358 GENERAL SCIENCE 



is composed has less density than the earth. How it is 

 that astronomers calculate the masses of the planets and 

 their satellites, their volumes and their relative densities, is 

 exceedingly interesting to those who have considerable 

 knowledge of mathematics. While the earth has but the 

 one satellite or moon, Jupiter has nine, and Saturn nine or 

 ten. Some of these have been discovered only within 

 recent years. 



SUMMARY 



In the changing positions of the moon relative to the direction of the 

 sun from the earth we can actually see the moon's revolution about 

 the earth. This change in distance of the moon from the sun must be 

 measured in degrees upon the curved surface of the sky. 



As the moon moves on in its orbit its illuminated side as lit up by 

 the sun is at times toward the earth. It then appears as a "full" 

 moon circular in outline. At other times the dark side is toward the 

 earth, while the crescent and gibbous phases are views of but portions 

 of the illuminated spherical surface. The change in extent of the sun- 

 lit side visible from the earth varies chiefly by reason of the moon's 

 changes of position in its orbit. 



Lack of water and of atmosphere, and a probable extreme range in 

 temperature from a time of two weeks of continuous sunshine to a two 

 weeks' absence of sunlight, makes it unlikely that any forms of life 

 such as are known here on earth can exist on the moon. 



The duration of the hours of moonlight, and the variation in its in- 

 tensity, depends as with sunlight upon how high up toward the zenith 

 of any observer the diurnal circles of the moon are. In this northern 

 hemisphere it is in the winter time that the full moons "run high" in 

 the heavens, and long bright moonlight nights occur. 



While the crescent moon as seen in the west at sunset is to the east 

 of the sun, and the crescent moon seen in the morning before sunrise 

 is to the west of the sun, it does not follow that the moon has passed 

 the sun and changed sides with regard to the sun. Such a change 

 does occur at the "dark of the moon," and just before "new moon." 

 In the former case what has occurred is that the waning crescent of 

 the "fourth quarter" is coming nearer and nearer to the sun, both its 



