xl INTRODUCTION TO ASTRONOMY. 



the wisdom of God. Thus, it was the simple circumstance of the fall of 

 an apple which led to the discovery of the laws upon which the Coper- 

 uican system is founded; and whatever credit this system had obtained 

 before, it now rests upon a basis from which it cannot be shaken. 



SECTION III. On the Earth. 



As the Earth is the planet in which we are the most particularly inte- 

 rested, we shall explain the effects resulting from its annual and diurnal 

 motions ; but for this purpose it will be necessary to make you acquainted 

 with the terrestrial globe. This globe, or sphere (fig. 8), represents the 

 earth. The line which passes through its centre, and on which it turns, 



Fig. 8. 

 i\ 



K 



is called its axis ; and the two extremities of the axis, A and B, are the 

 poles distinguished by the names of the north and the south pole. The 

 circle, C D, which divides the globe into two equal parts between the 

 poles, is called the equator, or equinoctial line; that part of the globe to 

 the north of the equator is the northern hemisphere ; that part to the south 

 of the equator, the southern hemisphere. The small circle, E F, which 

 surrounds the north pole, is called the arctic circle j that, G H, surround- 

 ing the south pole, the antarctic circle. There are two intermediate 

 circles between, the polar circles and the equator, that to the north, I K, 

 called the tropic of Cancer ; that to the south, L M, called the tropic of 

 Capricorn. Lastly, this circle, L K, which divides the globe into two 

 equal parts, crossing the equator, and extending northward as far as the 

 tropic of Cancer, and southward as far as the tropic of Capricorn, is called 

 the ecliptic. The delineation of the ecliptic on the terrestrial globe is not 

 without danger of conveying false ideas ; for the ecliptic (as has before 

 been said) is an imaginary circle in the heavens, passing through the 

 middle of the zodiac, and situated in the plane of the earth's orbit. 



In order to understand the meaning of the plane of the earth's orbit, let 

 us suppose a smooth, thin, solid plane cutting the sun through the centre, 

 extending out as far as the fixed stars, and terminating in a circle, which 

 passes through the middle of the zodiac. In this plane the earth moves 



