CHAPTER FIFTEEN 



THE OUTER PLANETS 



IN several ways the planets Mercury, Venus, and 

 Mars resemble the earth, and hence these four are 

 known as the terrestrial planets. Outside the orbits of 

 Mars and of the minor planets, four great worlds, 

 many times larger than the earth, revolve about the 

 sun. These are the planets Jupiter, Saturn, Uranus, 

 and Neptune. 



The years of these outer planets are longer than the 

 years of the planets we have studied because they have 

 farther to go in their revolution about the sun and their 

 motions along their orbits are slower. Thus the earth 

 moves i8 miles a second through space, while Neptune 

 moves only 3^ miles a second. In proportion to their 

 size the outer planets are much less dense than the 

 terrestrial planets. The earth, taken as a whole, is 

 more than 5^ times as dense as water. Saturn is lighter 

 than water, and Jupiter, the densest of the four outer 

 planets, is only i-J- times as heavy as water. 



The four terrestrial planets together have only three 

 satellites, two of them so small that no telescope would 

 reveal them if they were as far away as Jupiter. Twenty- 

 three moons belonging to the four outer planets are now 

 known, and it is probable that they have other smaller 

 satellites which have not been seen. 



JUPITER 



The diameter of Jupiter is about 86,500 miles. It 

 is 1309 times as large as the earth and contains 317.7 

 times as much material. It will rise as the sun sets 



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