PROPER MOTION OF THE STARS. 183 



cunsequeatly differ, at the least, in the ratio oi' 1 : 154), the 

 relative distanc3S also of the fixed stars from each other, and 

 the configuration of the constellations themselves, can not in 

 long periods remain the same. The Southern Cross will not 

 always shine in the heavens exactly in its present form, for 

 the four stars of which it consists move with unequal veloc- 

 ity in difterent paths. How many thousand years will elapse 

 before its total dissolution can not be calculated. In the re- 

 lations of space and the duration of time, no absolute idea 

 can be attached to the terms great and small. 



In order to comprehend under one general point of view 

 the changes that take place in the heavens, and all the mod- 

 ifications which in the course of centuries occur in the phys- 

 iog7iomic character of the vault of heaven, or in the aspect 

 of the firmament from any particular spot, we must reckon 

 as the active causes of this change: (1), the precession of 

 the equinoxes and the mutation of the earth's axis, by the 

 combined operation of which new stars appear above the 

 horizon, and others become invisible ; (2), the periodical and 

 non-periodical variations in the brightness of many of the 

 fixed stars ; (3), the sudden appearance of new stars, of 

 which a few have continued to shine in the heavens ; (4), 

 the revolution of ^telescopic double stars round a common 

 center of gravity. Among these so-called fixed stars, which 

 change slowly and unequally both in the intensity of their 

 light -and in their position, twenty principal planets move in 

 a more rapid course, five of them being accompanied by 

 twenty satellites. Besides the innumerable, but undoubt- 

 edly rotatory fixed stars, forty moving planetary bodies have 

 up to this time (October, 1650) been discovered. In the 

 time of Copernicus and of Tycho Brahe, the great improver 

 of the .sicience of observation, only seven were known. Near- 

 ly two hundred comets, five of" wliich have short periods of 

 revolution and are iuterior, or, in other words, are inclosed 

 within those of the principal planets, still remain to be men- 

 tioned in our list of planetary bodies. Next to the actual 

 planets and the new cosmical bodies which shine forth sud- 

 denly as stars of the first magnitude, the comets, when, dur- 

 ing their usually br.ef appearance they are visible to the na 

 kcd eye, contribute the most vivid animation to the r\Q\\2^ic- 

 ture — I had almost said the impressive landscape — of the 

 glarry heavens. 



The knowledge of the proper motion of the fixed stars is 

 closely connected historically with the progress of the sci' 



