192 PROGRESS OF SCIENCE IN THE CENTURY. 



six months later when the earth has travelled over 

 one-half of its orbit, another direction-line may be 

 observed; suppose the two direction-lines produced 

 till they meet, the point of intersection must be the 

 position of the star. Then we have a triangle whose 

 base is the diameter of the earth's orbit, and a geo- 

 metrical calculation enables us to determine the pro- 

 portion that the sides bear to this. 



The method of determining parallax is theoreti- 

 cally so simple that it could not but be known to 

 Copernicus and his followers. Indeed for three hun- 

 dred years before Bessel's success there were pains- 

 taking attempts to apply it, attempts which invaria- 

 bly ended in the disappointing result that the two 

 direction-lines from opposite ends of the earth's orbit 

 always seemed to be parallel. We know this to mean 

 that the star observed was too distant, or that the 

 instruments used were not precise enough, to show 

 appreciable parallax. 



As we have noted, Bessel succeeded and the im- 

 portance of the step thus taken is not affected by the 

 fact that his estimate of the distance of 61 Cygni 

 as 600,000 times that of the Sun is now reduced to 

 440,000. 



A few months after Bessel announced his dis- 

 covery, Henderson of Edinburgh published his esti- 

 mate of the distance of Centauri, which is, so far 

 as we know, the star nearest the solar system. Hen- 

 derson calculated its distance at 180,000 times that 

 of the Sun, this has now been extended to 270,000 

 times. 



Writing in 1885, Miss Clerke says: "The same 

 work has since been steadily pursued, with the gen- 

 eral result of showing that as regards their over- 

 whelming majority, the stars are far too remote to 



