THE EARTH AS A PART OF THE SOLAR SYSTEM 519 



in the sun in order to make these lines, because these lines 

 are the same as those made by certain substances found on 

 the earth. Such metals as iron and nickel have been found 

 to be present in the sun. It is to be expected that the 

 same elements would be found in both the earth and sun, 

 because it is believed that they came originally from the same 

 mass. The outer surface of the sun is made up of hot gases, 

 heated to an extremely high temperature. What are called 

 sunspots are frequently seen on the surface of the sun. 

 These are believed to be enormous. depressions or craters in 

 these gases. Farther in from the surface, these gases be- 

 come very dense and in the center may become liquid or 

 even solid. 



The sun is found to rotate on its axis like the earth, only its 

 period of rotation is longer. Observation of sunspots shows 

 that it takes the sun about twenty-five days to rotate once. 



Changes in the solar system. All the bodies of the solar 

 system are going through a series of changes. At one time 

 all were intensely hot. The sun 'is in that stage now, and 

 possibly Jupiter is. The earth represents a later stage, 

 when the crust is cool but the interior is hot as shown by 

 volcanoes. The moon represents a still later stage, for it 

 has lost nearly all its heat and is probably cooled all the way 

 through. In time the sun too will doubtless become cool 

 and cease to give off heat and light, but that is many million 

 years in the future. 



The seasons. Our change of seasons is due to the varying 

 amounts of heat we receive from the sun. In the summer 

 the north pole of the earth points towards the sun and during 

 the middle of the day the rays come down nearly straight. 

 In the winter the north pole of the earth points away from 

 the sun, which is low down in the sky at noon, and the 

 rays strike the earth obliquely, so that less rays strike a 

 given surface than in the summer when the rays are more 

 nearly vertical. Hence the earth receives more heat during 



