Page 243 hydrography 3385 



observer's sense of verticality is impaired by the vision of nonvertical parts of the 

 vessel. 



A sight should not be observed through the smoke or heat from the smokestack 

 because of the abnormal refraction created by the heat waves. 



A brisk breeze blowing in the observer's eyes is very disturbing, and when he has 

 to face the wind, every effort should be made to find shelter behind a wind deflector, 

 mast, or other part of the ship, where the celestial body can still be observed. 



No attempt should be made to take the individual measurements of a set of observa- 

 tions at regular intervals. Better results are obtained by marking each when the 

 observer feels that it can be measured most accurately, regardless of the irregularity of 

 the intervals between successive sights. 



E. RATING OBSERVATIONS 



At the time of observmg and not later, each observer should rate each set of sights, 

 according to his opinion, "excellent," "good," "fair," or "poor," the rating being entered 

 in the space provided on Form 719. This is a relative rating and is used later in giving 

 weight to each line of position when determining the probable position. It does not 

 matter if one observer's rating is "good," while another observer's rating of the same 

 star taken at the same time is only "fair." In rating sights the following factors should 

 be considered: 



(1) The relative distinctness of the horizon below each star observed. 



(2) The disturbing effect of the roll and pitch of the vessel and its effect on the height of eye. 



(3) The direction of the wind and whether or not it is blowing into the observer's eyes. 



(4) The rate of change in altitude of the star. 



(5) The relative distinctness of stars of different magnitudes. 



The observations may also be rated by using the altitude rate-of-change table, as 

 described in 3385. The use of these ratings in determining the probable position is 

 discussed in 3387. 



3385. Altitude Rate-of- Change Table 



In a given latitude, the rate of change of altitude of any celestial body is constant 

 for a given azimuth. This fact may be utilized to compute tables or construct graphs 

 from which the rate of change in altitude per minute or per second may be determined 

 for any observation. 



The accuracy of sextant observations at sea depends on a number of variable 

 factors, some of which are practically indeterminate. Some of these are the effect of 

 the pitch and roll of the vessel, the effect of a breeze blowing in an observer's eyes, the 

 rapidity of change in altitude of the observed body, the distinctness of the horizon, and 

 the experience of the observer. 



The accuracy of a series of observations on the same celestial body may be tested 

 by the altitude rate-of-change table. If the differences between successive observed 

 altitudes are compared with the mathematically correct differences in altitude for equal 

 intervals of time, the discrepancies between the observed and the true values are an 

 index of the accuracy of the observations. 



The data for the tables, or from which the graphs may be constructed, are found 

 by means of either of the following formulas: 



Change in altitude=15 cos sin a 



