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HYDROGRAPHIC MANUAL 



observations. When star sights are taken 

 from a vessel underway, the various sights 

 must be advanced (run up) or retired (run 

 back) to a selected dead-reckoning position. 



Each observer's star sights shall be used 

 independently to determine the probable 

 position of the ship, after which the results 

 may be compared and weighted if necessary 

 and an official ship's position adopted. Ob- 

 servations taken by different observers, by 

 different sextants, or at different times of the 

 day should never be combined to determine a 

 probable position. Each observer's sights 

 should be plotted on separate sheets of paper. 

 The dead-reckoning position, the adopted po- 

 sition of the ship, and the lines of position 

 after being run up or run back should be 

 inked. The name of each star observed and 

 its direction should be indicated alongside 

 each line of position. The observed lines of 

 position and the distance and direction each 

 was run up or back should be left in pencil. 

 The adopted position should be transferred 

 to the plotting sheet. If only one observer 

 obtains the astronomic position, the final 

 plot may be made on the plotting sheet 

 (see 5-127). 



There are several factors which influence 

 the accuracy of astronomic observations. 

 A change of 1 minute in altitude moves the 

 line of position 1 nautical mile. At the 

 Equator the altitude of a celestial body on 

 the prime vertical changes 1 minute of 

 arc in 4 seconds of time. For other latitudes 

 and azimuths the error in position caused 

 by an error in time is less, varying with the 

 cosine of the latitude and the sine of the 

 azimuth. All other conditions being equal, 

 there is likely to be a greater error in an 

 astronomic sight on a celestial body in an 

 east or west direction than on one in a north 

 or south direction. The reason for this is 

 twofold: an error in time will affect the 

 line of position proportionately more, and 

 the bodies change faster in altitude making 

 accurate observations more difficult. A dis- 

 tinct and clear horizon is essential for ob- 

 taining accurate observations. Evening star 

 sights should be taken as early as possible, 

 and morning sights as late as possible, to 



assure a well illuminated horizon. At the 

 time of observation and not later, each ob- 

 server should rate each set of sights, accord- 

 ing to his opinion, "excellent," "good," "fair," 

 or "poor." The rating should be based on 

 consideration of the following factors : 



(a) The relative distinctness of the hori- 

 zon below each star observed. 



(b) The disturbing effect of the roll and 

 pitch of the vessel. 



(c) The direction of the wind and wheth- 

 er or not it is blowing into the observer's 

 eyes. 



(d) The rate of change in altitude of the 

 star. 



(e) The relative distinctness of stars of 

 different magnitude. 



When the selected stars are symmetrically 

 arranged and at approximately the same 

 altitude, some of the errors of observation 

 are partly eliminated in the final plot. Ac- 

 cidental errors of observation are not elimi- 

 nated by this systemmatic arrangement. 

 Only observations made by experienced per- 

 sonnel who are able to obtain rapid and 

 consistently good results should be used. In- 

 experienced observers should be required to 

 take and compute sights, but the results 

 should not be used officially until they are 

 known to be trustworthy. 



For best results the celestial bodies to be 

 observed should be selected so that lines of 

 position will plot in a symmetrical quadri- 

 lateral. If more than four stars are observed, 

 it is preferable to combine lines of position 

 to a resultant rectangular figure of error 

 whose sides are roughly north-south and 

 east-west. The stars should be of about the 

 same magnitude and about the same alti- 

 tude. Observations on bodies at altitudes 

 between 15° and 20° usually give best results, 

 but altitudes between 10° and 35° may be 

 considered satisfactory. 



Due to the inherent inaccuracy of as- 

 tronomic sights, the lines of position cor- 

 rected for the run of the ship between sights 

 usually do not intersect at a point. Three 

 lines of position will form a triangle of er- 

 ror. The probable position of the ship 

 derived from a series of morning or evening 



