HEAVENLY BODIES 471 



ning motion like that of a top. As this speed increased portions 

 of this vapor were thrown off, as water is thrown off a grindstone, 

 or mud from an automobile tire. These rings of vapor which were 

 thrown off also started cooling and, retaining their whirling motion, 

 formed the earth and planets. They in turn threw off vapor because 

 of their speed, forming their moons (satellites). The great planet 



FIG. 361 The position of the earth at the winter solstice. What part of the 

 earth has continuous day? Night? Where would the day be longer than 

 the night? Night longer than the day? Why? Where would the sun just 

 " set " and begin rising again? 



Saturn is still very hot and is throwing off rings of vapor which may 

 some day become moons. 



The earth cooled off slowly until animal and plant life could 

 live upon it. 



In the diagram you will see the effect of heating and cooling 

 an apple. The earth's surface may be compared with that of the 

 apple. As it cooled off, many irregularities were formed which 

 produced continents and oceans, hills and valleys, mountains and 

 plains. 



Mercury. Mercury is so near the sun that it is almost impossible 

 to observe it carefully. One side of the planet has perpetual day 

 and the other perpetual night, which means that one side must be 

 intensely heated, while the other side maintains a very low tem- 

 perature. 



Venus. This planet is surrounded by clouds on which the sun 

 shines with the effect of giving Venus a very beautiful appearance, 



