THE CONQUEST OF TIME AND SPACE 



days by dead reckoning approaches a coast with quite 

 the same degree of satisfaction that he may entertain if 

 his log has been checked by observation of the sun or 

 stars. In case, however, a navigator is able to check 

 his reckoning by astronomical observations, aided by 

 the chronometer, he determines his location with great 

 accuracy. 



THE DEVELOPMENT OF THE SEXTANT 



The instrument with which such astronomical 

 observations are made is known as the sextant. Its 

 purpose is to measure with great accuracy the angle 

 between two objects, which in practice are the horizon 

 line on one hand and some celestial body, usually the 

 sun, on the other. The determination of the latitude 

 of the ship, for example, is a matter of comparative 

 ease, if the sun chances to be unobscured just at mid- 

 day. The navigator has merely to measure the exact 

 elevation of the sun as it crosses the meridian, — that 

 is to say when it is at its highest point, — and, having 

 made certain corrections for so-called dip and refrac- 

 tion, to which we shall refer more at length in a moment, 

 a very simple calculation reveals the latitude — that is 

 to say, the distance from the terrestrial equator. 



That the latitude of a ship could thus be determined, 

 with greater or less accuracy, has been familiar knowl- 

 edge to seamen from a very early period. It was by 

 the use of this principle that the earth was measured by 

 Eratosthenes and Posidonius in classical times, and 

 the sailors of antiquity probably carried with them a 



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