NAVIGATION. 83 



therefore, if you are satisfied with knowing your 

 latitude within i 21', the simple altitude of Polaris 

 gives it. But if you know the sidereal time of 

 your observation, even very roughly, say within 

 five or ten minutes of time, you can calculate the 

 correction required to give the true latitude from 

 the observed altitude of Polaris accurately enough 

 for practical purposes. 1 . This method is practised 

 very frequently at sea in the northern hemisphere. 

 The meridian altitude of any known star, or of the 

 sun, gives the latitude, for the Nautical Almanac 

 tells v you the distance 2 of the observed body from 



1 The greatest error in the deduced latitude due to error in your 

 reckoning of time is, of course, to be met if the observation is made 

 when the star is rising or sinking with the greatest rapidity that is 

 to say, when it has made a quarter of its revolution from the lowest 

 or highest points of its diurnal circuit. At such times there is an 

 error of 2' latitude for six minutes' error in your reckoning of time. 



2 The Nautical Almanac gives what is called the declinations of 

 stars and sun, that is, the angular distance north or south from the 

 celestial equator, this being a plane through the observer's eye perpen- 

 dicular to the axis of the earth's rotation. The north polar distance 

 is found by subtracting the declination from, or adding it to, 90, 

 according as it is north or south declination. Thus the declination 

 of Arcturus is 19 50' N. ; its north polar distance, therefore, is 

 70 10' N. Again, the declination of the sun to-day (Nov. u, 1875) 

 is 17 24' S ; his north polar distance, therefore, is 107 24'. 



G 2 



