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from the Greenwich meridian to the observer's meridian.
8.8.1. The following are examples of converting a body's celestial coordinates to its subpoint's terrestrial
coordinates. If the GHA is less than 180o, then the subpoint is in the Western Hemisphere and GHA
equals longitude. When the GHA is greater than 180o, the subpoint is in the Eastern Hemisphere and
longitude equals 360o – GHA. Again, Dec and latitude are equal (Figure 8.6).
Dec S13o-15'—Lat 13o-15' S
GHA 135o-00'—Longitude 135o-00' W
Dec N11o-32'—Lat 11o-32' N
GHA 290o-00'—Longitude 070o-00' E
Figure 8.6. Declination of a Body Corresponds to a Parallel of Latitude.
8.8.2. You will use two other hour angles in celestial navigation in addition to GHA (Figure 8.7), local
hour angle (LHA), and sidereal hour angle (SHA). LHA is the angular distance from the observer's
celestial meridian clockwise to the hour circle. LHA is computed by applying the local longitude to the
GHA of the body. In the Western Hemisphere, LHA equals GHA - W Long, and in the Eastern
Hemisphere, LHA equals GHA + E Long (Figure 8.8). When the LHA is 0, the body's hour circle and
the upper branch of the observer's celestial meridian are collocated, and the body is in transit. If the LHA
is 180, the hour circle is coincident with the lower branch of the observer's celestial meridian. SHA is
used with the first point of Aries. The first point of Aries is the point where the sun appears to cross the
celestial equator from south to north on the vernal equinox or first day of spring. Though not absolutely
stationary relative to the stars, Aries moves so slowly that we consider it fixed on the celestial equator
for as long as a year. The SHA is the angular measurement from the hour circle of Aries to the star's
hour circle (Figure 8.9). Aries and the stars move together so the SHA remains constant for a year.
204 AFPAM11-216 1 MARCH 2001
Figure 8.7. Greenwich Hour Angle.
Figure 8.8. Local Hour Angle.
AFPAM11-216 1 MARCH 2001 205
Figure 8.9. Sidereal Hour Angle.
Section 8D— Use of the Air Almanac
8.9. Basics. Although the Air Almanac contains astronomical amounts of data, most of it is devoted to
tabulating the GHA of Aries and the GHA and Declination (Dec) of the sun, moon, and the three
navigational planets most favorably located for observation. Enter the daily pages with Greenwich date
and GMT to extract the GHA and Dec of a celestial body.
8.10. Finding GHA and Dec. The GHA is listed for 10-minute intervals on each daily sheet. If the
observation time is listed, read the GHA and Dec directly under the proper column opposite the time.
8.10.1. For example, find the sun's GHA and Dec at GMT 0540 on 11 August 1995 (Figure 8.10). The
GHA is 263o-41' and Dec is N 15o-24'. (Extractions of GHA and Dec are to the nearest whole minute.)
To convert these values to the subpoint's geographical coordinates, latitude is North 15o-24'. When GHA
is greater than 180o, subtract it from 360o to get east longitude. The subpoint's longitude in this example
is (360o-00' minus 263o-41') East 96o-19'.
8.10.2. When you don't observe at a 10-minute interval, use the time immediately before the observation
time. Then use the Interpolation of GHA table on the inside front cover of the Air Almanac or the back
of the star chart and add the increment to the GHA (Figure 8.11).
8.10.3. For example, on 11 August 1995, you observe the sun at 1012 GMT. Enter Figure 8.10 to find
the GHA listed for 1010 (331o-11'). Since the observation was 2 minutes after the listed time, enter the
Interpolation of GHA table (Figure 8.11) and find the correction listed for 2 minutes of time (30'). Add
this correction to the listed GHA to determine the sun's exact GHA at 1012 (331o-41'). The Dec for the
same time is N 15o-21'. Thus, at the time of the observation, the subpoint of the sun is at latitude 15o-21'
N, longitude (360o-00' minus 331o-41') 028o-19' E.
206 AFPAM11-216 1 MARCH 2001
Figure 8.10. Daily Page From Air Almanac—11 August 1995.
AFPAM11-216 1 MARCH 2001 207
8.10.4. You can find the GHA and Dec of a planet in almost the same way as the sun. Because the
planet's Dec change slowly, they are recorded only at hourly intervals. Use the Dec listed for the entire
hour. For example, to find the GHA and Dec of Jupiter at 1109 GMT, 11 August 1995, enter the correct
daily page (Figure 8.10) for the time of 1100 GMT. The GHA is 240o-35' and the Dec is S20o-40'. Enter
the Interpolation of GHA table under sun, etc., (Figure 8.11) to get the adjustment for 9 minutes of time,
2o-15'. Therefore, GHA is 242o-50'. Jupiter's subpoint is at latitude 20o-40' S, longitude (360o-00' minus
242o-50') 117o-10' E.
8.10.5. If you need to find an accurate GHA and Dec without the Air Almanac, you can find the
　
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