OUR EARTH, A PLANET 



189 



tions over 16,000 feet high. Often smaller craters are 

 within the larger ones, and in many instances a single 

 mountain or groups of mountains rise from the floor 

 of a large crater. 



The moon is a dead, barren place. It has no atmos- 

 phere, no water, and no living things on it. Like the 

 planets it shines by reflected light. Since the moon ro- 

 tates once on its axis in the same period of time that 

 it revolves around the earth, it keeps the same side 

 or face towards the earth all the time. Have you tried 

 the experiment at the beginning of the topic which 

 shows why this is so? 



The moon completes one rotation on its axis in 

 about twenty-nine days. Therefore the sun shines con- 

 stantly on a region for two full weeks. During this 

 time the temperature is believed to reach or perhaps 

 exceed the boiling point of water. During the equally 

 long night of two weeks, the temperature falls below 

 256 F. 



The gravitational attraction at the surface of the 

 moon is only one-sixth that of the earth. A person 

 would weigh about one-sixth as much at the surface 

 of the moon as on the surface of the earth. A boy who 

 can throw a baseball one hundred feet high on the 

 earth could throw the same ball at least six hundred 

 feet high on the moon. A juniper on the moon could 

 jump six times as high and six times as far as he could 

 on the earth, assuming that he would be able to exert 

 the same physical activity on the moon as he can on 

 the earth. 



Sometimes we see the moon as a thin crescent and 

 sometimes it is big and round. Why'is this ? The moon 

 is not a self-luminous body but shines like the planets 

 by reflected sunlight. Also the moon revolves around 

 the earth. It is on account of these two facts that the 

 moon changes its appearance. 



Study Figure 310 carefully as you read the following 

 paragraphs and learn how the phases of the moon are 

 produced. Think of the sun as far off to the left. The 

 arrows show the rays coming from the sun. When the 

 moon is at E, the point in its revolution where it is 

 between the sun and the earth, the illuminated side of 

 the moon is turned from the earth and we cannot see 

 it. This is called new moon, which really means no 

 moon. 



About three days later the moon has moved in its 

 orbit to position F. Now we can see a very small part 

 of the illuminated side, and the moon appears as a 

 crescent. 



In about four more days the moon is at G. Then one 

 half of the illuminated side is visible, or the moon is 

 at first quarter. 



About fourteen days after the moon is new, it is at 

 A. In this position the earth is between the sun and the 

 moon, so that the entire illuminated side of the moon 



is visible to us. This is full moon. 



About twenty-one days after new moon, the moon 

 is at C, when again only one half of the illuminated 

 side is visible to us. This phase is called last quarter. 



About seven days after last quarter, the moon is 

 back at E and we have new moon again. The moon 

 completes a revolution about the earth in twenty-nine 

 and a half days. 



What are the effects of the moon upon the earth? 

 The moon reflects a large amount of sunlight upon 

 the earth, especially during full moon. However, in 

 comparison to the amount of sunlight received direct- 



Spring tides 



First day 

 New moon 



Sun -^= 



Neap tides 



Seventh day 

 First quarter 



Sun ->S 



Spring tides 



Fourteenth day 

 Full moon 



Sun --e= 



Neap tides 



Twenty-first day 

 Third quarter 



Sun -'.-= 



FIG. 311. CAUSES OF TIDES 



ly by the earth from the sun, it is of little importance. 

 The belief still persists, however, among many 

 farmers that the moon has a direct effect upon crops. 

 There is no foundation for this belief. It has been 

 estimated that the earth receives more light from 

 the sun in thirteen seconds than it does from the moon 

 in one year. Accurate observations and records of 

 the weather for long periods of time show no relation- 

 ship whatsoever between the phases of the moon and 

 climatic conditions. 



Tides caused by the moon are perhaps its most 



