CHAPTER XII 



OBSERVATIONS FOR LATITUDE 



By Circum-meridian Altitudes of Stars — By Circum-meridian 

 Altitudes of Sun. 



Astronomical observations are largely used in all descrip- General 

 tions of marine surveying. In all but small plans the eventual ^^^^ 

 scale of the chart is decided by the latitude and longitude, as 

 obtained by observations of sun or stars, and we have seen 

 that true bearings often enter largely into the construction of 

 charts. In running surveys, or in searching for or sounding 

 over shoals in mid-ocean, everytliing depends on the positions 

 astronomically found, and every method of correctly finding 

 the latitude and longitude is in requisition. In considering 

 this subject, we will take first shore observations with artificial 

 horizons, where we require results as accurate as we can obtain 

 with the sextant, to which instrument remarks will be confined, 

 excepting so far as the theodolite is used for true bearings, and 

 afterwards sea observations. 



In aU observations of the heavenly bodies, instrumental Eiimina- 

 errors, atmospheric effects, and personal differences largely E°^rs. 

 influence the results. No matter how correctly we may take 

 the actual observations, unless we can eliminate these variable 

 quantities, the positions obtained will be in error. 



On every occasion, therefore, where accuracy is aimed at, 

 the mode in which this elimination can be best carried out 

 must be considered. The general principle used in doing this 

 is to get two sets of observations for one result in such a 

 manner that the errors of all kinds will act in opposite direc- 

 tions in each set, and therefore disappear when the mean is 

 taken. The precise way in which this is done will be described 

 under each different observation. 



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