OUR EARTH, A PLANET 



187 



Secure a rubber ball about an inch and a half or two 

 inches in diameter, and through it stick a piece of wire 

 about six inches long. This ball represents the moon. Now 

 move the ball representing the moon around the earth about 

 six inches from it, from right to left as you face away from 



FIG. 307 



the sun. When the moon gets between the earth and sun, 

 observe what happens on the earth. 



Carefully observe the shadow as it begins and then as 

 it develops and finally passes off the earth. Repeat this and 

 make drawings to represent the appearance of the shadow 

 as it covers the earth. Make at least two drawings. If you 

 are using a globe for the earth, interchange it with the 

 ball representing the moon, and by moving the latter, show 

 how a lunar eclipse may be formed. 



Make two drawings, one showing how lunar and the other 

 how solar eclipses are formed. 



Solar eclipses can occur only at new moon. Give an ex- 

 planation of this. 



Lunar eclipses can occur only at full moon. Give an ex- 

 planation of this. 



Could a solar eclipse be observed at night or a lunar 

 eclipse in the day time? Explain your answer. 



FIG. 308 



Experiment 123. Why do days and nights change in 

 length from season to season? 



In a darkened room set up the equipment as illustrated in 

 Figure 308. The earth is represented by a two-inch rubber 

 ball or a tennis ball which has had a wire or knitting needle 



about six inches long pushed through it. The sun is placed at 

 the center of a circle with- a diameter of about fifteen inches 

 which has been drawn upon a piece of cardboard or paper. 

 This circle represents the earth's orbit or pathway about 

 the sun. At the quarter points label the orbit "North," "East," 

 "South," "West" as shown in the diagram. Draw a line 

 around the earth-ball halfway between the north and south 

 poles. This represents the equator. Now between the equa- 

 tor and north pole draw another line around the ball repre- 

 senting your latitude. On this line place a dot which might 

 represent your locality. 



Place the earth at the point of the orbit marked "South" 

 and have the axis point as nearly as possible to the direction 

 in which the North Star would be. How much of the surface 

 of the earth is lighted by the sun? 



Turn the earth on its axis in this position. Is the dot repre- 

 senting your locality in the light or shadow longer ? At this 

 season would the day or night be longer? What season are 

 you having when this condition occurs? 



Now move the earth through its orbit in the direction of 

 the arrow to a position east of the sun. Be sure that the axis 

 still points the direction of the North Star. How much of 

 the earth's surface is now lighted by the sun? 



Again turn the earth on its axis. Is the dot representing 

 your locality in the shadow or light longer? At this season 

 how would the length of day and night compare? What 

 season would this represent ? 



Again move the earth in its orbit until it is north of the 

 sun, and repeat the rotation on its axis at this point. 



How much of the earth's surface is illuminated ? Is day or 

 night longer in the locality where the dot is ? What season 

 are you having when the earth is north of the sun? 



Move the earth to a position in its orbit which is west of 

 the sun. Repeat the rotation of the earth on its axis. 



How much of the surface of the earth is here lighted by 

 the sun? How do the days and nights compare in length at 

 this season of the year? What season are you having? 



With the equipment study the reasons for the six-month 

 night and six-month day at the poles and determine when 

 each occurs. Study the length of day and night at the differ- 

 ent seasons of a city located on the equator. Repeat this for 

 a city located in the southern hemisphere. 



Write a short report, explaining clearly why days and 

 nights are of unequal length in the northern hemisphere at 

 different seasons of the year and also explaining the cause 

 of the long polar days and nights. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



What kind of heavenly body is our earth? Our 

 earth is a great, nearly round ball about 8,000 miles 

 in diameter. It is not a perfect sphere, because its 

 diameter at the equator is about twenty-seven miles 

 longer than its polar diameter. 



Only a few centuries ago most people imagined 

 the earth to be flat, and there still are a few persons 

 who argue that it is so. When Columbus made his 

 voyage to America, many persons believed that he 

 would fall off the side of the earth. There are, how- 

 ever, good reasons which cause us to believe it is 

 spherical in shape. (1) Ships sail completely around 

 the earth. In fact, such cruises are regularly advertised 

 today. Would this be possible if the earth were flat? 



