168 ANNUAL REPOT?T SMITHSONIAN' INSTITUTION, 19 28 



selected for us what is, perhaps, in some Avaj^s the most sensational 

 moment of all in the life of our race. 



The child sets its newly awakened mind to work to adjust and 

 coordinate a new array of facts. If the world was not made to 

 surround its cradle, what purpose can it serve? If the lights of the 

 great ships in the harbor were not designed to light its nursery at 

 night, what can the}' possibly be for? And, most interesting prob- 

 lem of all, if the world is such a big affair, can there be other cradles 

 and other babies? 



These remarks Avill have served their purpose if they suggest that 

 what I am rashly trying to set forth here should not be judged as a 

 finished science or the solution of a problem; it is rather the first 

 confused gropings of the infant mind trying to understand the world 

 outside its cradle. And if the impression produced by its first inex- : 

 perienced glance at the outer world had to be described in a single 

 word, it would probably select the word " immensity." 



THE IMMENSITY OF SPACE 



The immensit}^ of space is measured by the figures already men- 

 tioned. Light and wireless signals travel at the same rate because, 

 of course, they are essentially the same thing; and this thing takes 

 a seventh of a second to travel round the world, and probably some- 

 thing like 100,000 million years to travel round the universe. The 

 ratio of these times (2X10^^) measures the dimensions of the uni- 

 verse in terms of the familiar dimensions of the world; incidentally, 

 it also measures the expansion of our spatial ideas since Copernicus. 

 The disparity of size is too great to be easily visualized. Suppose 

 the size of our earth were represented by a single atom. Then the 

 range of vision of the biggest telescope is about represented by the 

 whole earth, and the size of the whole universe, according to the 

 theory of relativity, is represented by a stack of 1,000 million earths. 



Scarcely less bewildering than the immense extent of space is the 

 immense amount and varietj^ of matter it contains. The sun, which is 

 a million times as big as the earth and three hundred thousand times 

 as massive, proves to be something less than a grain of sand on the 

 seashore. It forms one of a family whose number must certainly be 

 counted in thousands of millions; Seares has estimated it at 30,000 

 millions. This is not the only family of stars in space. Each of the 

 great spiral and other extragalactic nebulae, such as are shown in 

 Plates 1, 2, and 3, is either a family of stars, or consists of stars in the 

 making, or of matter which is destined ultimately to form stars. 

 We can estimate the masses of these great nebulae by gravitational 

 means, and each is found to contain enough matter to make 1,000 

 million suns. This of itself will give some conception of the vast 



