THE LATITUDES AND LONGITUDES. 



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the zenith in the heavens of one place, from the zenith of another must necessarily 

 be the same in degrees as the distance between two places on earth. Thog n is 

 the zenith of P ; e is the zenith of E ; n is the same number of degrees from 

 as P is from E. This being clearly understood, it is evident tliat if we can, by 

 any means ascertain by observations, the distance from Z to n, we can infer at 

 once the distance from P to N, and hence, can. discover the distance from 

 P to E, or the latitude of the place. 



It is apparent, then, if we can observe the distance of the zenith of any 

 place from the celestial pole, that will give us the distance in degrees of the place 

 itself from the terrestrial pole, and by subtracting that from 9(P, we shall obtain 

 the distance of the place itself from the equator, or what is the same, its latitude. 

 As an example of this, lt us suppose that in measuring the distance from Z to 

 n we find it to be 50 ; "we infer, therefore, that since the distance of the zenith 

 from the pole is 50, the distance of the place from the terrestrial pole is also 50. 



But since the terrestrial pole is 90 D from the equator, it follows that the dis- 

 tance of the place from the equator must be 40, and it is north or south, ac- 

 cording as the zenith of the place is in the northern or southern hemisphere of 

 the firmament. 



Thus, then, it appears that the latitude of a place can always be found, provided 

 we can measure the distance of its zenith from the celestial pole ; and this, of 

 course, can always be done by the use of proper instruments, provided that the 

 zenith and the pole can be distinctly seen. Now the direction of the zenith 

 can always be determined by the plumb line ; but although the pole star is very 

 near the pole, it is not exactly at it ; there is, in fact, no star exactly at the 

 pole, and there being no visible object there, it is impossible to measure direct- 

 ly its distance from the zenith. This difficulty is eluded by measuring the 

 distance of the zenith from some star, or other celestial object, whose distance 

 from the pole happens to be known : for example, suppose that there were a 

 star directly between the zenith and pole, whose distance from the pole was 

 known to be 10. Then if we find by observing the distance of the zenith 

 from this star was 40, we should immediately infer the distance of the zenith 

 frum the pole to be 50. * 



It is ia;-fact, then, by this device that the latitude is always ascertained. By 

 various observations made by astronomers, the positions of most of the stars 

 and other celestial objects, with respect to the poles, are known and recorded ; 

 and when we desire to determine the latitude of any place, we measure the 

 distance of the zenith of that place from some celestial object whose position 

 with respect to the pole is known, and thence infer the position of the 

 place with respect to the terrestrial pole ; and from that deduce at once the 

 latitude. 



But our purpose would be equally served if we were supplied with the po- 

 sition of any visible object with reference to the celestial equator. Thus, if 

 we know the distance of the centre of the sun from the celestial equator, we 

 shall readily be able to find the latitude ; for it would only be necessary when 

 the sun is in, or very near the meridian, that is, at or near noon, to measure 

 the distance of the zenith of the place from the centre of the sun. This 

 would be done by measuring the distance of the zenith, first from the upper, 

 and then from the lower limb of the sun. The distance from the centre would 

 ( be the mean between these. 



Let us suppose, for example, that the sun being between the zenith of the 

 \ equator, we find that the distance from the zenith to the centre of the 

 ) sun is 20, and that we also ascertain from the table of the position of the 

 sun, that the distance of the centre of the sun at that time from the equa- 

 tor, is also 20, we should infer at once that the distance of the zenith 



