The Marine SextantGrunt Productions 2005
The Marine Sextant
Know the purpose of a marine sextant.
Apply proper procedures to determine the observed altitude (Ho) of a celestial body.
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The Marine Sextant
A marine sextant is nothing more than a device designed to measure the angle between two objects with a great deal of precision.
In celestial navigation, these two objects are
a celestial body (star, sun, moon, or planet)
the visible horizon
Grunt Productions 2005
Grunt Productions 2005
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...
Grunt Productions 2005
Grunt Productions 2005
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.
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These corrections account for
index error (error in the sextant itself)
difference between visible and celestial horizon, due to the observer’s height of eye
adjustment to equivalent reading at the center of the earth and the center of the body
refractive effects of the earth’s atmosphere
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Index Correction
There may be some error present in the sextant itself; this is known as index error.
This is easily determined by setting the sextant to zero and observing the horizon; if there is no error, the view looks like the next slide...
Grunt Productions 2005
Grunt Productions 2005
Index Correction
However, often there is a slight error. In this case, the view looks a little different…
Grunt Productions 2005
Grunt Productions 2005
Index Correction
To account for this error, we apply an index correction (IC).
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Dip Correction
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.
The need for this correction is illustrated on the next slide...
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Grunt Productions 2005
Dip Correction
The dip correction is dependent upon the observer’s height of eye.
Values of the dip correction are tabulated inside the front cover of the Nautical Almanac.
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Apparent Altitude
Now, by applying the IC and the dip correction, we can determine the apparent altitude (ha).
Simply put,
Altitude Correction
The last major correction accounts for the refractive effects of the earth’s atmosphere.
This correction is known as the altitude correction and is tabulated inside the front cover of the Nautical Almanac.
The next slide illustrates the need for this correction...
Grunt Productions 2005
Grunt Productions 2005
Determination of Ho
The corrections needed to convert from the sextant altitude (hs) to observed altitude (Ho) are
IC (sextant error)
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 non-standard conditions of temperature or pressure.
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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, and 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
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Use of a Strip Chart
To walk us through the calculation, 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...
Grunt Productions 2005
Grunt Productions 2005