12 



THE OPEN SKY 



The speed of the planets in their orbits and the length of 

 their paths about the sun vary widely. Mercury travels 

 through space about eight times as fast as Neptune, and 

 completes its comparatively short trip around the sun in 

 about 88 days. Neptune requires 164 years to traverse 

 its vast orbit once. 



Astronomers have never satisfactorily determined what 

 the length of day is on Mercury, Venus, Uranus, or Nep- 

 tune the two 

 planets closest to 

 the sun, and the 

 two most distant. 

 A day on Mars dif- 

 fers but little in 

 length from the 24- 

 hour day of the 

 earth, but Jupiter 

 and Saturn whirl 

 completely around 

 on their axes once 

 in about every ten 

 hours. The change 

 of place of planets 



A LARGE METEORITE . . . 



in their relations to 



A shooting-star which fell to the earth. 



each other and to 



the stars is owing to their respective motions about the 

 sun. 



The three planets which shine most brightly for us are 

 Venus, Jupiter, and Mars. To the naked eye Venus is the 

 most magnificent planet in the solar system, exceeding in 

 light and beauty the brightest sfcar. It is therefore called 

 by the name of the Roman goddess of beauty. Jupiter, 



