abstracts: navigation 233 



NAVIGATION. — The chart as a means of finding geographical position 

 by observations of celestial bodies in aerial and marine navigation. 

 G. W. Littlehales. Proc. U. S. Naval Inst. 44: No. 3. March, 

 1918. 



Building upon the principle that at any instant of time there is a 

 series of positions on the earth at which a celestial body appears at 

 the same given altitude and that these positions lie in the circumference 

 of a circle marked out by a radius arm whose pivot is that geographical 

 position which has the body in its zenith and whose length is the same 

 arc-measure as the zenith distance or the complement of the altitude 

 the method proceeds to recognize that the difference of the simultaneous 

 altitudes of the same celestial body at two geographical positions is 

 the shortest great circle arc-distance between the circles of equal alti- 

 tude passing through the two places. By supplying the altitudes 

 and azimuths of the celestial bodies as they would appear at stated 

 intervals of time in a chosen geographical position within the limits 

 of the chart, an observer, in a position as yet unknown, having measured 

 altitude of a celestial body, may at once lay down the locus of his 

 position by comparing the altitude so measured with the tabulated 

 altitude of that body and laying off the difference between the measured 

 and tabulated altitude as an intercept from the chosen geographical 

 position in the direction of the azimuth of the celestial body and toward 

 or away from the bearing of the body according as the measured alti- 

 tude was higher or lower than the tabulated altitude. 



In the illustrative specimen, consisting of a map of the United States, 

 a large compass diagram has been centered at the middle position 

 in latitude 39° and longitude 97°, since the attending tabulation is 

 with reference to this point; and since all altitude-differences are laid 

 off from there, circumferences of equal distances from this point have 

 also been delineated, in order that, with a given altitude-difference, 

 the observer may at once proceed to find the point through which his 

 locus is to be drawn at right angles to the intercept of altitude-difference 

 by passing out by the amount of the altitude-difference to the proper 

 drawn or intermediate circumference along the compass-radial indicated 

 by the azimuth ascertained from the bordering tabulation. G. W. L. 



