AIR NAVIGATION 453 



ed, a small quantity dependent on the latitude of the observer and 

 the dedination of the sun.^^ 



An example of such a table (for declination 23°) is shown in Figure 

 4 on page 454. It will be noted that the changes in azimuth due to 

 changes in latitude are comparatively small. The effect of declina- 

 tion is smaller still. (For instance at latitude 80° N., with an hour 

 angle of five hours, or 75°, and with a declination of 20° N., the azi- 

 muth is 78° 47' S by W, and, with a declination of 23° N., the azimuth 

 is 79° 23' S by W). Consequently the tables are very compact, es- 

 pecially as it is necessary to give the corrections only to the nearest 

 half degree. 



By Sun Compass 



The same principle is employed in the construction of the clock- 

 work solar compasses^^ that were used in the recent flights over the 

 north pole. The clockwork mechanism compensates for the gradual 

 change in the hour angle. 



Clockwork solar compasses are satisfactory, no doubt, for flying 

 along a meridian ; but it is likely that they would become unnecessarily 

 complicated for use in a flight undertaken in any other direction. 



Steering a Course by Ranging In 



Though astronomical or magnetic methods are used extensively 

 for finding direction, they are not so practical for steering a course. 



In the case of astronomical methods a straight line or great circle 

 course involves a frequent change in the angle subtended by the 

 heavenly body and the direction of flight. This is due partly to the 

 apparent movement of the heavenly body and also, except in flying 

 along a meridian, to the convergence of the meridians. The latter 

 reason also applies to the change in angle between the direction of the 



16 Azimuth tables of an extremely compact nature are to be found in the Norwegian fisheries 

 almanac for latitudes 70° to 90° (see "Retvisende peiling av sol, maane eller stjerne paa hpiere bredder 

 end 70° N.," pp. 51-61 in "Norsk Fiskeralmanak 1923, redigert av Trygve Haaland," Selskabet for de 

 Norske Fiskeriers Fremme, Bergen). They are not, however, well adapted for use in aircraft because, 

 before the azimuth is finally extracted, it is necessary to extract two other quantities from the tables 

 and do three or four additions and subtractions. 



"Arctic Azimuth Tables for Parallels of Latitude between 70° and 80''," by Lieutenants Seaton 

 Schroeder and Richard Wainwright, was published at Washington in 1881 (Bur. of Navigation, U. S. 

 Hydrogr. Office {Publ.\ No. 66). The tables "were prepared for the use of the U. S. S. Rodgers in her 

 search for the Arctic steamer Jeannette. As time did not permit of computing the azimuths for the 

 whole time that heavenly bodies of northern declination would be visible in such high latitudes, they 

 are given, at intervals of ten minutes, for six hours out of the twenty-four — that is (in the case of the 

 sun), from s a. m. to 8 A. M., and 4 p. M. to 7 p. M." These tables also are not very well adapted for use 

 in aircraft as each table is for a separate degree of latitude. As one degree of latitude is covered within 

 an hour in an aircraft traveling north or south, it is much more convenient to have each table for one 

 degree of declination, especially as the sun's declination is not likely to change more than a degree dur- 

 ing a flight across the polar regions. 



16 The instruments used by the first Amundsen-Ellsworth flight and the flight of the Norge were 

 designed by Boykow and made by Goerz, while Commander Byrd used an instrument designed by 

 A. H. Bumstead of the National Geographic Society. (See, for the latter. Fig. 2 in Commander 

 Byrd's article, above.) 



