8 THE OPEN SKY 



turus which reaches the eye to-night left that star more 

 than thirty-five years before the battle of Lexington and has 

 been traveling toward us ever since at the rate of about 

 16 billion miles a day. Other stars are so much farther 

 away that it is impossible to measure their distances. No 

 wonder the lights of the stars are so. dim to us that they fade 

 away at the brilliant rising of the morning sun. 



Experiment 1. Early on a clear evening when the stars are 

 shining brightly locate the Big Dipper. (See page 10.) Carefully 

 determine its position by standing in a definite place and sighting 

 along the side of a high building or lofty tree. Make a sketch of 

 the position of the Dipper and some of the stars near it. Several 

 hours later in the evening stand in the same place and determine 

 in a similar way the position. Make a sketch. Has the position 

 of the Dipper changed in relation to your line of sight? What 

 caused the change? Has its position changed in relation to the 

 other stars? Locate some other constellations and make similar 

 determinations. 



All the stars appear to be fixed in their relative places. 

 In the northern hemisphere the stars at the north appear 

 to go around in a circle. The other stars appear to rise in 

 the east and to set in the west just as the sun does. If 

 we observe the stars that rise to the northeast, east, and 

 southeast we shall find that they are above the horizon for 

 different lengths of time. 



The ancients noticed these facts and explained them by 

 saying that the earth was at the center of a hollow sphere, 

 upon the inner surface of which were the stars, and that 

 this sphere was continually revolving about the earth, 

 and also slightly changing its position with respect to the 

 earth. We of the present day know that it is the earth that 

 is turning on an imaginary axis and also gradually changing 



