186 



EARTH AND HEAVENLY- BODIES 



TOPIC 3. OUR EARTH, A PLANET 



SUGGESTED PROBLEMS AND QUESTIONS 



1. What kind of heavenly body is our earth? 



2. Why does the moon appear to change its 

 shape ? 



3. What are the effects of the moon upon the 

 earth? 



4. What are the causes of eclipses? 



5. What causes day and night? 



6. What causes the seasons of the year? 



7. How is time kept? 



SUGGESTIONS AND HELPS FOR STUDY 



1. In studying the material of this topic you will 

 find the diagrams and pictures helpful. Study them 

 closely. 



2. It is difficult at first to understand why the moon 

 always turns the same face toward the earth. This is 

 because the moon turns once on its axis while it is 

 making a revolution about the earth. See if you can 

 demonstrate this point by an experiment. 



3. In studying this topic try to form a mental pic- 

 ture of the motions of the earth and moon in space. 

 It will help to clarify some of the more difficult points. 



4. You may find the following new words and 

 phrases in this study : 



axis an imaginary line which passes through a body 

 upon which the body is supposed to turn or rotate. 



equinox the time when the sun crosses the equator, oc- 

 curring about March 21 and September 22 each year. 



neap tides tides occurring at the first and third quarters 

 of the moon. 



spring tides tides occurring at new and full moon. 



Bo/S head represents earth Lamp represents 

 Full moon _^\F' rs *" Quarter-_^=^_ 



Third QuarYer 



Tennis ball 

 represents moon 

 new moon 



FIG. 305 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS 1 



Experiment 121. Does the moon actually change its 

 shape? 



Secure an electric-light globe and a tennis ball or base- 

 ball as pictured in Figure 305. The electric light is to il- 

 lustrate the sun, you are the earth, and the ball represents 



1 See workbook, p. 68. 



the moon. A candle may be used in place of the electric 

 light for the sun. With the light well above your head and 

 to your back, hold the ball at arm's length, sufficiently high 

 to receive light from the lamp. Observe the amount of the 

 ball that is lighted. Slowly turn from right to left, holding 

 the ball exactly in front of you until you have made one 

 complete turn about. Observe the amount of illumination 

 of the moon and how it changes as you turn. Repeat this 

 several times until you see clearly why the moon appears 

 to change shape as it turns about the earth. 



Again repeat the turning, but this time sketch in your 

 notebook the appearance of the part lighted at each quarter 

 turn. 



At the start of this experiment the moon is represented 



at quarter. When a quarter turn has been made from 



the starting position it has reached quarter. Another 



quarter turn brings it to Full moon is quarter 



and new moon is 



Full moon occurs when 



quarter. New moon occurs when 



o 



FIG. 306 



In figure 306 the inner circle shows the amount of the 

 moon lighted at different times in its trip around the earth. 

 Copy the figure in your notebook and shade in the dark 

 portion of the moon in each of the outer circles so that it 

 shows the shape of the moon, as seen from the earth, to 

 correspond with the amount of illumination in each circle 

 shown in the inside ring. 



Experiment 122. What causes eclipses? 



In a partly darkened room set an electric lamp on a desk 

 as shown in Figure 307. Again this represents the sun. A 

 candle may be used in place of the lamp. 



On a piece of light cardboard draw a circle about six 

 inches in diameter and place this three or four feet away 

 from the sun. This represents the earth; if you wish you 

 may draw in the outlines of the continents to make it more 

 real. A geographical globe may be used instead of the card- 

 board earth. Set the cardboard upright as shown in the 

 illustration. 



