The Heavens 155 



see the whole of the lighted part and we have a full moon, which 

 shines brilliantly all night. The moon's revolution up to this 

 point is often referred to as the waxing of the moon. The period 

 from half moon to full moon is known as the second quarter. 



The moon, continuing in her course around the earth, now 

 shows phases in a reverse order from those in the second quarter. 

 The part of the lighted side visible to us grows less night by 

 night until after about seven days she again resembles a half 

 circle, and half of the lighted hemisphere is again visible to us. 

 This period from full moon to half moon is known as the third 

 quarter. 



The visible portion continues to decrease until only a cres- 

 cent is seen with the cusps pointed toward the west. Finally the 

 lighted side is wholly invisible to us, and a complete revolution has 

 been made. This period from half moon to new moon is known 

 as the fourth quarter. The last half of the revolution is often 

 referred to as the waning of the moon. The period of time in 

 passing through one revolution from new moon to new moon is 

 about 29^2 days. This period constitutes a lunar, or moon, 

 month. 



The Tides. Among the interesting sights at the seashore 

 are the rise and fall of the ocean level at regular intervals. These 

 regular movements of the ocean are called tides. A rising tide is 

 called a flood tide and a falling tide is called an ebb tide. 



Tides are caused by the attraction, or pulling force, exerted 

 by the sun and by the moon. Since this attraction lessens rapidly 

 as distance increases, the nearby moon exerts far more pull than 

 the greater, but far distant, sun. If the moon stood still, we 

 would have a high tide and a low tide every 12 hours, but, owing 

 to the changing position of the moon with relation to the earth, 

 the tide rises for 6 hours, 13 minutes and then falls for the same 

 period, causing a high tide every 12 hours, 26 minutes. 



When moon and sun are in a position to exert their 

 attracting forces at the same point or at opposite points on the 

 earth, we have extreme high tides, or spring tides. When the 

 attractive forces of sun and moon act at right angles, we have 

 low, or neap, tides. 



