Solar System. * 3$ 



They perform their revolutions also in 

 very different periods of time. The time 

 of performing their revolutions round 

 the sun is called their year, and the time 

 of performing their revolutions on their 

 axes their daij. 



The axis of a planet is an imaginary 

 line conceived to be drawn through its 

 center, about which it revolves as if on a 

 real axis. The extremities of this line t 

 terminating in opposite points of the 

 planet's surface are called its poles. A 

 bowl whirled from one's hand into the 

 open air turns round such a line with- 

 in itself, whilst it moves forward; and 

 such are the lines we mean when we 

 speak of the axes of the heavenly bodies, 



Venus and Mercury being nearer to 

 the sun than our earth, are called inferior 

 planets, and all the rest, which are with- 

 out the earth's orbit, are called superior 

 planets. 



The sun is placed near the common 

 center, or rather in the lower* focus of 



* If a thread be tied loosely round two pins 

 stuck in a table, and moderately stretched by the 

 point of a black-lead pencil carried round by an 

 even motion and light pressure of the hand, an 



