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timing mechanism, or averaging is necessary. It is continuously integrating altitude against elapsed time.
After at least 30 seconds, it may be stopped at any time up to 2 minutes. The average altitude is read
directly from the counter. A half-time clock will indicate the half time of the observation. The time
indication may be added directly to the time of starting the observation to compute the mean time of the
observation. At the end of the observation, the averager energizes a solenoid (actuates a lever if
unmodified) which drops a shutter across the field of view, indicating the end of the observation.
Although it is possible to utilize an instantaneous shot, the normal timed observation lasts for 2 minutes.
It is impossible to time any observation for less than 30 seconds using the sextant timer.
13.4.11. A heading scale shutter (diffuser lever) control (11) provides a convenient means of blocking
out the bright illumination on the azimuth scale for night celestial observations. The objective lens (12)
is located just above the heading scale shutter control. The lens aligns the azimuth scale of the sextant
with the longitudinal axis of the aircraft. The lens can be rotated with the fingers in order to calibrate the
azimuth scale on a known bearing while looking through the eyepiece. The objectives lens can remove
up to 2o azimuth error in the azimuth ring. A locking ring beneath the lens prevents accidental
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movement. A dial lamp located on the right side of the sextant provides three beams of light to
illuminate the averager indicators, the altitude counter, and the watch clip. The watch clip is made to
hold an old-fashioned pocket watch.
13.5. Electrical Cables. Cables provide power for sextant operation and illumination. One "Y" cable
provides power from the mount to the sextant for illumination and averager operation.
13.6. Sextant Case. The case provides shock-absorbent storage for the sextant when it is not in use. The
sextant fits into formfitting foam blocks and is secured by straps. The case also contains spare bulbs for
sextant illumination and provides storage for the electrical cable.
Section 13B— Errors of Sextant Observation
13.7. Basics. If collimation of the body with the bubble and reading the sextant were all that had to be
done, celestial navigation would be simple. This would mean LOPs that are accurate to within 1 or 2
miles could be obtained without any further effort. Unfortunately, considerable errors are encountered in
every sextant observation made from an aircraft. A thorough understanding of the cause and magnitude
of these errors, as well as the proper application of corrections to either Hc or Hs, will help minimize
their effects. Remember that any correction applied to the Hs may be applied to the Hc with a reverse
sign. Accuracy of celestial navigation depends upon thorough application of these corrections together
with proper shooting techniques. The errors of sextant observation may be classified into four groups:
(1) parallax, (2) refraction, (3) acceleration, and (4) instrument.
13.8. Parallax Error. Parallax in altitude is the difference between the altitude of a body above a bubble
horizon at the surface of the earth and its calculated altitude above the celestial horizon at the center of
the earth. All Hcs are given for the center of the earth. If the light rays reaching the earth from a celestial
body are parallel, the body has the same altitude at both the center and the surface of the earth. For most
celestial bodies, parallax is negligible for purposes of navigation.
13.8.1. Parallax Correction for the Moon. The moon is so close to the earth that its light rays are not
parallel. The parallax of the moon may be as great as 1o; thus, when observing the moon, a parallax
correction must be applied to the Hs. This correction is always positive (+) and varies with the altitude
and with the distance of the moon from the earth. The correction varies from day to day because the
distance of the moon from the earth varies. Corrections for the moon's parallax in altitude are given on
the daily pages of the Air Almanac and are always added, algebraically, to sextant altitudes. The values
of parallax for negative altitudes are obtained from the Air Almanac for the equivalent positive altitudes.
13.8.2. Semidiameter Correction. This correction is found on the daily pages of the Air Almanac.
Apply it when shooting the upper or lower limb of the moon or the sun.
13.8.2.1. It is more likely to occur on observations of the moon because, when the moon is not full
(completely round), the center is difficult to estimate. Shoot either the upper or lower limb and apply the
semidiameter correction listed on the Air Almanac page for the time and date of the observation.
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