1882.] on the Prober Motions of the Stars. 117 



respect to the ecliptic, and he obtained a further confirmation of this 

 result by examining the account of an occultation of Aldebaran by the 

 moon, observed at Athens in the year 509 a.d. ^ 



A few years after Halley announced this important fact, Bradley 

 made his famous discovery of the aberration of light, and its effect 

 upon the apparent place of a star; and subsequently the same 

 astronomer discovered the apparent sidereal movement depending on 

 he nutation of the earth's axis. The astronomer could now ascertain 

 tJie true place of a star m the heavens with a precision to which the 



nfl LT .i^r'^^f '^'i^*' ""^^^ ^^^" ^^ comparison, and it seemed 

 piobable that ere long the great problem of the proper motions of the 

 stars might be attacked with some hope of success 



To ascertain the proper motion of a star it is necessary to have 

 two well determined places of the star separated from each other by a 

 sufficiently great interval of time. Down to the middle of the last 

 century no such materials may be said to have existed, if we except 

 a tew isolated cases such as those referred to by Halley, for the nro- 

 bable errors m the observed places of a star far exceed in magnitude 

 the minute quantity which was the object of inquiry. To Bradley is 

 due a great work of observational astronomy which has constituted the 

 basis of the more extensive investigations of the present day relating 

 to the proper motions of the stars. This consisted in a series of star 

 observations executed by that astronomer at the Royal Observatory 

 Greenwich from 1750 to 1762, but which it was reserved for Bessel' 

 the great German astronomer, to reduce, and finally to publish in the 

 year 1818. ^ A comparison of those star places with the corresponding 

 results obtained at the Greenwich Observatory in the present century 

 by Sir George Airy, the late Astronomer Eoyal, has conducted 

 astronomers to important conclusions respecting the proper motions 

 of the stars. Materials tending to elucidate the same great question 

 have also been derived from the star observations of several other 

 astronomers of the present century. 



[The lecturer here exhibited a diagram containing the following 

 Illustrations of the proper motions of the stars :— 



star. Magnitude. ProperMotioa 



Thousand Years. 



Sirius 



Procyon 



Arcturus .. 



a Centauri 



Capella [ " 



Rigel 



Antares ]] * 



Groombridge, 1830 , . .*.' " " 7 ,71 n^ 



O^Eridani ;. 4 ^^00 



Lalande, 27,744 .... ' q 



Lalande, 30,044 .. •• •• 



Lalande, 30,694 .. .. [[ \\ q J^^/. 



Weisse's Bessel XVIL, 322 .. .. 7 J^^g-, 



1360 



1210 



2230 



3710 



250 



20 



30 



4100 

 1681 

 1607 



