The Planets 233 



has the longest orbit of all the planets, requires 165 

 years to complete a revolution ; the other planets have 

 periods between these extremes, according to their 

 distance from the sun. If you will watch Jupiter year 

 after year, you may see that each year it moves among 

 the stars about 30 degrees, or about one twelfth of the 

 whole way around. Its period of revolution is not quite 

 12 years. If you see it in a certain direction this year 

 in October at eight o'clock, next year you will see it in 

 the same position at eight o'clock in November and the 

 following year in December; thus in 12 years it will 

 have made a complete revolution in the heavens. 



By taking hold of a string that is attached to a ball, 

 you can whirl the ball around your hand in any direction 

 you please. You can imagine eight balls without strings 

 whirling around your hand at different distances. They 

 might all whirl in the same direction or they might 

 have quite different directions, one moving in a north- 

 and-south plane, another east and west, others in planes 

 inclined at large angles to both of these. If your hand 

 were touching the floor the balls might roll in circles 

 of different size, all the circles having your hand as the 

 center. They might all go one way around, or some one 

 way, others the opposite way. 



The eight planets revolve around the sun almost in 

 one plane, and they all revolve in one direction. The 

 earth is one of them, and the plane in which it moves 

 is called the plane of the ecliptic. The planes in which 

 the others move are inclined only a little to the plane of 

 the ecliptic. It would be useless to try to find a planet 

 anywhere in the heavens except near the ecliptic. 



