Some Things to Observe and Explain 213 



When a boy turns completely around, so as to face 

 in all directions, he is rotating. When he walks around 

 a tree, keeping at nearly the same distance from it, he 

 may be said to revolve around it. If he were to rotate 

 and revolve at the same time, his motions would be like 

 the motions of the earth. If the boy who is both rotat- 

 ing and revolving will notice, at the time he is facing 

 the tree, some distant object in line with it, and then 

 when next facing the tree will look at the same distant 

 object again, he will see that it is no longer in line. 

 This will enable him to understand why the stars change 

 their position with reference to the sun all through the 

 year. Each night they rise about 4 minutes earlier 

 than on the preceding night. 



FIG. 132. On the moon a boy could lift 6 times as much as he can lift on 

 the earth. Can you explain why this is the case ? 



