THE UNIVERSE 7 



9. Constellations. From the earliest times the stars have 

 been favorite objects of speculation and study, and much was 

 known about them even before the invention of the telescope 

 and spectroscope made more intimate study possible. Long 

 before the dawn of history, man had looked up at the evening 

 sky and fancying the stars grouped in forms resembling earthly 

 objects, had given the names of these objects to the groups. 

 Such groups are known as Constellations. Among the more 

 striking of these are the Great Bear, the Scorpion, the Northern 

 Crown, the Southern Cross, the Pleiades, Sagittarius, Orion, 

 Booetes, and Pegasus. Others may be easily distinguished 

 on a clear night. The seven principal stars in the constella- 

 tion of the Great Bear are known to nearly every one as the 

 "Big Dipper." The constellations are not really related 

 groups of stars, although they appear so when viewed from the 

 earth. All are moving in various directions through the 

 heavens, and in time, although it may be millions of years 

 hence, they will have different positions from those they now 

 occupy. 



10. Motion of the Constellations. Observers of the heavens 

 soon discover that the constellations, though retaining their 

 places with respect to one another, do not have a definite posi- 

 tion in the sky but steadily drift westward. This apparent 

 motion is due to the revolution of the earth about the sun 

 which thus causes us to view them from a different position in 

 space each time we see them. They are, however, to be found 

 in the same positions on the same dates each year because at 

 the end of a year we have returned to the exact spot from 

 which we viewed them a year earlier. The drifting westward 

 or, rather, our change in position with regard to the constella- 

 tions, causes one after another to disappear in the west while 

 new ones appear in the east. For a part of each year, there- 

 fore, some of the constellations are invisible. Among those 

 most conspicuous in our summer skies are Booetes, the North- 



