 

 second corresponds at least to eight billions (8,000,000,000,000) of leagues. 

 Every year, then, the 6l8t of the Swan moves at least 4,000,000,000,000 of 

 leagues. And this is what was called a fixed star ! 



