V9 



a. CENTAURI, WITH A GEAPHIC PEOJECTION OF 



ITS ORBIT PROM ITS APPARENT CURVE. 



Bt a. B. Biggs. 



Several circumstances invest the star Alpha Gentauri, one of 

 the brightest in our southern sky, with special interest. It 

 was one of the earliest whose annual parallax (from which is 

 deduced its distance) was approximately ascertained. It is, 

 so far as at present known, by much the nearest of the fixed 

 stars to our system. It is perhaps the most magnificent of 

 double stars. And (what invests it with special interest to us) 

 it is, from its great southern declination, invisible to the 

 Observatories of the Northern Hemisphere. Science is 

 therefore dependent entirely upon southern observations for all 

 that can be known of the relative movements of its components. 

 Its distance from the solar system is about 225,000 times that 

 of the earth from the sun. 



Various attempts have been made from time to time to 

 determine its orbit and period, the latter varying from 75 

 years by Powell in 1854, to 88i years by Dr. Doberck in 1879. 

 My own observations of the star extend from about the latter 

 date to the present time. In venturing to attack the problem, 

 I have therefore the advantage of combining my own more 

 recent measures with the anterior ones of other observers. 



A brief description of my method will furnish some criterion. 

 of the value of the result. I first drew up a table of every 

 observation I could find recorded (including those in Crossley 

 Gr. and Ws work on double stars). These I laid off in a 

 curve, on plotting paper (ruled decimally in millimetre squares) 

 taking dates and position-angles for ordiuates an co-ordinates. 

 The distance measures were treated in the same way. After 

 smoothing each curve into reasonable symmetry, I drew up a 

 table for every 5th degree of position-angle, with date, as 

 ascertained from the curve. The next process was to draw a 

 circle, with radii to every 5th degree, the centre being the 

 locus of the principal star A. On these radii I laid off by dots 

 the distance answering thereto by date, of the star B. A 

 figure was thus obtained approximating to that of a long ellipse, 

 answering to the apparent curve of the star B round its 

 primary. But it was not a true illipse, and it ought to have 

 been. I wasted much time in trying to construct an ellipse 

 that should fairly represent a mean of all the different positions, 



