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PROFESSOR A. S. EDDINGTON, F.R.S., ON 



some at least of the fixed stars were in motion. The star on 

 which in particular he based this conclusion was Arcturus ; 

 there was no doubt that this star was changing its position with 

 respect to the surrounding stars. It is interesting that this 

 famous old star — mentioned, as we know, in the Book of Job — 

 should be the one to open up a new branch of astronomy. 



Now what does the change of position amount to ? We now 

 know that Arcturus is exceptionally fast-moving, but not the 

 fastest ; in fact, about twenty stars are known to exceed it in 

 speed. I am speaking here of apparent rate of progress across 

 the sky, not the actual velocity in miles per hour. The 

 apparent rate is, of course, influenced by the nearness or distance 

 of the star. The quickest of all is a telescopic star in the 

 Southern Hemisphere (C.Z. 5h. 243), which travels at the rate 

 of nine seconds of arc per year. As that may not convey much 

 impression to you, I will put it another way. You know Orion 

 and the three stars that form his belt. I will use the belt as a 

 sort of standard race-track in the sky. The fastest star would 

 take 1,050 years to travel from one end to the other of Orion's 

 belt. That does not seem a very rapid rate, but still it is some- 

 thing quite appreciable without need for specially refined 

 measures. Arcturus would take about 3,000 years to do the 

 same course. But speeds so great as this are quite exceptional ; 

 a sort of average motion would be about one-twentieth of a 

 second per year, or Orion's belt in 180,000 years. That is 

 getting down to something very minute, but still it is quite 

 practicable to detect this and even considerably smaller move- 

 ments with certainty. 



We have now at our disposal the measured movements of 

 some thousands of stars, which we may proceed to examine. 



There are a number of cases in which these motions reveal at 

 once connections between stars which are certainly widely 

 separated from one another, and between which we should 

 scarcely have expected that any relation could exist. Incidentally 

 we find that many pairs of stars near together in the sky move 

 along together, and in such a group as the Pleiades all the stars 

 have a common motion ; but this tendency to a common 

 motion is found in some much more widely scattered stars. If 

 we select a certain region of the sky comprising Perseus and 

 parts of the surrounding constellations, and take, not all the 

 stars, but those characterized as particularly white-hot — as we 

 should say, stars of the helium type of spectrum — it is found 

 that these stars by their movements are sorted out into two 

 distinct groups. The stars of one set are moving moderately 



