DISTANCES OF THE STARS. 191 



words, " had arrived at no available result."* The observa- 

 tions taken from August, 1837, to October, 1838, by means 

 of the great heliometer erected in 1829, first led him to the 

 parallax of 0"-3483, which corresponds with a distance of 

 592,200 mean distances of the earth, and a period of 9| 

 years for the transmission of its light. Peters confirmed this 

 result in 1842 by finding 0"'3490, but subsequently changed 

 Bessel's result into 0"-3744 by a correction for temperature. 1 

 The parallax of the finest double star of the southern hem- 

 isphere (a Centauri) has been calculated at 0"'9128 by the 

 observations of Henderson, at the Cape of Good Hope, in 



39 millions de lieues soient vus de la 61" du Cygne sous un angle de 

 plus d'une demi-seconde. Mais une base vue perpendiculairement sou- 

 tend un angle d'une demi-seconde quand on est eloigne de 412 mille 

 fois sa longueur. Done la 61 e du Cygne est au moins a une distance 

 de la terre egale a 412 mille fois 39 millions de lieues." " During the 

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

 Mathieu and myself very carefully observed the altitude of the star 

 above the horizon, at Paris. At the latter period its altitude only ex- 

 ceeded that of the former by 0"-66. An absolute parallax of only a 

 single second would necessarily have occasioned a difference of l"-2 

 between these heights. Our observations do not, therefore, show that 

 a semi-diameter of the earth's orbit, or thirty-nine millions of leagues, 

 are seeu from the star 61 of Cygnus, at an angle of more than 0"-5. 

 But a base viewed perpendicularly subtends an angle of 0"'5 only when 

 it is observed at a distance of 412,000 times its length. Therefore the 

 star 6 1 Cygni is situated at a distance from our earth at least equal to four 

 hundred and twelve thousand times thirty-nine millions of leagues." 



* Bessel, in Schum., Jahrb. 1839, s. 39-49, and in the Astr. Nachr., 

 No. 366, gave the result 0"-3136 as a first approximation. His later and 

 final result was 0"-3483. (Astr. Nachr., No. 402, in bd. xvii., s. 274.) 

 Peters obtained by his own observations the following, almost identical, 

 result of 0"-3490. (Struve, Astr. Slell, p. 99.) The alteration which, 

 after Bessel's death, was made by Peters in Bessel's calculations of the 

 angular measurements, obtained by the Konigsberg heliometer, arises 

 from the circumstance that Bessel expressed his intention (Astr. Nachr., 

 bd. xvii., s. 267) of investigating further the influence of temperature 

 on the results exhibited by the heliometer. This purpose he had, in 

 fact, partially fulfilled in the first volume of his Astronomische Untersuch- 

 ungen, but he had not applied the corrections of temperature to the ob- 

 servations of parallax. This application was made by the eminent as- 

 tronomer Peters (Ergdnzungscheft zu den Astr. Nachr., 1849, s. 56), 

 and the result obtained, owing to the corrections of temperature, was 

 0"-3744 instead of 0"-3483. 



t This result of 0"-3744 gives, according to Argelander, as the dis- 

 tance of the double star 61 Cygni from the sun, 550,900 mean distances 

 of the earth from the sun, or 45,576,000 miles, a distance which light 

 traverses in 3177 mean days. To judge from the three consecutive 

 statements of parallax given by Bessel, 0"-3136, 0"-3483, and 0"-3744, 

 this celebrated double star has apparently come gradually nearer to us 

 in light passages amounting respectively to 10, 9\, and 8 T 7 ff yeara 



