The Heavens 161 



The Sidereal System. The sidereal system consists of the 

 stars great blazing suns, all of which, except our own sun, are 

 much farther away from the earth than any planet. 



Stars and Constellations. By means of the telescope, an 

 instrument for seeing distant objects clearly, astronomers have 

 discovered that there are in the heavens millions of bodies called 

 stars. Without the aid of the telescope, the eye is able to see 

 only about 7,000 of these, even under the most perfect conditions. 

 But conditions seldom are perfect. Dust and vapor in the atmos- 

 phere, and the light of the moon, interfere so that usually only 

 about one-third of these 7,000 stars are visible. 



Yerkes Observatory. 



THE SEVEN STARS OF THE BIG DIPPER 



The two stars farthest from the handle point to the North Star. 



In past ages, the Egyptians, Greeks and Romans observed the 

 stars only with the naked eye. They thought they saw in groups 

 of stars outlines of mythical heroes, animals, and other objects. 

 Such groups of stars are known as constellations. There are 

 nearly fifty of them, among which are the Great Bear, the Little 

 Bear, Lyra, Cassiopeia, and the Pleiades or Seven Sisters. 



The Great Bear. One of the most interesting objects in the 

 northern sky is the Big Dipper. It is always visible in our latitude 

 when the sky is clear at night. It lies in the constellation of the 

 Great Bear (Ursa Major). This constellation contains 133 stars 

 that may be seen with the naked eye. The seven stars which 



