466 LITTLEHALES: AZIMUTH OF CELESTIAL BODIES 



of the observer and the meridian of the point of intersection with 

 the equator of the great-circle arc passing through the geographical 

 position of the observer and the geographical position of the 

 observed celestial body. With this difference of hour-angle 

 together with the latitude of the observer, the abridged azimuth 

 tables, or the column of 0° of declination of the present solar 

 azimuth tables, may be entered in the usual columns of time and 

 latitude, and the required azimuth will thus be found. 



The present great-circle sailing charts issued by the Hydro- 

 graphic Office are constructed upon the gonomonic projection, 

 and hence afford the means of accomplishing this purpose. The 

 procedure is simple. It consists of plotting the latitude of the 

 observer on any selected meridian and the observed celestial 

 body in a latitude equal to the declination on a meridian whose 

 difference of longitude from the meridian selected to represent 

 the meridian of the observer is equal to the hour-angle of the 

 observed celestial body, and, having drawn a straight Une through 

 the two positions thus plotted, noting the difference of longitude 

 between the intersection with the equator of the straight line so 

 drawn and the meridian selected to represent the meridian of the 

 observer. If the geographical position of the observed body 

 lies on the opposite side of the equator from the position of the 

 observer and beyond the limits of the chart, plot the latter point 

 as if it were on the same side of the equator and note on the equator 

 the longitudes of both points; then join these four points diagon- 

 ally by straight lines^ and the longitude of the intersection of the 

 diagonal lines will give the longitude of the required intersec- 

 tion with equator of the great-circle arc joining the geographical 

 positions of the observer and the observed celestial body. 



When either the latitude of the observer or the declination 

 of the observed celestial body is higher than 60°, there will be 

 instances arising in which even the most ample of these charts 

 will not suffice to extend, to its intersection with the equator, 

 the required great-circle arc passing through the geographical 

 position of the observer and the geographical position of the 

 observed celestial body. In such cases resort may be made to a 

 gnomonic chart, known as Godfray's great-circle sailing chart, 



