JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. IV NOVEMBER 4, 1914 No. 18 



ASTRONOMY. — A special inethod of finding the Sumner line J 

 G. W. LiTTLEHALES, Hydrograpliic Office. 



\Mienever the hour-angle of the observed celestial body is 

 within two minutes of being a multiple of ten minutes at the 

 time of observation, an easy solution presents itself of the prob- 

 lem of finding the azmiuth and altitude for the purpose of laying 

 down the Smnner line by the Saint Hilaire method. In such 

 a case, we may assume the position of the observer to be in a 

 latitude represented by the integral number of degrees of lati- 

 tude next to the latitude by dead reckoning and in a longitude 

 which will make the local hour-angle of the observed body an 

 exact multiple of ten minutes, and we may then, by availing 

 ourselves of smiple resources, find the azimuth and altitude 

 that the observed body would have to an observer situated in 

 the assum.ed geographical position. 



Since the United States Hydrographic Office has completed 

 the publication of the azimuths or true bearings of celestial 

 bodes for all declinations up to 70°, north and south, at inter- 

 vals of ten minutes of hour-angle for the entire circuit of the 

 heavens, and for parallels of latitude of the observer extending 

 to 70° from the equator, the value of the azimuth, required in 

 the circumstances presented, may be taken out from these 

 tables without any interpolation, save alone between adjacent 

 columns for the declination. The parts of the astronomical tri- 



'The Sumner line is a locus of the observer's geographical position comprising 

 so much of an arc of the circle of equal altitude as covers the limits of uncer- 

 tainty in reckoning. 



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