3387 HYDROGRAPHIC MANUAL PaGE 248 



altitudes between 10° and 35° may be consid^-ed satisfactory. The lower limit may 

 be reduced in tropic latitudes, for good results have been obtained there with altitudes 

 as low as 5°. In higher latitudes the lower limit may have to be increased if there is 

 abnormal refraction. 



The group of celestial bodies selected need not be the same for all observers but no 

 advantage is gained in using different ones. The tendency, at least in the evening, is 

 for one obsei-ver to observe stars which have already been foinid by some other 

 observer. 



When observations on bright stars or planets have to be combined with those on 

 faint stars, comparable conditions may be approximated by observing the bright stars 

 first, as soon as they become visible m the telescope in the evening. In the morning the 

 sequence should be reversed, the faint stars being observed first and the bright ones 

 later when they have dimmed somewhat. Observations should not be made when the 

 horizon is indistinct, but better results can be obtained under such conditions by using 

 a sextant equipped with a binocular telescope. Observations should never be made at 

 night, using a moonlit horizon, because of the likelihood of a false horizon. 



B. STAR IDENTIFICATION 



An observer experienced in taking stellar observations should have no difficulty in 

 identifying the principal stars in the heavens on a clear night, but an observer who has 

 never taken such observations or who does so only at infrequent intervals must have 

 some method of identifying the stars which he observes. Even one familiar with the 

 heavens occasionally takes an observation on a star which he cannot identify, this 

 occurring most frequently when most of the heavens is obscured by clouds and there 

 is an insufficient pattern of stars visible in the vicinity of the one obs(^rved to make 

 identification certain. 



As a matter of routine, beginners should always note the approximate azimuth of 

 each observed star, and experienced observers should note the azimuths of any observed 

 stars whose identification is doubtful. Observed stars may be identified visually, 

 graphically, or by computation. H. O. Chart No. 2100 may be used for visual identi- 

 fication. The pattern of the stars in any constellation or any section of the heavens 

 compared with the same area on the star chart will disclose in most cases the identity 

 of the star observed. Unfortunately, this chart includes only the brighter stars and a 

 few of the fainter ones which form the patterns of the various constellations. 



Stars may be identified by computation, using H. O. Publication No. 127, Star 

 Identification Tables, or the star identification table following each degree of latitude 

 in H. O. Publication No. 214, Tables of Computed Altitude and Azimuth. Entering 

 the tables with the observed altitude and azimuth of the celestial body, its hour angle 

 and declination may be found. Interpolation by inspection will suffice. The hour 

 angle is converted into right ascension, and with the decimation and right ascension 

 the body may be identified in the Nautical Almanac. 



If no tables are available tlu' right ascension and declination of an unidentified 

 star can be computed by the formulas and methods given on page 187, Bowditch, 1938 

 edition, under the heading "Star Identification." 



The brighter stars may be identified by the use of the Rude Star Finder and 

 Identifier, H. O. No. 2102a or H. O. No. 21026. The latter is based on the same 

 principle as 2102a but the star chart and the templates are on a smaller scale and the 

 results cannot be obtained as accurately as from 2102cr. 



