190 



EARTH AND HEAVENLY BODIES 



noticeable effects. People living near the ocean ob- 

 serve the waters of the open sea move in and rise on 

 the shore, and then recede again. Twice every twenty- 

 four hours and fifty-one minutes these phenomena 

 take place. Tides are caused by the attraction of the 

 moon and to a lesser extent by the attraction of the 

 sun. The waters on the side of the earth nearest the 

 moon are pulled outward, causing high tide. The 

 waters on the directly opposite side of the earth, be- 



FIG. 312. ECLIPSES OF MOON AND SUN 



cause of their greater distance from the moon, are 

 attracted much less and lag behind, causing a high 

 tide at that point. Low tides occur at places just half- 

 way between these two points. The highest tides, 

 called spring tides, occur when the sun, moon, and 

 earth are in a straight line. The lowest tides, called 

 neap tides, occur when the sun is at right angles to a 

 line connecting the earth and the moon. See Figure 

 311. How often every month do neap tides and spring 

 tides occur? The word spring in connection with tides 

 has nothing to do with the season of the year called 

 spring. 



Tides have considerable practical value. In some 

 places the energy of tides is used to run water wheels 

 and machinery. High tides deepen certain shallow 

 harbors, making it possible for large ships to enter. 

 By keeping the water in motion tides help to keep 

 some bays and harbors from becoming stagnant and 



impure. They wash the ocean shore twice a day ana 

 remove wastes that would otherwise accumulate. Cer- 

 tain fishing industries such as oyster and clam fishing 

 secure beneficial effects from tides. 



What are the causes of eclipses? Have you ever 

 observed an eclipse of the moon or an eclipse of the 

 sun ? The a/icients were much afraid during an eclipse, 

 which they thought was caused by an angry god try- 

 ing to blot out their source of light. Today most peo- 

 ple have no fear or dread of these phenomena because 

 we know their real causes, and we are able to cal- 

 culate their appearances hundreds of years in advance. 

 An eclipse of the moon occurs whenever the earth 

 gets directly between the sun and the moon (Fig. 

 312). At this time the earth shuts off the sun's rays 

 from the surface of the moon, which appears dark. 



The eclipse of the sun is. caused when the moon 

 comes between the sun and the earth (Fig. 312). Most 

 people have never seen an eclipse of the sun. Because 

 the sun is a great deal larger than the moon and also 

 because the moon is considerably smaller than the 

 earth, the shadow cast by the moon covers only a 

 small strip of the earth. If we happen to be inside this 

 strip, none of the sun is in view, and we call the 

 phenomenon a total eclipse of the sun. When a part 

 of the sun remains visible it is called a partial eclipse. 



What causes day and night? We commonly say 

 that the sun rises in the east and sets in the west. 

 But is the sun moving or are we moving? At night 

 the stars also seem to move from east to west. Do the 

 sun, the moon, and the stars really revolve about the 

 earth once a day? They appear to, and the people of 

 ancient times thought that they actually did. They 

 were deceived. Have you ever sat in a passenger train 

 and looked at another train nearby when both were 

 standing still ? When your train started to move it 

 appeared as if the other train were moving back- 

 ward, but in reality your train was moving forward, 

 and the other train was standing still. It is just the 

 same with the earth and the stars. It looks as if the 

 sky, the sun, and the stars were moving from east to 

 west, but actually the earth is turning from west to 

 east. Carefully performed experiments have shown 

 that the earth rotates on its axis once every twenty- 

 four hours. 



Since the earth rotates constantly on its axis, only 

 one half the earth can receive direct rays from the sun 

 at a time. Therefore night and day follow each other. 

 At the equator of the earth the day is twelve hours 

 long and the night twelve hours long. At the north 

 pole and the south pole the day is six months long and 

 the night six months long. Those of us who live in 

 the north temperate zone know that the length of our 

 days and nights varies. We shall see why in the next 

 few paragraphs. 



