Our Earth and Its Fellow Planets [ 387 



another. Charts which indicate the positions of these heavenly 

 bodies will help you to recognize them and also indicate where to 

 look for them as their positions change. You can find sky charts 

 in many newspapers and in publications devoted to astronomy. 



When your child has looked carefully at timely charts, he will 

 be thrilled as he traces with you the paths of the planets. Over a 

 period of time he will have the exciting experience of seeing them 

 travel along with the stars, then seemingly move backward then 

 soon turn again and go forward in their original direction. 



WHY THE EARTH is "DIFFERENT" 



It is quite natural for a child to suppose that since our 

 earth is one of a group of planets, the others are worlds just like 

 our own. When he learns that this is not so, he will surely be 

 intensely curious about the difference between the earth and the 

 other planets. 



Is it because the other planets are too far away from the sun 

 or too near it that he could not live on them? Some planets are 

 larger than the earth (which has a diameter of nearly eight thou- 

 sand miles) and others are smaller. Is the size of a planet important 

 in regulating the possibility of life on it? 



Why Life is Possible on our Earth: Most scientists agree that 2 

 combination of favorable factors makes our form of life possible 

 on earth. 



Among these are the size and weight of the materials of which 

 the earth is made. They are just right to produce the gravity that 

 is needed not too great to crush us by our own weight, and yet 

 great enough for the earth to hold sufficient atmosphere to sustain 

 life. 



Another condition is that we are neither too close to, nor too 

 far away from, the sun; and the length of our seasons is apparently 

 stimulating to organic life. Not only are these particular condi- 

 tions missing on other planets, but they have other conditions 

 definitely unfavorable to our form of life. 



