AIR NAVIGATION 447 



circle of equal altitude must cut this meridian at right angles; and, if 

 the sun is observed or some star with a low declination, it is a sufficient 

 approximation when flying within five degrees of the pole to draw the 

 position line as a straight line ; however, when farther away than this 

 or when the declination of a star is high, it is necessary to take the 

 curve of the position line into consideration. This may be done in 

 several ways, though perhaps the simplest is to have templates made 

 of a selection of suitable curves.^ 



The plotting of this position line (for examples, see Fig. 2) gives 

 an observer an independent check on the accuracy of his dead reckon- 

 ing, but by itself it is not an independent method of position finding. 

 If, however, position lines are plotted at about hourly intervals it 

 will be possible to obtain a rough indication as to the course being 

 taken. This applies to sun observations; but, when the flight is under- 

 taken at night or when the moon is available, the method becomes 

 much more efficient, as it is then possible to observe altitudes of two 

 or more heavenly bodies in rapid succession at azimuths which will 

 make the plotted position lines intersect with well-conditioned cuts. 

 When only one position line is plotted it is best that it should cut 

 the line of flight at an angle of 45°. Should it cut the latter at right 

 angles then it will be a good check only on the distance flown. Should 

 it on the other hand lie more or less parallel to the line of flight then 

 it is only a check on direction. 



Suitable Sextants and Their Accuracy 



In order to observe an altitude of a heavenly body the only in- 

 strument at all practical for use in aircraft is a sextant.^ Of the many 

 forms of this instrument probably the most convenient is the bubble 

 sextant, as by using it dependence on a natural horizon and correc- 

 tions for dip are avoided. 



If on the other hand the natural horizon can be seen — and it is 

 understood that in clear weather an ice field provides an excellent 

 horizon when one is flying below looo feet and that in foggy weather 

 the bank of fog does the same provided the aircraft can rise to 4000 

 feet — then the Baker sextant^ is well worth the attention of polar 



3 For use with the American Geographical Society's navigational chart of the Arctic Basin (of 

 which Fig. 2 is a section and Fig. i in Dr. Bauer's paper, above, is a reduced facsimile) the writer has 

 had prepared celluloid sheets (9x5 inches) in which 10 curves have been cut. This number has been 

 found sufficient for navigational purposes within the Arctic Circle when the declination of the heavenly 

 body observed is not more than 25°. A small table for choosing the nearest correct curve with altitude 

 and declination as arguments has also been prepared. 



« K. H. Beij: Astronomical Methods in Aerial Navigation, Natl. Advisory Committee for Aero- 

 nautics Rept. No. 198, Washington, 1924; Air Navigation Instruments ... By Command of 

 the Air Council . . Air Ministry (Directorate of Research), Air Publication 803, H. M. Sta- 



tionery Office, London, 1924. 



5 Beij, op. cit.; Air Publication 803; T. Y. Baker and L. N. G. Filon: Position Fixing in Aircraft 

 During Long-Distance Flights Over the Sea, Trans. Royal Aeronaut. Soc, No. 2, 1920. 



