Of the following five altitude corrections, the first three apply to
observations of all celestial bodies. The last two corrections are
applicable only when the observed body belongs to the solar system.
Figure 9-9 illustrates the correction problem. To obtain the true altitude,
you must correct the sextant altitude of any celestial body for:
1. Index error, which is the constant instrument error caused by a lack
of perfect parallelism between the index mirror and horizon glass when
the sextant is set at 0°.
2. Refraction, which is the deviation of rays of light from a straight
line caused by Earths atmosphere.
3. Dip of the horizon, which is the difference in direction between the
visible and celestial horizons caused by the observers height above the
If the observed body belongs to the solar system, corrections must also
be made for:
4. Parallax, which is caused by the proximity of bodies of the solar
system to Earth, resulting in a difference in altitudes measured from the
surface of Earth and from the center of Earth. Such an occurrence is not
true of other heavenly bodies whose distance from Earth is considered
5. Semidiameter, which results from the nearness of bodies of the solar
system, which makes it necessary to consider the observed bodies as
appreciable size instead of as mere points of light; for example, stars.
The sextant altitude of such a body is obtained by bringing its disk
tangent to the horizon. Semidiameter correction must be applied to find
the altitude of the center.