408 ] Recognizing the Stars 



of the moon can be seen anywhere on the side of the earth where 

 it is night. This is explained by the fact that the earth's shadow 

 is large enough to cover the whole moon (during a lunar eclipse) , 

 whereas the moon's shadow is small and covers only a small part 

 of the earth. 



How the Sun is "Blotted Out": "How can the moon ever 'blot out* 

 the entire sun, if the sun is so much bigger?" a child may wonder. 

 If he holds a dime at arm's length he will find that the small 

 coin seems to more than cover the moon. This same principle 

 operates when we see the nearby moon between us and the distant 

 sun. While the sun's diameter is four hundred times larger than 

 that of the moon, the sun is also four hundred times farther away, 

 and the two disks seem of equal size. 



ATOMIC ENERGY AND THE SUN 



Children take a lively interest in the information that the 

 sun's energy is atomic, and that this giant heavenly body was 

 producing atomic energy before the earth began. Long ago people 

 believed that the sun was merely a great burning mass; but if this 

 had been true, it would have burned itself out in less than two 

 thousand years. When scientists realized that this idea must be 

 wrong, they were puzzled about the secret of the sun's energy 

 until they discovered, quite recently, that an atom could be split, 

 releasing an enormous amount of energy. This solved the puzzle 

 of the sun's energy! 



The term "atom" (taken from a Greek word meaning "indiv- 

 isible") had been chosen for what was believed to be the tiniest 

 possible unit of matter. We now know that these units are made 

 of still smaller particles that are in motion and are constantly 

 changing into new forms. The change may be sudden and violent, 

 as in an atomic bomb, or slow and gradual. 



In the case of the sun, hydrogen atoms are changing into helium 

 atoms. To be specific, four hydrogen atoms are changed into one 

 helium atom, and one per cent of their weight is converted into 

 atomic energy. This means that the sun, with its countless active 

 atoms, is constantly growing smaller, or losing weight; but it is so 



