LESSON 19:
The Marine Sextant, and
Determination of Observed Altitude
• Learning Objectives
– Know the purpose of a marine sextant.
– Apply proper procedures to determine
the observed altitude (Ho) of a celestial
body.
The Marine Sextant
• A marine sextant is nothing more than a
device designed to measure, with a
great deal of precision, the angle
between two objects.
• In celestial navigation, these objects are
– a celestial body (star, sun, moon, or planet)
– the visible horizon.
Use of the Sextant
• A sextant is used to determine the
sextant altitude (hs) of a celestial body.
• First, we have to decide which stars to
observe; this is done using a Rude
Starfinder or other methods.
• When making an observation, the star
should look as shown in the next slide...
Determination of Observed
Altitude (Ho)
• We must make some corrections to
hs to come up with the Ho, which we
need to use the altitude-intercept
method.
Determination of Observed
Altitude (Ho)
• These corrections account for the
following:
– index error (error in the sextant itself)
– difference between visible and celestial
horizon, due to the observer’s height of eye
– adjustment to the equivalent reading at the
center of the earth and the center of the body
– refractive effects of the earth’s atmosphere
Determination of Ho
• The corrections needed to convert
from the sextant altitude (hs) to
observed altitude (Ho) are
1. Index Correction (IC) - sextant error
2. Dip (D) - height of eye
3. Altitude Correction (Alt Corr) refractive effects of the atmosphere
1. Index Correction (IC)
• Error present in the sextant itself is
known as index error (IC).
• This error is easily determined by
setting the sextant to zero and
observing the horizon; if there is no
error, the view looks like that of the
following slide...
Index Correction
• Often, however, the sextant has a
slight error. In this case, the view is
as follows:
Index Correction
• To account for this sextant error, we
apply an index correction (IC).
• This correction number is a function
of the individual sextant itself.
2. Dip Correction (D)
• Next, we must account for the difference
between the celestial horizon and the
visible horizon, due to our height of eye.
• This is known as the dip correction (D).
• Values of the dip correction are
tabulated inside the front cover of the
Nautical Almanac.
Apparent Altitude
• Now, by applying the index correction (IC)
and the dip correction (D), we can
determine the apparent altitude (ha).
ha = hs + IC + D
• Note that this is not yet the observed
altitude (Ho) required for our calculations.
3. Altitude Correction
• The third correction accounts for the
refractive effects of the earth’s
atmosphere.
• Known as the altitude correction, it is
tabulated inside the front cover of the
Nautical Almanac.
Ho = ha + Alt Corr
Altitude Correction
Determination of Ho
• Again, the corrections needed to
convert from the sextant altitude (hs)
to observed altitude (Ho) were
– IC (index correction, from sextant error)
– D (dip, from height of eye)
– Alt Corr (altitude correction, from
refractive effects)
Additional Corrections
• These corrections are all that are
needed under normal circumstances
to determine Ho of a star.
• An additional correction is required if
the observation is made under nonstandard conditions of temperature
or pressure.
Additional Corrections
• If we are using the sun, moon, or planets,
the problem becomes a bit more
complicated.
• In addition to the corrections we already
mentioned, we must also accout for
– horizontal parallax (sun, moon, Venus, Mars)
– semidiameter of the body (sun and moon)
– augmentation (moon)
Additional Corrections
• These additional corrections make
determination of Ho for the sun,
moon, and planets generally more
difficult than those for a star.
• For simplicity’s sake, we’ll stick to
determination of Ho for a star.
Use of a Strip Chart
• To aid in making any calculations in
celestial navigation, we normally use
a form called a strip chart.
• An example of a strip chart used for
calculating Ho of Dubhe is shown on
the next slide...