360 ^ SCIENTIFIC EECOKD FOR 1885. 



position, but of such small aiuount as to defeat any attempt at the tle- 

 termiuatiou of au orbit. The stars have a commou proper motion of 

 about — 0^-1720 in rij»ht ascension and +l"-S95o in declination, and this 

 peculiarity led jto the suspicion of a measurable parallax. The com- 

 parison stars used by Dr. Lamp were DM. +59°, 1913, and 1919, of 9*4 

 and 7-8 magnitude, respectively, and the mean result obtained is 



n = 0"-34 rt0"-034 

 The position for 1885 is E. A. = 18Hl^-5', Decl. = -f 59° 2V. 



DOUBLE STARS. 



The orbit of Gl Ci/gni. — Prof. C. F. W. Peters, of Kiel, has published in 

 IsTos. 2708-9 of the Astronomische Nachrichten an elaborate determination 

 of the orbit of the double star 61 Cygni. This, as he remarks, had not 

 hitherio been satisfactorly accomplished, although very numerous obser. 

 vations have been made, extending over an interval of more than one 

 hundred and thirty years, principally on account of the peculiarity that 

 the api)arent path of the companion star is very nearly rectilinear in 

 direction and uniform in amount, seeming to indicate that the apparent 

 motion of the double star is common to both components, and that 

 there is no perceptible motion of one with reference to the other. - - - 

 The most recent investigation is that of Prof. O. Struve, published in 

 the twenty-seventh volume of the Memoirs of the Imperial Academy of 

 St. Petersburg, in 1880, which, founded on the observations (carefr.lly 

 cleared from systematic errors) made atDorpat and Pnlkowa in the fifty 

 years from 1828 to 1878, represented them by a circular orbit. This, 

 however, i>roceeded, as Professor Peters points out, on the inadmissible 

 assnmi)tion that the angular velocity of the star, with regard to the 

 center of the circle, was uniform. Before commencing the present in- 

 vestigation, he reduced all the available observations to a common 

 epoch (the beginning of 1850). It soon appeared that it was not pos- 

 sible, on o^ccount of the very small curvature of the path, to obtain an 

 elliptic orbit by the application of the ordinary methods. Professor 

 Peters therefore determined in the first i^lace the circular orbit which 

 should best represent the observations, and proceeded afterwards to 

 deduce an elliptical one. The eccentricity of this ellipse is 0-17, corre- 

 sponding to an eccentric angle of 10°, and the semi-major axis 29"-48. 

 The period of the companion star in the elliptic orbit thus calculated 

 amounts to 782 C years. If the path be accepted as thus calculated by 

 Professor Peters, it becomes possible to determine an approximate 

 value of the sum of the masses of the components. This he finds to be 

 0-820 or 0-200, in terms of the sun's mass according as 0''-37 or 0"-54 be 

 taken as the parallax of the star. As a mean, therefore, the «um of the 

 masses would be equal to about half the mass of the sun, whilst the 

 mean distance of the two stars from each other would be about seventy 

 times thtit of the earth, or two and one-third times that of lNei)tune. 



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