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procedures and applicable tables in Pub. No. 249, Volume 1 for Aries or Volume 2 or 3 for the sun.
8.11. Finding GHA and Dec of Moon. The moon moves across the sky at a different rate than other
celestial bodies. In the Interpolation of GHA table, the intervals for the moon are listed in the right
column where the values for the sun, Aries, and the planets are in the left column.
8.11.1. The interpolation of GHA table is a critical table and the increment is opposite the interval in
which the difference of GMT occurs. If the difference (for example, 06'-31" for the moon) is an exact
tabular value, take the upper (or right) of the two possible increments (that is, 1o-34'). The up or right
rule applies to all critical tables.
8.11.2. For example, at 1136 GMT on 11 August 1995 you observe the moon. The following
information is from the Air Almanac (Figures 8.10 and 8.11):
GHA of moon at 1130 GMT 163o 16'
GHA correction for 6 minutes 1o 27'
GHA 164o 43'
Dec S8o 00'
Thus at 1136Z, the moon's subpoint is located at S 8o-00', longitude 164o-43'W.
8.12. Finding GHA and Dec of a Star. The stars and the first point of Aries remain fixed in their
relative positions in space, so the gas of the stars and Aries change at the same rate. Rather than list the
GHA and Dec of every star throughout the day, the Air Almanac lists the GHA of Aries at 10-minute
intervals and gives the sidereal hour angle (SHA) of the stars. The GHA of a star for any time can be
found by adding the GHA of Aries and the SHA of the star. The GHA of a star is used to precomp any
star that falls within 29o (declination) of the equator using Volume 2 or Volume 3.
8.12.1. The table, STARS, is inside the front cover of the almanac and on the back of the star chart. This
table lists navigational stars and the following information for each star: the number corresponding to
the sky diagram in the back, the name, the magnitude or relative brightness, the SHA, the Dec, whether
used in Pub. No. 249, and stars that can be used with Dec tables. NOTE: If you need a higher degree of
accuracy, the SHA and Dec of the stars are listed to tenths of a degree in the Air Almanac's appendix.
8.12.2. For example, at 0124 GMT on 11 August 1995, you observe Altair. To find the GHA and Dec
look at the extracts from the tables in Figures 8.11 and 8.12.
208 AFPAM11-216 1 MARCH 2001
GHA at 0120 GMT: 339o-03'
GHA correction for 4 minutes: 1o-00'
GHA for 0124: 340o-03'
SHA Altair 62o-21'
GHA for Altair at 0124: 42o-24'
Dec Altair N8o-52'
Thus the subpoint of Altair is 08o-52' N 042o-24' W.
Figure 8.11. Interpolation of Greenwich Hour Angle, Air Almanac.
AFPAM11-216 1 MARCH 2001 209
Figure 8.12. Sidereal Hour Angle Obtained From Table.
8.13. Summary. All the celestial concepts and assumptions you've learned may help you obtain a
celestial LOP. A celestial LOP is simply a circle plotted with the center at the subpoint and a radius
equal to the distance from the observer to the subpoint. To accurately compute this distance and the
direction to the subpoint of the body, you must initially position the subpoint and then measure the
angular displacement of the body above the horizon. GHA and Dec position the body, and the sextant
measures the height above the horizon. A basic knowledge of celestial theory and LOPs will help you
appreciate celestial navigation and detect errors. The next section explains how angular displacement is
measured.
Section 8E— Celestial Horizon
8.14. Basics. You use a sextant to measure a body's angular displacement above the horizon. The
celestial horizon is a plane passing through the earth's center perpendicular to the zenith-nadir axis. The
visual horizon approximates this plane at the earth's surface. Figure 8.13 depicts the zenith-nadir axis
and the celestial horizon. The angular displacement you see through a sextant is the height observed, or
Ho. Ho is measured along the vertical circle above the horizon. The vertical circle is a great circle
containing the zenith, nadir, and celestial body. The body's altitude is the same whether measured at the
earth's surface from an artificial horizon or at the center of the earth from the celestial horizon because
these horizons are parallel and the light rays from the body are essentially parallel. Figure 8.14 shows
210 AFPAM11-216 1 MARCH 2001
that the infinite celestial sphere makes the difference in angle for light rays arriving at different points on
the earth infinitesimal.
Figure 8.13. Celestial Horizon is 90o From Observer Zenith and Nadir.
Figure 8.14. Parallel Lines Make Equal Angles With Parallel Planes.
8.14.1. The angle between light rays is called parallax. In Figure 8.15, parallax is shown at its
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