336 THE WORLD MACHINE 



hardly more than an average of one a year since Bessel's 

 day. 



Still, it is a beginning enough to make certain that a true 

 parallactic shift of the stars may be found, has been found. 

 What Aristarchus, what Tycho, what Galileo, what Hooke, 

 Halley, Bradley, Herschel, and a long line of investigators could 

 not discover, Struve, Bessel, and Henderson had attained. 

 They swept away the last objection that might lie against the 

 Aristarchan-Coppernican scheme. For all thinking minds Brad- 

 ley's discovery of aberration, the queer little annual shift of the 

 stars in their places, had been definite proof. In no other way 

 than by supposing the annual motion of the earth could this 

 indubitable shift be explained. There could, indeed, be little 

 doubt in an intelligent mind after Cassini's demonstration of the 

 true distance and grandeur of the sun. It was wholly incon- 

 ceivable that a body a million times the bulk of the earth should 

 revolve about our midget planet. 



After 1840 or 1850, when the results of Bessel and the others 

 had been fully confirmed, doubt became mere obtusity. Human 

 certitude can never be absolute ; it can only approach infinite 

 probability. It is practically infinite probability that the truth 

 of the Coppernican theory will never be seriously questioned 

 again so long as the world shall last. 



The discovery of stellar parallax brought more than simple 

 proof of theory ; it gave us our first insight into the true grandeur 

 of the stars. 



So long as man had no means of knowing their distance, 

 it was open to suppose that the brighter stars are the nearer 

 stars, that they are all more or less of a size. It was equally 

 open to suppose that they were all at the same distance a 

 very great distance, no doubt and that they vary in size as 

 they vary in brightness to our eyes. Neither supposition in 

 any wise represented the fact. 



The nearest star, alpha Centauri, as convergent lines of study 

 make clear, is something very close to a duplicate of our own 

 sun. It is about the same in size, temperature, and brightness. 

 We may therefore suppose that its evolution has been very 

 nearly parallel to our own. It is not very fanciful to think 

 that the discovery of the nearness of our sun was made upon 

 one of the planets of alpha Centauri at about the same period 



