SIZE AND AGE OF THE UNIVERSE— JEANS 125 



trade-winds and seen the clouds moving in endless procession from 

 east to west as a result of the very rotation he was trying to discredit. 



It was not until 1543 that these arguments were refuted by Coperni- 

 cus. Ptolemy's argument had been that the earth cannot be rotating, 

 because if it were it would fly to pieces ; thus the nightly motion of the 

 stars must result from the rotation of the heavens themselves. Yet, if 

 the whole heavens rotated once every 24 hours, they must have an even 

 higher tangential velocity than he, Copernicus, wished to attribute to 

 the earth. Why then, asks Copernicus, do not the heavens themselves 

 fly to pieces? It was a shrewd thrust, but Copernicus was betrayed 

 into pursuing his stricken enemy too far. For he went on to inquire 

 whether the heavens really could be expanding under the centrifugal 

 force of their rotation ; and his argument has a strange ring of 1935 

 about it. He scornfully asks what the heavens could possibly be ex- 

 panding into, for as thej^ are the whole universe, there can be no 

 space beyond them into which they could expand. 



The theories of Copernicus fared better than those of Aristarchus, 

 the two principal reasons for their greater success being that printing 

 and the telescope had been invented in the meantime. Two-thirds 

 of a century after Copernicus published his book, the telescope of 

 Galileo had virtually established the truth of his doctrines, and the 

 sun replaced the earth as the fundamental unit of the universe. Ten 

 years before Galileo had looked through his first telescope, Giordano 

 Bruno was maintaining that the stars were similar objects to the 

 earth, moon, and planets — as Pythagoras had conjectured 2,000 years 

 before. Ten jesivs after, Kepler was saying that they must be similar 

 objects to the sun ; and this led to the first real comprehension of the 

 immensity of space. For, if the stars were intrinsically as bright as 

 the sun, they must be at stupendous distances to look so much fainter 

 than the sun. We receive approximately 100,000 million times as 

 much light from the sun as we do from a first-magnitude star such as 

 Altair, Betelgeux, or Aldebaran. Thus, if these stars are comparable 

 with the sun in luminosity, they must needs be about 320,000 times 

 as distant — no smaller distance would be compatible with their f aint- 

 ness. In modern terminology, these first-magnitude stars would be 

 at distances of approximately li/^ parsecs or 5 light-years. 



We know now that this method of . calculation cannot lead to 

 very accurate results, at any rate in its present crude form, because 

 the supposition that the stars are all of the same candlepower as the 

 sun is very far from the truth— some have more than 10,000 times the 

 candlepower of the sun, while others have less than a ten-thousandth 

 part. But the method admits of almost endless refinement, and in 

 its modern form provides the most useful, and indeed almost the only, 

 method for estimating the distances of very remote objects. 



