THE EVER-WIDENING WORLD OF STARS. 41 



While the Ptolemaic system of astronomy was ac- 

 cepted there were no means of forming any trustworthy 

 views respecting the extent of the stellar universe. If 

 the earth were ever at rest we could never know how 

 far the stars are from us ; and therefore the old astrono- 

 mers were free to invent whatever theories they pleased 

 as to the scale on which the sidereal scheme is con- 

 structed. It was only when the earth was set free by 

 Copernicus from the imaginary chains which had been 

 conceived as holding it in the centre of the universe 

 that it became possible to form any conception of the 

 distances at which the stars lie from us. Indeed Tycho 

 Brahe immediately pointed this out as an overwhelm- 

 ing objection against the new theory. ' Are we to 

 suppose,' he argued, ' that the stars are placed at such 

 enormous distances from us as to seem wholly un- 

 changed in position while the earth sweeps round the 

 sun in an orbit millions of miles in diameter ? If this 

 amazing theory were true, the stars would be hundreds 

 of millions of miles from us, a view which is utterly 

 monstrous and incredible.' 



But strange as this new view appeared, it gradually 

 gained ground. It became presently so well established 

 that when Cassini discovered that the earth travels in 

 a much wider orbit than Tycho Brahe had supposed 

 so that the stars were at once thrown many hundreds 

 of millions of miles farther from us astronomers still 

 held to the new order of things. 'With Briarean 

 arms,' as Humboldt has described their labours, the 

 fellow-workers of Cassini thrust farther and farther 



