THE UNIVERSE 9 



hundred and forty times the bulk of all the planets combined. 

 If the sun were represented by a globe twenty-five feet in 

 diameter, the earth, on the same scale, would be a ball three 

 inches in diameter. The sun is the source of light and heat for 

 the planets, and the center about which they all revolve, being 

 held in their courses by that mysterious force called gravity, 

 which acts between all the heavenly bodies with a strength that 

 is in proportion to their masses and their distances from one 

 another. 



13. The Planets. Of the eight planets, the four outermost 

 are larger than the earth. Jupiter, the giant planet, is about 

 fourteen hundred times as large. The average distance from 

 the earth to the sun is 92,800,000 miles. Neptune, the farthest 

 planet, is thirty times as far, or nearly three billion miles. All 

 the planets move about the sun from east to west in periods in 

 proportion to their distance from it. Mercury makes a com- 

 plete revolution in eighty-eight days, while Neptune requires 

 one hundred and sixty-five years for the journey. The two 

 planets nearest the sun have no moons and the earth has only 

 one, but Mars has two, Jupiter nine, Saturn ten, Uranus 

 four, and Neptune one. In addition, Saturn has a broad lumi- 

 nous belt of rings about its equator. 



14. The Moon. The earth's only satellite, the moon, is a 

 much smaller body which revolves about it just as the earth 

 itself revolves about the sun. It is much nearer to us than 

 the sun, being in fact, only two hundred and forty thousand 

 miles away. It makes one revolution about the earth in 

 twenty-nine days, and revolves on its own axis once during 

 that time, in consequence of which, we only see one side of 

 it. The configuration of the visible side, however, is as well 

 known as similar areas on the earth. The moon shines by 

 reflected sunlight, but the amount of moonlight we receive 

 varies with the amount of the illuminated surface visible. 

 When the sun, moon, and earth are in such a position that we 



