408 GENERAL SCIENCE 



ejected a large amount of lava and mud which covered a 

 large area; these are evidences of mountain-making. 



These mountains are old, but the coal which is now 

 mined in the Appalachian and Rocky Mountains is older 

 and was formed long before the mountains were made. 

 This gives us a suggestion that the earth is many millions 

 of years old. 



282. The Earth and Sun. During these millions of 

 years while the earth was in process of changing its sur- 

 face, it has been moving around the sun, from which it gets 

 heat and light. The path of the earth around the sun is 

 called the earth's orbit. The time required for the earth 

 to go once around its orbit is called a year. This orbit, 

 though not quite circular, is about 184,000,000 miles in 

 diameter, and the sun is always approximately 92,000,000 

 miles distant. The diameter of the earth is about 8,000 

 miles and the diameter of the sun is about 860,000 miles. 



283. The Earth and Moon. The moon is a spherical 

 body which moves around the earth and it is about 

 240,000 miles distant and about 2,000 miles in diameter. 

 The moon has mountains and volcanic peaks on it; these 

 can be seen with a large telescope. There is no evidence 

 of any water or life on the moon. The moon goes around 

 the earth once in 28 days, making a lunar month, and it 

 turns once on its axis during the same time, and on that 

 account always keeps the same part turned toward us. 

 The moon's phases are due to its positions with respect 

 to the sun. When the moon and sun are in the same direc- 

 tion from us, the moon is said to be dark and cannot be 

 seen because it passes across the sky with the sun; during 

 this time the sun is shining on the part of the moon turned 

 away from us. When the sun and moon are in opposite 

 directions, or nearly so, from us the moon passes across 



