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Subtract the correction from the Hs when shooting the upper limb; add the correction to the Hs when
shooting the lower limb. Reverse the sign if applying the correction to the Hc.
13.8.2.2. Listed on the same page is the semidiameter correction for the sun, which is applied the same
way as for the moon.
284 AFPAM11-216 1 MARCH 2001
EXAMPLE: Using Figure 13.5 extract the corrections for the upper limb of the moon as observed on 11
August 1995 at 1100Z is 33o41'. Apply these corrections as:
Hs 33o41'
Parallax +49'
Semidiameter -16'
Ho 34o14'
13.9. Atmospheric Refraction Error. Still another factor to be taken into consideration is atmospheric
refraction. If a fishing pole is partly submerged under water, it appears to bend at the surface. The
bending of light rays as they pass from the water into the air causes this appearance. This bending of the
light rays, as they pass from one medium into another, is called refraction. The refraction of light from a
celestial body as it passes through the atmosphere causes an error in sextant observation.
13.9.1. As the light of a celestial body passes from the almost perfect vacuum of outer space into the
atmosphere, it is refracted as shown in Figure 13.6 so that the body appears a little higher above the
horizon than it really is. Therefore, the correction to the Hs for refraction is always negative. The higher
the body above the horizon, the smaller the amount of refraction and, consequently, the smaller the
refraction correction. Moreover, the greater the altitude of the aircraft, the less dense the layer of
atmosphere between the body and the observer; hence, the less the refraction.
13.9.2. The appropriate correction table for atmospheric refraction is listed inside the back cover of all
four books used for celestial computations; namely, the Air Almanac and each of the three volumes of
Pub. No. 249. This table, shown in Figure 13.7, lists the refraction for different observed altitudes of the
body and for different heights of the observer above sea level. The values shown are subtracted from Hs
or added to Hc.
13.10. Acceleration Error. Presently, the only practical and continuously available reference datum for
the definition of the true vertical is the direction of the gravitational field of the earth. Definition of this
vertical establishes the artificial horizon. It is also fundamental that the forces caused by gravity cannot
be separated by those caused by accelerations within the sextant. A level or centered bubble in the
sextant indicates the true vertical only when the instrument is at rest or moving at a constant velocity in
a straight line. Any outside force (changes in GS or changes in track) will affect the liquid in the bubble
chamber and, consequently, displace the bubble.
13.10.1. When the sextant is moved in a curved path (Coriolis, changes in heading, rhumb line) or with
varying speed, the zenith indicated by the bubble is displaced from the true vertical. This presents a false
artificial horizon above which the altitude of the celestial body is measured. Since the horizon used is
false, the altitude measured from it is erroneous. Therefore, the accuracy of celestial observations is
directly related to changes in track and speed of the aircraft. Acceleration errors have two principal
causes: changes in GS and curvature of the aircraft's path in space.
AFPAM11-216 1 MARCH 2001 285
Figure 13.5. Correction for Moon's Parallax.
286 AFPAM11-216 1 MARCH 2001
Figure 13.6. Error Caused by Atmospheric Refraction.
Figure 13.7. Corrections for Atmospheric Refraction.
13.10.2. The displacement of the liquid and the bubble in the chamber may be divided into two vectors
and each vector may be considered separately. These vectors may be thought of as a lateral vector (along
the wings) and a longitudinal vector (along the nose-tail axis of the aircraft). Any change in GS can
AFPAM11-216 1 MARCH 2001 287
cause a longitudinal displacement. This change can be brought about by a change in the airspeed or the
wind encountered, or the change in GS brought about by a change in heading due to other factors (gyro
precession, rhumb line error, etc.). A lateral displacement results from a number of causes, most of
which will occur in spite of any efforts to hold them in check. These causes are Coriolis, rhumb line, and
wander errors.
13.11. Coriolis Force. Any free-moving body traveling at a constant speed above the earth is subject to
an apparent force that deflects its path to the right in the Northern Hemisphere and to the left in the
Southern Hemisphere. This apparent force and the resulting acceleration were first discovered shortly
before the middle of the 19th century by Gaspard Gustave de Coriolis (1792-1843) and given
　
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