THE ROTATION OF THE GALAXY ^ 



By A. S. Eddington 

 Plumian Professor of Astronomy in the University of Camhridge 



Early in 1718 Edmund Hallcy communicated to the Royal Society 

 the paper announcing his discovery of the proper motions of the 

 stars, under the title " Considerations on the Change of the Latitudes 

 of some of the Principal Fixt Stars." Referring to a compariscni he 

 had made of modern places of stars with the ancient observations 

 collected in Ptolemy's Almagest, he wrote : 



I was suiiDrized to find the Latitudes of three of the principal Stars of Heaven 

 directly to contradict the supposed greater Obliquity of the Ecliptick, which 

 seems confirmed by the Latitudes of most of the rest, they being set down in 

 the old Catalogue as though the Plain of the Earths Orb[it] had changed its 

 Situation, among the fixt Stars, about 20' since the time of Hipparchns. . . . 

 Yet the three Stars PaUlicium or the Bulls Eye, Sirius and Arcturus do con- 

 tradict this rule directly. . . . What shall we say then? It is scarce credible 

 that the Antients could be deceived in so plain a matter, three Observers con- 

 firming each other. Again these Stars being the most conspicuous in Heaven, 

 are in all probability nearest to the Earth, and if they have any particular 

 Motion of their own, it is most likely to be perceived in them, which in so long 

 a time as 180<) Years may shew it self by the alteration of their places, though 

 if be utterly imperceptible in the space of a single Century of Years. . . . This 

 Argument seems not unworthy of the Royal Society's Consideration, to whom 

 T humbly offer the plain Fact as I find it, and would be glad to liave their 

 opinion. 



Two hundred years have gone by, and now we are faced with a 

 great accumulation of data concerning these apparent movements 

 of the stars. This has been supplemented, mainly during the last 

 20 years, by extensive determinations of their velocities in the line of 

 sight by use of the spectroscope. We have, therefore, a mine of ma- 

 terial from which we are trying to learn what we can of the nature of 

 the motions of the stars as a system and to reach some kind of dynam- 

 ical theory of what is going on. A caution must be given at the out- 

 set. According to modern views the dimensions of our galaxy are 

 immense; and although our survey of stellar motions extends over 



1 Reprinted by permission from The Rotation of the Galaxy, brfng the Halley lecture 

 delivered on May 30, 19.30, by A. S. Eddington, Oxford University Press. 1930. 



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