STRUCTURE OF THE GALAXY PLASKETT 



191 



traveling at the tremendous speed of 186,000 miles per second will 

 cover in a year, represents about 6 million million miles. To get 

 a concrete illustration, the nearest fixed star, a Centauri, in the 

 Southern Hemisphere and not visible in thege latitudes, is 414 light- 

 years distant, or 25 million million miles. It is very easy to say 25 

 million million or to see it as 25 followed by 12 ciphers, but not 

 quite so easy to realize the tremendous distance involved. Some 

 help may be gained by calculating how long it would take an ex- 

 press train traveling 60 miles an hour, day and night, to cover this 

 distance. Such a conveyance would take some 57,000,000 years to 

 comj)lete the journey. Perhaps a still better illustration is that of 

 the spider web, the very fine filament used in the reticles of transits 

 or measuring microscopes, of which it is estimated that 2 pounds 

 would stretch around the earth, 25,000 miles. At this rate it would 

 require no less than 1,000,000 tons of spider web to reach a Centauri 

 and hence 1,600 million tons to span Newcomb and Seeliger's esti- 

 mate of the diameter of the galaxy. 



At the risk of boring you, but in the hope of " getting across " 

 some conception of the immensity of stellar distances, and the sparse- 

 ness of the stars in space, I shall show some figures made by me 

 some 4 years ago to illustrate the scale of the universe. The first 

 is a table (table 1), starting with the smallest known particle, the 

 electron, and proceeding by 21 steps each 100 times the preceding, 

 to the largest known entity, the Einstein universe. It should be stated, 

 of course, that these dimensions were those extant when the drawings 

 were made, but some of them are quite different now, especially the 

 dimensions of the universe, and in the present rapidly changing state 

 of quantum and relativity theory may be again different next year. 



Table 1. — Scale of universe 



1. 6X10-" cm. 

 1. 6X10-11 

 1. 6X10-9 

 1.6X10-7 

 1. 6X10-5 

 1. 6X10-3 

 1.6X10-1 

 1.6X10 

 1. 6X103 

 1. 6X105 

 1. 6X10^ 

 1.6X10" 

 1. 6X1011 

 1. 6X1013 

 1. 6X1015 

 1. 6X1017 

 1.6X1019 

 1. 6X1021 

 1. 6X10" 

 1. 6X1025 

 1. 6X10" 

 1. 6X102« cm. 



Electron. 



Atom. 



Soap bubble. 



Tissue paper. 



Earth. 



Earth-moon. 

 Earth-sun. 

 Solar system. 



Nearer stars. 



Local cluster. 



Galaxy. 



Nearer spirals. 



6 X range 100" telescope. 



Einstein universe. 



