45 



at intervals of a few years of fresh catalogues of star-places 

 has long been a staple item of the Observatory programme. 

 The necessity for the continued renewal of these celestial 

 surveys became evident from the time of Halley, who pointed 

 out, in 1718, that the title " fixed stars " was a misnomer, 

 as applied to at least four of the brightest of the heavenly 

 host, for Sirius, Arcturus, Betelgeux and Aldebaran were 

 in motion on the celestial sphere. The place of a star, 

 therefore, could not be determined once for all and assumed 

 to be unchangeable ; it must be continually revised. 



The next stage in this enquiry was reached when the elder 

 Herschel drew the inference from the proper motions of 14 

 stars that there was an apparent drift of the stars in general 

 towards the constellation Cam's Major, such as would be caused 

 by an actual motion of the sun and its attendant planets 

 towards the opposite point of the heavens, namely, the con- 

 stellation Hercules. And in the latter half of the igth century 

 a number of astronomers, of whom Airy was one, made fresh 

 determinations of the place of the " Apex of the Sun's Way." 

 By this time the proper motions of many hundreds of stars 

 had been ascertained, Bradley 's observations, made more 

 than one hundred years earlier, affording the groundwork 

 for that evaluation in a large number of cases. A third 

 stage was reached when a number of stars were seen to be in 

 associated motion; one of the most familiar instances of 

 such association being given by five of the seven stars of the 

 Plough; another by the stars of the Pleiades; and a third 

 by those in the head of Taurus. 



Early in the present century the study of proper motions 

 entered on its fourth stage, by the discovery of JACOBUS 

 CORNELIUS KAPTEYN that these motions were by no means 

 haphazard, but, when treated in the mass, showed a strongly 

 marked tendency to alignment in two directions. The 

 later development of this investigation has been due to 

 ARTHUR STANLEY EDDINGTON, Chief Assistant at Greenwich 

 (1906-1913). Looked at from the point of view of the centre 

 of gravity of the stars considered, the two star-streams 

 must be moving in directions exactly opposite to one another. 

 " The direction of relative motion of the two streams is a 

 very important and fundamental axis, for it is an axis of 

 symmetry of distribution of stellar motions." It lies accur- 

 ately in the plane of the Milky Way. " If, as seems to be 

 the case, the number of stars belonging to the two streams 

 is the same, the velocities of the two streams referred to 

 the mean of the stars must be equal. The results, regarded 



