12 Dr. J. Oroll on the Origin of Nebula'. 



with certainty as to the rate of motion of the greater number 

 of the stars. 



There seems to be a somewhat prevailing misapprehension 

 regarding the extent of our knowledge of stellar motions. 

 Before we can ascertain the rate of motion of a star from its 

 angular displacement of position in a given time, we must 

 know its absolute distance. But it is only of the few stars 

 which show a well-marked parallax that we can estimate the 

 distance; for it is now generally admitted that there is no re- 

 lation between the apparent magnitude and the real distance 

 of a star. All that we know in regard to the distances of the 

 greater mass of the stars is little else than mere conjecture. 

 Even supposing we knew the absolute distance of a star and 

 could measure its amount of displacement in a given time, 

 still we could not be certain of its rate of motion unless we 

 knew that it was moving directly at right angles to the line 

 of vision, and not at the same time receding or advancing 

 towards us ; and this we could not determine by mere obser- 

 vation. The rate of motion, as determined from its observed 

 change of position, may be, say, only twenty miles a second, 

 while its actual velocity may be ten times that amount. 



By spectrum-analysis it is true we can determine the rate 

 at which a star may be advancing or receding along the line 

 of sight independently of any knowledge of its distance. But 

 this again does not give us the actual rate of motion, Unless we 

 are certain that it is moving directly to or from us. If it is 

 at the same time moving transversely to the observer, its 

 actual motion may be more than a hundred miles per second, 

 while the rate at which it is receding or advancing, as de- 

 termined by spectrum-analysis, may not be 20 miles a 

 second. But in many cases it would be difficult to ascertain 

 whether the star had a transverse motion or not. A star, for 

 example, 1000 times more remote than « Centauri (that is, 

 twenty thousand billion miles), though moving transversely to 

 the observer at the enormous rate of 100 miles per second, 

 would take upwards of 30 years to change its position so much 

 as 1", and. 1800 years to change its position V; in fact we 

 should have to watch the star for a generation or two before 

 we could be certain whether it was changing its position or 

 not. And even after we had found with certainty that the 

 star was shifting, and this at the rate of 1' in 1800 years, 

 we could not, without a knowledge of its distance, express the 

 angle of displacement in miles. But from the apparent 

 magnitude or brilliancy of the star, we could not determine 

 whether its distance was 10 times, 100 times, or 1000 times 

 that of a Centauri; and consequently we could form no con- 



