THE CONQUEST OF THE ZONES 



it would seem as if the simplest possible method of 

 determining longitude would be found through observa- 

 tion of the sun's meridian passage. The user of the 

 sextant on shipboard always makes, if weather permits, 

 a meridian observation of the sun, and such observa- 

 tion gives him an accurate gauge of the altitude of the 

 sun at its highest point and hence of his own latitude. 

 By adjusting the arm of the sextant with which this ob- 

 servation is made, the observer is able to determine the 

 exact point reached by the sun in its upward course with 

 all requisite accuracy. 



But, unfortunately for his purpose, the sun does not 

 poise for an instant at the apex of its upward flight and 

 then begin its descent. On the contrary, its orbit being 

 circular, the course of the sun just at its highest point is 

 approximately horizontal for an appreciable length of 

 time, and while the observer therefore has adequate 

 opportunity to measure with accuracy the highest 

 point reached, he cannot possibly make sure, within 

 the limits of a considerable fraction of a minute, as to 

 the precise moment when the center of the sun is on 

 the meridian. He can, indeed, determine this point 

 with sufficient accuracy for rough calculations, but 

 modem navigation demands something more than 

 rough calculations, inasmuch as a variation in time of 

 one minute represents one-quarter of a degree of longi- 

 tude, or fifteen nautical miles at the equator, and such 

 uncertainty as this would imply can by no means be 

 permitted in the safe navigation of a ship that may be 

 passing through the water at the rate of a nautical 

 mile in less than three minutes. 

 voL.vn-3 [33] 



