3386 hydrogrAphic manual Page 244 



in which 0=latitude and a=azimiith of body from the pole, and the resultant change 

 in altitude is in minutes of arc per minute of time, or seconds of arc per second of time; or 



^=4 sec 4> cos a 

 in which A' = the number of minutes of time corresponding to 1° change in altitude, or 

 similarly, the number of seconds of time for 1 minute change in altitude; and a being 

 as above. 



A table prepared for each 5° azimuth and for two or three selected latitudes will be 

 found satisfactory. The table inay be entered by interpolation, or a graph for each 

 latitude may be constructed with the azimuth for one argument and time for the other 

 argument. 



The difference between any two consecutive observations of a series should rarely 

 differ from the computed value by more than 15 seconds of arc. 



The altitude rate-of-change table may be used to w^eight observations, for it is 

 obvious that the more adverse most of the conditions of observation are, the greater 

 will be the discrepancies between the observed and the computed differences. In 

 addition to serving as a means of judging the relative quality of the observations, the 

 use of the rate-of-change table will detect small accidental errors in the recorded times 

 or the readings of the sextant. 



3386. Sun Sights 



Wlien the sun is visible, observations should be made on it at least five different 

 times during the day. The early morning and late afternoon sights should be taken 

 preferably when the altitude of the sun is between 12° and 25°. The noon sight should 

 be taken at local apparent noon and the mid-morning and mid-afternoon sights should 

 be taken at times approximately lialfway between the noon sight and the early morning 

 and late afternoon sights. 



The meridian altitude sight at noon slioidd be taken independently by at least 

 three observers. Sights at the other four times should be taken independently by 

 at least two observers although, of course, not necessarily at the same times. 



It is only when clear skies are prevalent that the sun sights can always be taken 

 near the times desired. In stormy and overcast weather sights may be impossible, 

 but in partly cloudy weather the observers must be on the watch to take advantage 

 of the sun's momentary appearance through a break in the clouds in order to obtain 

 the sight as nearly as practicable at the time desired. The quality of the observation 

 is probably more important than the time at which it is taken, and if a poor or only fair 

 sight is obtained near the time desired but conditions are better some little time later, 

 the sight should be repeated and the first observation rejected. 



The most valuable lines of position are those resulting from observations when the 

 sun is on the prime vertical and the meridian. In certain latitudes and at certain seasons 

 the sun is never on the prime vertical and at other times its altitude when near this 

 azimuth is not appropriate for a sight. It is considered more important to observe the 

 five sights at the times of day and altitudes prescribed above than it is to try to obtain 

 sights when the sun is on the prime vertical. 



3387. Position From Star Rights 



Each observer's morning and evening star sights shall be used ijidependently to 

 determine a probable position of the ship, after which the results should be compared 

 and weighted if necessary and an official ship's position adopted. The star sights of 

 one observer should never be combined with those of others in determining positions. 



