Radical changes in the Nautical Almanac to meet the needs of the airman are open to criticism by the mariner; however, in order to meet the demands of aviation personnel, it has been decided to design an air almanac especially suited for their use. Since the principle of celestial air navigation is the same as that for marine navigation, and since the same accuracy is built into the Air Almanac, it may be possible that the Air Almanac will ultimately replace the Nautical Almanac for marine use also.
The principal features incorporated in the Air Almanac, appearing in 1933, are:
- The elimination of equation of time, right ascension, and sidereal time.
- The elimination of the present sunrise and sunset, and moonrise and moonset tables. The sunrise and sunset tables are replaced by a simple diagram from which the local civil time of sunrise or sunset are taken, using the date and latitude as arguments. The latitude is carried to 75°, for both north and south latitudes. The present moonrise and moonset tables are incorporated directly in the expanded lunar ephemeris, giving at a glance whether or not the moon is available, and giving the phenomena for south as well as for north latitudes.
- The lunar ephemerides are given for every ten minutes as in the Lunar Ephemeris for Aviators. This practically eliminates all tedious interpolations.
- Instead of tabulating the equation of time of the sun, and the right ascension for the other bodies from which the hour angle is computed, the G.H.A. is given direct for convenient intervals of G.C.T.
- The tabulation of the elements is given for every 10-minute interval for the moon, for every hour for the sun, and for every day for the stars and for the planets. Suitable interpolation tables are included so that an additive correction to the tabulated G.H.A. gives the exact G.H.A. without further mental calculations. The declination is given for each tabulation, and therefore requires practically no interpolation.
- The utilization of both front and back cover pages. This gives four ready indexed pages which give the interpolation tables for the sun, moon, and stars, and the altitude correction tables for the bubble sextant for all bodies.
- A slight reduction in the dimensions of the present Nautical Almanac, with approximately the same number of pages.
In order to show the clear-cut saving which results in the use of the Air Almanac, some examples will be given by the Air Almanac compared with the Nautical Almanac. It is estimated that about 25 per cent of the work of determining a fix is saved by using the Air Almanac. An additional saving may be made by the proper use of a second setting watch to get the exact second of G.C.T. If the moon is observed at an even 10-minute interval, no correction is made to the tabulated hour angle and declination. If the sun or star or planet is observed on the even minute, the correction is facilitated.
As an accuracy test of the sunrise diagram to be included in the Air Almanac, as drawn to double scale, five problems were worked by five experienced men. This was the first attempt by each, and the use of the diagram required practically no instructions.
These results demonstrate that the diagram will afford ample accuracy. It will be noted that in 25 separate solutions only 1 differed by 1 minute, and only 3 other solutions differed 0.5 minute from the values given in the tables.
The appearance of the sunrise and sunset diagrams is shown in Fig. 1 which gives the sunrise for March 21 to September 21. To use, enter with proper date, move up or down to the observer’s latitude on the curves, thence to either border scale and read the local civil time of sunrise. To convert civil to standard or zone time, add 4 minutes for each degree west of the standard meridian, and subtract 4 minutes for each degree east of the standard meridian.
The duration of twilight is graphed with the same arguments as are used in the sunrise diagram, and is used in the same way. The duration of twilight instead of the local civil time is picked off. Six pages of instructions and diagrams together with the data inserted in the moon ephemeris, serves to replace about 42 pages of the Nautical Almanac.
The separate tables of transits of stars and planets are not required in the Air Almanac since the tabulation of hour angle gives a convenient means for computing the times of transits. The G.H.A. for 0 hours G.C.T. is given for each day. The Greenwich transit for any day is the instant the body transits the meridian of Greenwich. The tabulated hour angle subtracted from 360° therefore gives the hour angle to be swept through before the time of transit, and the interpolation tables provided gives this elapsed time direct. For example, if a star’s G.H.A. on any date is 240°, the time of its Greenwich transit is found by subtracting 240 from 360, giving 120°, and then by entering the star hour angle interpolation table on page 2, pick out the time corresponding to an hour angle of 120°, which in this case is 7 hrs. 59.6 min. or the G.C.T. of Greenwich transit.