Page 249 hydrography 3387 



Star finders and identifiers serve another purpose besides identification. At 

 morning sights any star to be observed is readily visibk^ and its altitude may be meas- 

 ured and followed while waiting for the horizon to brighten sufficiently to take the 

 observation. At evening sights the best results are obtained while the sky is so bright 

 that the stars are difficult to find unless their approxmiate altitudes and azimuths are 

 known. If certain stars are selected for observation in advance, their approximate 

 altitudes and azimuths may be determined by the use of tlie star finders for given times 

 of observations. A table may be made of the stars expected to be observed, listing 

 the predicted altitudes of each at 2- or 3-minute intervals throughout the period when 

 each star will probably be observed. From this table the altitude of a star at any 

 given instant may be found by interpolation and set on the sextant, and with the 

 sextant pointed in the predicted direction the star should be found in the field of view. 



For greatest precision the construction of a special star chart is recommended, if 

 it is expected that a large niunber of stellar observations will be taken in one latitude 

 during a long period of time. This should preferably be an aluminum-mounted sheet 

 on which all stars brighter than about magnitude 4K> should be plotted according to 

 right ascension and declination. The largest convenient scale should be used. To fit 

 this chart, templates similar to those of the Rude Star Finder may be made by using 

 the values from various nautical tables used to compute astronomic sights. For ex- 

 ample, H. O. Publication Nos. 203 and 204, Sumner Line of Position, may be used. 

 The required values may be plotted directly on the star chart, smooth curves drawn 

 through them, and the results engraved on a celluloid template by tracing. Any one 

 template will give correct values only for the latitude for which it is constructed, but 

 for this latitude the altitude of any star should be predictable within at least 10 minutes 

 of arc. For most projects not more than two templates should be required. 



The center of the template should always be placed on the star chart at the latitude 

 for which it was constructed. For other latitudes the predicted altitudes must be cor- 

 rected for the difference between the latitude of observation and the latitude of the 

 template. This correction is equal to the dift'erence in minutes between the latitude 

 of observation and the latitude of the template, multiplied by the cosine of the azimuth 

 of the star measured in degrees from the elevated pole of the observer. This correction 

 may be derived mathematically by using a table of natural cosines, as found in table 31 

 of Bowditch, and a slide rule. The values of the corrections for differences of 30 

 minutes or less may be found in the table entitled "Altitude Correction for D. R. 

 Latitude" in the back of H. O. Publication No. 214. The proper sign to be used in 

 applying the correction will be self-evident ; where the latitude of observation is greater 

 than the latitude of the template for a star north of the prime vertical, the correction 

 is additive and for one south it is subtractive. 



C. DAYTIME OBSERVATIONS ON STARS AND PLANETS 



Although the stars and planets can rarely be seen with the naked eye during day- 

 light, the brighter ones can frequently be observed by making use of their precomputed 

 altitudes and azimuths to find them in the sextant (see B above). The planet Jupiter 

 can frequently be observed m the daytime on an azimuth that will make a line of 

 position from it cross advantageously with one from a sun sight. The planet Venus, 

 when near quadrature, can be observed during the day, and a line of position from it 

 will often serve the same purpose. Even Polaris, whose magnitude is only 2.1, has 

 been observed during the daytime. Lines of position obtained from daytime observa- 

 tions on stars and planets should be crossed with sun sights taken at the same approxi- 



465.'?8i;— 44 18 



