46 



MOTIONS OF THE DOUBLE STARS. 



of place of the whole solar system would re- 

 main unknown to us, were it not that the won- 

 derful perfection of modern astronomical instru- 

 ments for taking measurements, and the ad- 

 vances of the astronomy of observation, ren- 

 der our progress obvious towards distant stars 

 as towards objects on a coast apparently in 

 motion. The proper motion of the 61st star in 

 the constellation of the Swan, for example, is 

 so considerable, that in the course of 700 years 

 it will have amounted to a whole degree. 



The measure or quantity of alteration in the 

 heaven of the fixed stars — of alteration in the 

 relative positions of the self-luminous stars to 

 one another — can be determined with more of 

 certainty than the phenomenon itself can be 

 genetically explained. Even after we have al- 

 lowed for all that belongs to the precession of 

 the equinoxes and the nutation of the earth's ax- 

 is, as consequences of the influence of the sun 

 and moon upon the spheroidal figure of our plan- 

 et, to the propagation or aberration of light, and 

 to the parallax produced by diametrically oppo- 

 site positions of the earth in its orbit round the 

 sun — when a correction has been made for 

 each and all of these particulars, there is al- 

 ways a quantity in the remaining annual mo- 

 tion of the fixed stars, which is the conse- 

 quence of the translation of the whole solar 

 system in space, and which is the consequence 

 of the proper and actual motion of the stars 

 themselves. The difficult numerical separa- 

 tion of these two elements, of the proper from 

 the apparent motion, has been made possible 

 by the careful specification of the directions in 

 which the motions of the several stars take 

 place, and by the reflection that, were all the 

 other stars absolutely at rest, they would ap- 

 pear to recede perspectively from the point to- 

 wards which the sun was moving in his course. 

 The final result of the investigation, which the 

 calculus of probabilities confirms, is this : that 

 both the stars and our sun change their place in 

 the Universe. From the admirable researches 

 of Argelander("), who in Abo extended and ma- 

 terially improved upon the labours begun by the 

 elder Herschel and Prevost, it appears that the 

 sun is in motion towards the constellation of 

 Hercules, very probably towards a point in this 

 constellation, which lies in a combination of 

 537 stars (for the equinox of 1792-5) in 257° 

 49' Right Ascension ; -f 28° 49'-7 Declination. 

 In this class of investigations it is always matter 

 of great difficulty to separate the absolute from 

 the relative motion, and to determine what be- 

 longs to the solar system in particular and alone. 



If the non-perspective proper motions of the 

 stars be considered, many of them appear group- 

 wise opposed in their directions ; and the data 

 hitherto collected make it at least not necessary 

 to suppose that all the parts of our astral sys- 

 tem, or the whole of the star-islands which fill 

 the universe, are in motion about any great, 

 unknown, luminous, or non-luminous central 

 mass. The longing to reach the last or high- 

 est fundamental cause, indeed, renders the re- 

 flecting faculty of man as well as his fancy dis- 

 posed to adopt such a supposition. The Stagi- 

 rite himself has said — "All that is in motion 

 refers us to a Mover, and it were but an endless 

 adjournment of causes were there not a prima- 

 ry immoveable Mover"("). 



The manifold changes of place exhibited by 

 the fixed stars in groups, not parallactic mo- 

 tions, dependent on changes in the position of 

 the observer, but actual and ceaseless motions 

 in universal space, reveal to us in the most 

 incontrovertible manner, through a particular 

 class of phenomena, namely the motions of the 

 double stars, and the measure of their slower 

 or more rapid motions in different parts of their 

 elliptical orbits, the empire of the laws of grav- 

 itation beyond the limits of our solar system, 

 in the remotest regions of creation. The curi- 

 osity that is inherent in the nature of man 

 needs not any longer to seek satisfaction upon 

 this field of inquiry in gratuitous assumptions, 

 in the limitless ideal-world of analogies. By 

 the progress of the astronomy of observation 

 and calculation, it stands at length even here 

 upon stable ground. It is not so much the 

 numbers of the double and multiple stars that 

 have been discovered (2,800 to the year 1837 !) 

 circulating about a centre of gravity lying be- 

 yond the confines of either or any of them, that 

 excites our amazement ; it is the extension of 

 our knowledge of the fundamental force of the 

 whole material world, the indications of the 

 universal dominion of mass-attraction, that ar- 

 rest us, and that belong to the most brilliant 

 discoveries of our age. The time of revolution 

 of double stars of different colours presents the 

 greatest imaginable diversity ; it extends from 

 a period of 43 years, as in jy Coronae, to one of 

 several thousands, as in 66 Ceti, 38 Gemino- 

 rum, and 100 Piscis. Since Herschel's meas- 

 urements in 1782, the nearest leader in the tri- 

 ple system of C, Cancri, has now accomplished 

 more than a complete revolution. By a skilful 

 combination of observations of altered distan- 

 ces and angles of positionC*), the elements of 

 the orbits of more than one of the double stars 

 have b^en discovered — nay, conclusions as to 

 the absolute distance of double stars from the 

 earth, and comparisons of their masses with 

 the mass of the sun, have even been made. 

 But whether here, and in our solar system, the 

 quantity of matter is the sole measure of the 

 force of attraction, or whether specific attrac- 

 tions, not in proportion to the mass, are at the 

 same time efficient, as Bessel first showed, is 

 a question the solution of which it remains with 

 late posterity to accomplish("). 



If we compare our sun, with the other so- 

 called fixed stars in the Astral system to which 

 we belong, with other self-luminous suns, there- 

 fore, we discover, in connection with several 

 of them at least, ways opened up, which ena- 

 ble us to approximate, within certain extreme 

 limits, to a knowledge of their distance, of 

 their volume, of their mass, and of the rapidity 

 with which they change their places. If we 

 assume the distance of Uranus from the sun, 

 at 19 of the distances of the earth from the sun 

 then is the central body of our planetary sys- 

 tem 11,900 Uranus distances from the star a 

 Centauri, almost 31,300 of these distances 

 from 61 Cygni, and 41,600 of the same meas- 

 ures from a Lyrs. The comparison of the 

 volume of the "sun with the volume of fixed 

 stars of the first magnitude, depends on an ex- 

 tremely uncertain optical element ; viz., the 

 apparent diameter of the fixed stars. If, with 

 Herschel, we assume the apparent diameter of 



