A formidable and mighty air conflict rages along the entire battle front of the Old World, necessitating for the victorious combatants all expertness in the clever tricks of rapid maneuvering, together with every known short cut for determining the wide changes in geographical position. Celestial navigation plays an important role, and, as a matter of interest, there are portrayed here the different methods of procedure employed by the opposing flyers to win advantage. Our naval and commercial pilots are now familiar with the new American Air Almanac of 1941, with its convenient material, excellent arrangement, and ease of manipulation.
The United States was the pioneer in the production of the Air Almanac. In 1932, the U. S. Navy brought forth the initial edition of this type of work containing a new feature, in its tabulation of the Greenwich hour angle for all celestial bodies. The original Almanac, while containing the necessary material, proved to be ill arranged, unsuitable, and impractical for air purposes. In 1936 France, after some study of the American method, improved the Almanac greatly by furnishing perforated sheets together with all the data for sun, moon, planets, and stars recorded on one daily sheet. The elements of the celestial bodies were tabulated for every 20 minutes of Greenwich mean time and opposite, in juxtaposition, was tabulated the corresponding Greenwich sidereal time. Every heavenly body, including the sun, was recorded for its versascension (360° minus right ascension). This combination of Greenwich sidereal time with versascension supplied the required Greenwich hour angle for any heavenly body. Therefore the complete process was standardized and every kind of observation was worked and solved in the same identical manner.
For unknown reasons, the British lagged behind in the production of an air almanac. In 1937, after an exhaustive study of both the American and French editions, they incorporated the better ideas of both publications and issued a trial almanac for their flyers. In 1939, after all its flaws were completely ironed out and the chaff eliminated in service tests, they produced an air almanac, which for size, convenience, necessary material, and excellent arrangement appeared ideal. The American Air Almanac of 1941 is almost an exact replica of this British edition.
The German Air Almanac is unfamiliar to American aviators. It was also issued in 1939, and is identical in size with the British edition. It is printed quarterly, has perforated pages; the daily work sheet contains all elements of sun, moon, planet, stars and Aries, and is tabulated for every 20° (lh20m) of Greenwich civil time, thus making but 18 tabulations of Greenwich hour angle. There is given on the same page an auxiliary interpolation table from 1° (4m) to 20° for all utilized celestial bodies. A critical form of table is also shown for the declination, tabulated for G.H.A. from 40° (2h40m) to 360°. The time of sunrise, sunset, and duration of twilight is indicated for different latitudes, also moon- rise and moonset for Greenwich, with interpolation corrections for 10° of longitude, and the moon and sun’s horizontal parallax and semidiameter. On the reverse side of the daily sheet the G.H.A. is tabulated for the 16 brightest stars for G.C.T. 20° (lh20m) to 360°. A page on the back cover of the Almanac gives for all other navigational stars the sidereal hour angle, which, combined with the hour angle of Aries, gives the G.H.A. of any recorded star.
[GRAPH 1,2,3]
In the German Almanac, the Greenwich hour angle for all bodies is tabulated westward from the lower branch of the Greenwich meridian, hence differs from all other types of Air Almanacs by 180°. Thus where the American tabulates the G.H.A. as 270°, the German records it as 90°, which appears to be a good suggestion, since all time begins at midnight. The German aviator uses a navigation watch reading to degrees, minutes, and tenths of arc and is set to G.C.T., somewhat like the type shown in the illustration. The dial consists of three circles of different colors with watch hands of corresponding color. The inner circle is red with a red pointing hand, the middle circle is black and white, with a black pointing hand, while the outer circle is blue with a blue pointing hand. A fourth hand acts as a stop watch. The middle circle is graduated in steps of 10° of arc from 0° to 360° and the black pointer makes one revolution in a day. The red pointer of the interior (red) circle makes one revolution in the same time that the black pointer covers one division on the middle circle. Three hundred divisions on the outer circle (blue) are each equal to an arc of 0.2' each, and the blue pointer that moves over it divides the degrees marked on the inner circle into minutes and tenths of arc. Thus the reading on the black hand is 70°, on the red hand 8°, on the blue hand 15/8 or the total reading is 78°-15.'8.
[IMAGE]
With the hour angle and declination found from the Air Almanac, an epitomized navigational table is entered to give directly the altitude and azimuth from which the line of position is plotted. The Germans use the “F” table, a process similar to that of H.O. 208. The British and Americans adopt the fast process as given in H.O. 214.
The following problems demonstrate the operation of the tables:
[EQUATIONS]