192 



ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 3 



The dimensions are given in centimeters, inches, miles, and light- 

 years, and for comparison corresponding familiar objects are given 

 to the right. The astronomical end of the scale is illustrated by fur- 

 ther figures, each having its side 100 times the length of the preced- 

 ing and each showing in the right-hand lower corner the size of the 

 preceding figure. Starting with a sketch of the "Western Hemisphere 

 just nicely fitting in a square with a side 10,000 miles, it and the next 

 two steps, the earth-moon system 1,000,000 miles and the sun-earth 

 100,000,000 miles are sufficiently familiar not to need illustrating. 

 The next step (fig. 1), again 100 times the length of the preceding 



Figure 1. — Solar system. 



or 10,000,000,000 miles, easily contains the planetary system but only 

 part of the orbits of some of the long-period comets. The next (fig. 

 2), of 1,000,000,000,000 miles, or one-sixth of a light-year as you will 

 remember, shows the whole planetary system as a dot in empty space. 

 How empty space really is will be better appreciated when it is 



learned that the sun in this figure would be an invisible dot .„„ „^^ 



^ 400,000 



inch in diameter and that you would have to travel 25 times as far 



in any direction before you would encounter a single star. Yet so 



tremendous is its extent that it is estimated there are some 200,000,- 



000,000 stars in the galaxy. 



