AN ADDRESS ON ASTROPHYSICS. 313 



for the two methods are in fact mutually helpful and mutually de- 

 pendent: the motion of every star involves both components. 



In this connection two points call for appreciation: First, the 

 motion of the solar system is a purely relative quantity. It refers to 

 the group of stars used in the solution. We could easily select twenty 

 or thirty of these stars whose velocities were such that the deduced 

 motion would be reversed 180° from that given by the entire list of 

 stars. We want to know the solar motion with reference to the entire 

 sidereal system. A satisfactory solution of the problem demands that 

 we use enough stars to be considered as representative of the whole 

 system. Second, the great sidereal problems require that observational 

 data for their solution should cover the whole sky. Until one year 

 ago radial velocity measures were confined to the northern two thirds 

 of the celestial sphere. Further attempts to deduce the solar motion 

 from northern observation alone would not be justified. Observations 

 in the southern third of the sky were needed, not only to represent that 

 large region in the solution, but in order that the unknown systematic 

 errors which affect the northern observations, as well as the southern, 

 might be eliminated, through the symmetrical balancing of the ma- 

 terial. Fortunately the energetic and wise policy of the Cape Ob- 

 servatory and the generosity of Mr. D. 0. Mills have provided two 

 complete equipments, which are now busily engaged in supplying the 

 southern data required. The Mills spectrograph in the northern 

 hemisphere has secured about three thousand spectrograms of approxi- 

 mately five hundred stars, and the Mills spectrograph in the southern 

 hemisphere has secured four hundred spectrograms of one hundred 

 and twenty-five stars. The number of stars not on the Mills list, and 

 accurately observed with other high-dispersion spectrographs, is not 

 known, but it is probably between one hundred and +wo hundred. We 

 may reasonably expect that, in two or three years, as many as eight 

 hundred well-determined radial velocities may be brought to bear upon 

 pressing sidereal problems. 



It is a frequent question : Is the solar system moving in a simple 

 orbit, and will it eventually return to the part of its orbit where it is 

 now? The idea of an affirmative answer to this question is very prev- 

 alent in the human mind. It is natural to think that we must be 

 moving on a great curve, perhaps closed like an ellipse, or open like a 

 parabola, the center of mass of the universe being at the curve's prin- 

 cipal focus. The attraction which any individual star is exerting 

 upon us is certainly very slight, owing to its enormous distance; and 

 the combined attractions of all the stars may not be very much greater ; 

 for since we are somewhere near the center of our stellar system, the 

 attractions of the stars in the various directions should nearly neutralize 

 one another. Even though we may be following a definite curve at 



VOL. LXVI. — 21. 



