Marine sextant lrg

Grunt Productions 2005

The Marine SextantThe Marine Sextant

A Brief By Lance Grindley



Learning ObjectivesLearning Objectives Know the purpose of a marine sextant.Know the purpose of a marine sextant. Apply proper procedures to determine the Apply proper procedures to determine the

observed altitude (Ho) of a celestial body.observed altitude (Ho) of a celestial body.



A marine sextant is nothing more than a A marine sextant is nothing more than a device designed to measure the angle device designed to measure the angle between two objects with a great deal of between two objects with a great deal of precision.precision.

In celestial navigation, these two objects In celestial navigation, these two objects are are • a celestial body (star, sun, moon, or a celestial body (star, sun, moon, or

planet)planet)• the visible horizonthe visible horizon


Use of the SextantUse of the Sextant

A sextant is used to determine the sextant A sextant is used to determine the sextant altitude (hs) of a celestial body.altitude (hs) of a celestial body.

First, we have to decide which stars to First, we have to decide which stars to observe; this is done using a Rude observe; this is done using a Rude Starfinder or other methods.Starfinder or other methods.

When making an observation, the star When making an observation, the star should look as shown in the next slide...should look as shown in the next slide...


Determination of Observed Determination of Observed Altitude (Ho)Altitude (Ho)

We must make some corrections to hs to We must make some corrections to hs to come up with the Ho, which we need to come up with the Ho, which we need to use the altitude-intercept method. use the altitude-intercept method.


Determination of Observed Determination of Observed Altitude (Ho)Altitude (Ho)

These corrections account forThese corrections account for• index error (error in the sextant itself)index error (error in the sextant itself)• difference between visible and celestial difference between visible and celestial

horizon, due to the observer’s height of horizon, due to the observer’s height of eyeeye

• adjustment to equivalent reading at the adjustment to equivalent reading at the center of the earth and the center of the center of the earth and the center of the bodybody

• refractive effects of the earth’s refractive effects of the earth’s atmosphereatmosphere


Index CorrectionIndex Correction

There may be some error present in the There may be some error present in the sextant itself; this is known as index error.sextant itself; this is known as index error.

This is easily determined by setting the This is easily determined by setting the sextant to zero and observing the horizon; sextant to zero and observing the horizon; if there is no error, the view looks like the if there is no error, the view looks like the next slide...next slide...



However, often there is a slight error. In However, often there is a slight error. In this case, the view looks a little different…this case, the view looks a little different…



To account for this error, we apply an To account for this error, we apply an index correction (IC).index correction (IC).


Dip CorrectionDip Correction

Next, we must account for the difference Next, we must account for the difference between the celestial horizon and the between the celestial horizon and the visible horizon, due to our height of eye.visible horizon, due to our height of eye.

This is known as the dip correction.This is known as the dip correction. The need for this correction is illustrated The need for this correction is illustrated

on the next slide...on the next slide...


Dip CorrectionDip Correction

The dip correction is dependent upon the The dip correction is dependent upon the observer’s height of eye.observer’s height of eye.

Values of the dip correction are tabulated Values of the dip correction are tabulated inside the front cover of the inside the front cover of the Nautical Nautical AlmanacAlmanac..


Apparent AltitudeApparent Altitude

Now, by applying the IC and the dip Now, by applying the IC and the dip correction, we can determine the apparent correction, we can determine the apparent altitude (ha).altitude (ha).

Simply put,Simply put, ha = hs + IC + dipha = hs + IC + dip


Altitude CorrectionAltitude Correction

The last major correction accounts for the The last major correction accounts for the refractive effects of the earth’s refractive effects of the earth’s atmosphere.atmosphere.

This correction is known as the altitude This correction is known as the altitude correction and is tabulated inside the front correction and is tabulated inside the front cover of the cover of the Nautical AlmanacNautical Almanac..

The next slide illustrates the need for this The next slide illustrates the need for this correction...correction...


Altitude CorrectionAltitude Correction


Determination of HoDetermination of Ho

The corrections needed to convert from The corrections needed to convert from the sextant altitude (hs) to observed the sextant altitude (hs) to observed altitude (Ho) arealtitude (Ho) are• IC (sextant error)IC (sextant error)• Dip (height of eye)Dip (height of eye)• Altitude (refractive effects)Altitude (refractive effects)


Additional CorrectionsAdditional Corrections

These corrections are all that are needed These corrections are all that are needed under normal circumstances to determine under normal circumstances to determine Ho of a Ho of a starstar..

An additional correction is required if the An additional correction is required if the observation is made under non-standard observation is made under non-standard conditions of temperature or pressure.conditions of temperature or pressure.


Additional CorrectionsAdditional Corrections If we are using the sun, moon, or planets, If we are using the sun, moon, or planets,

the problem becomes a bit more the problem becomes a bit more complicated.complicated.

In addition to the corrections we already In addition to the corrections we already mentioned, we must also accout formentioned, we must also accout for• horizontal parallax (sun, moon, Venus, horizontal parallax (sun, moon, Venus,

and Mars)and Mars)• semidiameter of the body (sun and semidiameter of the body (sun and

moon)moon)• augmentation (moon)augmentation (moon)


Additional CorrectionsAdditional Corrections

These additional corrections make These additional corrections make determination of Ho for the sun, moon, determination of Ho for the sun, moon, and planets generally more difficult than and planets generally more difficult than those for a star.those for a star.

For simplicity’s sake, we’ll stick to For simplicity’s sake, we’ll stick to determination of Ho for a stardetermination of Ho for a star


Use of a Strip ChartUse of a Strip Chart

To walk us through the calculation, we To walk us through the calculation, we normally use a form, called a strip chart.normally use a form, called a strip chart.

An example of a strip chart used for An example of a strip chart used for calculating Ho of Dubhe is shown on the calculating Ho of Dubhe is shown on the next slide...next slide...

Marine sextant lrg

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