ASTRONOMY. 415 



The fixed points round which this sphere is 

 supposed to turn are the poles ; a line drawn from 

 one to the other is called the axis of the sphere ; 

 and round this line the heavens seem to turn every 

 day. 



To understand this more clearly, we must have 

 recourse to a figure, or diagram. Let H O (Plate 

 23. fig. 1.), represent the circle of the horizon, 

 seen edgeways, when it will appear as a straight 

 line ; let H P FO R Q be the complete sphere of 

 the heavens, of which we shall suppose H P E O 

 to be the visible hemisphere, and HQRO the in- 

 visible hemisphere ; then P will be the pole, or 

 fixed point, among the stars, visible to us, round 

 which they all appear to turn; and R will be the 

 opposite pole, or fixed point, in the sphere ; the 

 line P R will be the axis of the sphere. If through 

 the centre of the sphere C, there be drawn a line 

 Q E, it will represent the edge of a great circle, at 

 equal distances from both poles, and at right angles 

 to the axis called the equator, because it divides the 

 heavens into two equal parts. 



If H O be the horizon, the highest point, or 

 that immediately over our heads, as M, is called 

 the zenith ; and the opposite point in the sphere, 

 or lowest point N, is called the nadir. 



The rising and setting of the sun are the two 

 most remarkable circumstances to be observed in 

 the heavens. He rises in the east, mounts to the 

 highest point in the arch which he describes, and 

 descends in the west. The highest point to which 

 he reaches is naturally called the mid-day point. 

 If a great circle be traced through this point and 

 the zenith, it is called the meridian of the place ; 

 and all the stars must cross this circle, or meridian, 

 twice in the twenty-four hours; but those that go 



