45o POPULAR SCIENCE MONTHLY. 



stood that the length of this line is taken to represent the actual mo- 

 tion; the latter would be infinitesimal as compared with its length; we 

 use it only to show direction. We may, however, use the line to rep- 

 resent on a magnified scale the actual amount of the motion during 

 any unit of time, say, one year. It may be divided into two com- 

 ponents; one, S, in the direction of the line of sight from us to the 

 star, which for brevity we shall call the radial line, and the other, S M, at 

 right angles to that line. 



It must be understood that, as the term 'proper motion' is com- 

 monly used, only the component S M, can be referred to, because the 

 radial component, S R, does not admit of being determined by telescopic 

 vision. As we know from the preceding chapters, it can in the case of 

 the brighter stars be determined by spectroscopic measurement of the 

 radial motion. At present we leave this component out of consid- 

 eration. 



The visible component, S M, can also be resolved into two perpen- 

 dicular components, the one east and west on the celestial sphere, the 

 other north and south. The former is the proper motion in right 

 ascension (the measured motion in this coordinate being multiplied by 

 the co-sine of the declination to reduce it to a great circle), and the 

 other is the proper motion in declination. In star catalogues these 

 two motions are given, so far as practicable. Thus, altogether the 

 actual motion of a star in space may be resolved into three components: 

 that of right ascension, that of declension, and the radial component. 



An additional consideration is now to be added. The proper mo- 

 tion of a star, as observed and given in catalogues, is a motion relative 

 to our system. It has been shown in a former chapter that the latter 

 has a proper motion of its own. When account is taken of this, and 

 the motions are all reduced as well as we can to a common center of 

 gravity of the whole stellar system, we conceive the observed proper 

 motion of the star to be made up of two parts, of which one is the 

 actual motion of the star relative to the common center, and the other 

 due to the motion of the sun, carrying the earth with it. The direction 

 of the latter appears to us opposite that of the motion of the sun. The 

 sun's motion being directed to the constellation Lyra, it follows that the 

 component in question in the case of the stars is directed toward the 

 opposite constellation, Argo. This component, as we know, is termed 

 the parallactic motion, being dependent on the distance or parallax 

 of the star. 



As in the case of other proper motions, we may measure the 

 parallactic motion either in angular measure, as so many seconds per 

 century, or in linear measure, as so many kilometers per second. The 

 relation of the two measures depends on the distance of a star. The 

 simplest conception of the relation may be gained by reflecting that the 



