174 



THE NATIONAL GEOGRAPHIC MAGAZINE 



Photograph trom Yerkes Observatory 



A YKRK1JS PHOTOGRAPH OF SOME OP THE NEBUI,^ IN THF, PI.FJADES 



Imagine a drop of water expanded into a sort of supersteam so attenuated that it would 

 fill a globe sixty-two miles in diameter. It is believed that some of the nebulae may be com- 

 posed of gases as rare as that. 



Referring to the picture of the heavens 

 on page 171, and to the map accompany- 

 ing it, let us survey the sky as it will 

 appear at the hours and on the dates 

 given therewith. 



Of course, the Great Dipper will first 

 claim our attention, as it is the princi- 

 pal "landmark" of the heavens. It will 

 be seen westward from the Pole Star, 

 with its Pointers "guiding the eye to 

 Polaris and its handle sweeping in a 

 broad curve toward Arcturus and Spica. 



TTIP, GREAT DIPPER 



The star at the bend of the handle of 

 the Great Dipper is known as Mizar. 

 Insignificant though it looks in its small- 

 ness, it radiates more than a hundred 

 times as much light as the sun, and is 

 nearly five million times as far away. 



Its light has to travel three-quarters of 

 a century to reach the earth. It is a 

 great triple luminary. The combined 

 mass of two of its members is many 

 times as great as that of our sun ; they 

 swing around their common center of 

 gravity every twenty days. 



Following the line of the Pointers 

 eastward, one's eye picks up Polaris, the 

 only bright star in its neighborhood. 



Shining down upon us from a point 

 almost midway between the zenith and 

 the northern horizon ill the latitude of 

 Washington, this humble star of the 

 second magnitude tells little of its glory. 

 Yet it is so distant that the light-waves 

 entering the eye as one looks at it today 

 left it forty-five years ago and have been 

 traveling at the rate of more than eleven 

 million miles a minute to reach us. 



