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Altitude Corrections, Continued
Name
Parallax
Description
Parallax is the difference between the altitude of a body, as measured
from Earth’s center, and its altitude (corrected for refraction and dip) as
measured from Earth’s surface. Altitude from the center of Earth is
bound to be greater than from the surface. Consequently parallax is
always a plus correction.
Parallax increases from 0° for a body directly overhead to a maximum
for a body on the horizon. In the latter instance, it is called horizontal
parallax (HP). Parallax of the Moon is both extreme and varied
because of its changing distance from Earth in its passage through its
orbit. Parallax of the Sun is small; parallax of the planets is even
smaller. For the stars, parallax is so tiny it is negligible.
Semidiameter
The true altitude of a body is measured to the center of that body.
Because the Sun and Moon are of appreciable size, the usual practice is
to observe the lower limb. Therefore, semidiameter correction must be
added. It follows, then, that if the upper limb of either body is
observed, the semidiameter correction is subtractive. Semidiameter
correction amounts to about 16 minutes of arc for either the Sun or
Moon. Stars are considered as points, and they require no semidiameter
correction. When observing a planet, the center of the planet is visually
estimated by the observer, so there is never a semidiameter correction.
Remarks
In concluding the subject of altitude corrections, remember that some
tables for altitude corrections (the Nautical Almanac, for example)
combine two or more of the corrections for refraction, parallax, and
semidiameter.
The correction for height of eye (dip) appears in a separate table for use
with all bodies. Index error, which is impossible to include in such
tables, should always be determined, recorded, marked plus or minus,
and applied before any of the tabulated corrections.
9-22
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