THE EARTH AS A PART OF THE SOLAR SYSTEM 527 



are watched for several weeks their position in the sky with 

 reference to the stars changes, while the stars always keep 

 the same position with reference to each other. The planets 

 also show a change in brilliancy and apparent size from year 

 to year, because their distance from the earth changes, ac- 

 cording to whether the earth and planet are on the same side 

 or opposite sides of the sun. Mars when farthest away from 

 the earth is seven times as far as when nearest the earth. 

 Those planets which are nearer the sun than is the earth, 

 Mercury and Venus, are never seen at any great distance 

 away from the sun, so that they are seen only for a few hours 

 before sunrise or a few hours after sunset. Venus is the one 

 most commonly seen as a " morning " or " evening " star. 



The source of the light seen from the planets is the same 

 as from the moon, reflected sunlight. It is thought that 

 perhaps Jupiter may give out a small amount of light on 

 account of its own luminosity. 



These planets vary greatly in size. The two smallest, 

 Mercury and Mars, are about one half the diameter of the 

 earth. Venus is about the same size; while Jupiter, the 

 largest, is about ten times as large as the earth. Mercury 

 is the nearest to the sun of all the planets, less than one half 

 as far as the earth. Neptune is the farthest away, about 

 thirty times the distance of the earth. The time that it 

 takes each planet to make a single revolution around the 

 sun depends on the distance from the sun, those farther 

 away taking a longer time. The period for Mercury, the 

 planet nearest the sun, is about three months; while that 

 for Neptune, the one farthest away from the sun, is one 

 hundred sixty-five years. 



Most of the planets have moons. Saturn has ten. Two, 

 recently discovered, are very small. Another interc 

 feature about Saturn is its rings, which extend in broad, 

 thin belts around the circumference of the planet. These 

 are very beautiful when seen through a telescope. 



