THE SIZE AND AGE OF THE UNIVERSE ^ 



By SiE James Je:ans, F. R. S. 



[With 2 plates] 



It has often been said that the history of the race is that of the in- 

 dividual writ large, and this remark is specially applicable to the 

 question of the size of the universe. The new-born child is unable to 

 form an adequate conception of the size of the world, probably because 

 it takes its cradle or its nursery as its unit of measurement. It was the 

 same with the human race in its infancy. Taking for gi-anted that the 

 earth was the central and most important part of the universe, it some- 

 what naturally supposed that the earth was comparable in size with 

 the whole universe. 



EARLY DISCUSSIONS OF THE PROBLEM 



Peering into the dimly lit recesses of early science, we see the gradual 

 crumbling away of this belief. In the sixth century B. C, Pythagoras 

 taught that the earth was globular in shape ; and in the fourth century 

 B. C, Heraclides of Pontus explained that the apparent rotation of 

 the heavens arose from the rotation of this globular earth under the 

 stars. Such teachings as these inevitably led men to revise their esti- 

 mates both of the relative size and relative importance of the earth. 

 In the third century B. C, we find Aristarchus of Samos making the 

 first attempts to estimate the size of the universe by the really scien- 

 tific method of exact measurement. He saw that when the moon was 

 exactly half illuminated, the line from the sun to the moon must be 

 perpendicular to the line from the moon to the earth. Thus in the 

 triangle formed by the sun, the earth and the moon, one angle is a right 

 angle, while another, that at the earth, can readily be measured by 

 observation taken on earth. In this way Aristarchus hoped to obtain 

 the relative lengths of the sides of the triangle in question, and so also 

 the relative distances of the sun and moon. 



His theory was perfect, but his observations very faulty. Actually 

 the angle at the earth differs from a right angle by only about 9 



^ Discourse delivered at the Royal Institution on Nov. 29, 1935. Reprinted by periais- 

 sion from Nature, vol. 137, no. 3453, Jan. 4, 1936. 



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