326 HOW LATITUDE IS RECKONED 



as a matter of fact, it consists only of the sixth part of a circle. The optical 

 principle upon which the instrument is founded (that of double reflection) 

 permits of half a degree of the arc being numbered and considered as a whole 

 degree. Thus, in the sextant what is really only an arc of 60 degrees is 

 divided into 120 equal parts, each of which does duty as a degree. 



The optical principle upon which the sextant is founded is thus ex- 

 pressed in scientific language: "If a ray of light suffers two successive re- 

 flections in the same plane by two plane mirrors, the angle between the first 

 and last direction of the ray is twice the angle of the mirror." 



What the sextant does, expressed differently, is to solve the astronomical 

 triangle, one point of which is the pole, the second the observed heavenly body, 

 which is the sun, and the third the zenith, which is the point directly over 

 the head of the observer. What the observer seeks to find out from his read- 

 ings of the sextant is the sun's altitude. Once he gets that he can get all the 

 other necessary data from the so-called "Nautical Almanac," a government 

 publication, revised for each year, which is among the most treasured posses- 

 sions of every navigator and explorer. 



By latitude is meant the angular distance between the horizon and the 

 level of the observer. In making observations at sea the actual horizon — 

 that is, where the sky and the water meet — is used. On shore, however, ob- 

 servers make use of an artificial horizon. Ordinarily this consists of a cast- 

 iron trough, containing pure mercury, which is protected from disturbances 

 from the wind by an angular glass roof. A form of artificial horizon more 

 suitable for the needs of explorers is that known as Capt. George's, since it 

 is more compact and more easily carried. In place of mercury, molasses, crude 

 oil and other substances may be used in the artificial horizon. 



What is known as the "meridian altitude," or the sun's position at noon, 

 is the best for getting the latitude, hence it is that observations are usually 

 taken when the chronometer of the explorer or navigator tells him that it is 

 noon. At that time the error which an observer is likely to make in deter- 

 mining the longitude is a matter of small importance. 



The two things that an observer must know in order to get his latitude 

 are the altitude of the sun, which he gets by means of his sextant, and the decli- 

 nation of the sun, which he gets from his Nautical Almanac. By declination 

 of the sun is meant its angular distance north or south of the celestial equator 

 — i. e., a circle reaching to the heavens which is in the same plane as the 

 equator of the earth. 



