THE EARTH. 19 



What is there remarkable about Saturn ? 



Which of the planets is the farthest from the sun ? 



How long is Saturn in performing its revolution 1 



What are comets ] 



What is the nature of their course ? 



Are their motions regular 1 



What are the fixed stars ] 



What number may be seen by the naked eye ? 



How many have been discovered with the telescope '? 



What is a constellation? 



LESSON IV. 



FORM AND MAGNITUDE OF THE EARTH. 



IT is difficult to discover the form of the earth 

 by merely looking upon it, because we can see 

 but a small part of it at once, and because we are 

 too near to it. The general form of an object is, 

 however, to be ascertained by the form of its 

 shadow. Now, whenever the earth, in its journey 

 round the sun, happens to be in a straight line 

 between the sun and the moon, it always casts a 

 circular shadow upon the face of the moon, and 

 thus we know that the earth is round, like a 

 globe. 



The circumference of the earth is about 25,000 

 miles. It is possible, then, to travel round the 

 earth. If you turn your back on your home, 

 and continue to travel with your face to the w^st, 

 the quarter in which the sun sets, you will arrive 

 at home again from the east, or the quarter in 



