XIX.] THE FIGURE OF THE EARTH. 323 



Here the earth is represented as hanging in the centre of a 

 great space, which is bounded on all sides by a starry vault. 

 Let a person be standing on the earth at the point O. 

 That point in the heavens which he sees directly over- 

 head, when he looks up, is called the zenith; and the 

 opposite point, which is immediately beneath his feet, and 

 which it is therefore impossible for him to see, since the 

 solid earth stands in his way, is called the nadir?- The 

 direction of the straight line joining these two points is the 

 direction in which a plumb-line hangs when the plummet is 

 free. An imaginary plane passing exactly midway between 

 the zenith and the nadir constitutes the horizon? 



It was explained on p. 9, that very near to the north 

 pole of the heavens there is a star called the pole star. 

 That point on the horizon which is immediately beneath 

 the north celestial pole is the true north, and the other 

 cardinal points on the earth's surface are also referred 

 to the horizon. Now, suppose a person at O in Fig. lor, 

 observes how high the pole star is above the northern 

 horizon ; and that tvo persons travel from this point, — 

 one going due north, and the other due south — and that 

 they observe, at ditferent times, the apparent altitude of 



1 Nadir and ziiiith are words of Arabic origin. 



'^ There are, in fact, two kinds of horizon. It was said above (p. 320) 

 that the horizon is th« circle which limits a person's vision, wherever he 

 may happen to be. This circular boundary, so far as it is formed by 

 the surface of the earth, is distinguished as the apparent, or sensible, 

 horizon. Tb.e great plane which is shown in Fig. loi jDassing through 

 the centre of the earth, and extended to the celestial sphere, is dis- 

 tinguished as the plane of the true or rational horizon, which is an 

 imaginary circle, dividing that sphere into two equal halves, one above 

 and one below the true horizon. Practically these two horizons coincide, 

 for the distances of the stars from us are so great, that, if the apparent 

 horizon were extended until it met the celestial sphere, it might be 

 regarded as coincident with the rational horizon, to which it would be 

 .sensibly parallel, though separated from it by half the earth's diameter. 



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