A marine sextant is used to measure the angle between celestial bodies and the visible horizon in celestial navigation. It determines the sextant altitude (hs) which is then corrected for index error, dip due to observer height, and atmospheric refraction to calculate the observed altitude (Ho) used for navigation. The document outlines the procedures for using a sextant including determining the index correction, applying the dip correction, and making additional corrections when observing bodies other than stars to account for factors like parallax. A strip chart example is provided to walk through the full Ho calculation process.

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The Marine SextantThe Marine Sextant
A Brief By Lance Grindley

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The Marine SextantThe Marine Sextant
 Learning ObjectivesLearning Objectives
 Know the purpose of a marine sextant.Know the purpose of a marine sextant.
 Apply proper procedures to determine theApply proper procedures to determine the
observed altitude (Ho) of a celestial body.observed altitude (Ho) of a celestial body.

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The Marine SextantThe Marine Sextant
 A marine sextant is nothing more than aA marine sextant is nothing more than a
device designed to measure the angledevice designed to measure the angle
between two objects with a great deal ofbetween two objects with a great deal of
precision.precision.
 In celestial navigation, these two objectsIn celestial navigation, these two objects
areare
• a celestial body (star, sun, moon, ora celestial body (star, sun, moon, or
planet)planet)
• the visible horizonthe visible horizon

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Use of the SextantUse of the Sextant
 A sextant is used to determine the sextantA 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 toFirst, we have to decide which stars to
observe; this is done using a Rudeobserve; this is done using a Rude
Starfinder or other methods.Starfinder or other methods.
 When making an observation, the starWhen making an observation, the star
should look as shown in the next slide...should look as shown in the next slide...

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Determination of ObservedDetermination of Observed
Altitude (Ho)Altitude (Ho)
 We must make some corrections to hs toWe must make some corrections to hs to
come up with the Ho, which we need tocome up with the Ho, which we need to
use the altitude-intercept method.use the altitude-intercept method.

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Determination of ObservedDetermination 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 celestialdifference between visible and celestial
horizon, due to the observer’s height ofhorizon, due to the observer’s height of
eyeeye
• adjustment to equivalent reading at theadjustment to equivalent reading at the
center of the earth and the center of thecenter of the earth and the center of the
bodybody
• refractive effects of the earth’srefractive effects of the earth’s
atmosphereatmosphere

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Index CorrectionIndex Correction
 There may be some error present in theThere 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 theThis 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 theif there is no error, the view looks like the
next slide...next slide...

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Index CorrectionIndex Correction
 However, often there is a slight error. InHowever, often there is a slight error. In
this case, the view looks a little different…this case, the view looks a little different…

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Index CorrectionIndex Correction
 To account for this error, we apply anTo account for this error, we apply an
index correction (IC).index correction (IC).

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Dip CorrectionDip Correction
 Next, we must account for the differenceNext, we must account for the difference
between the celestial horizon and thebetween 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 illustratedThe need for this correction is illustrated
on the next slide...on the next slide...

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Dip CorrectionDip Correction
 The dip correction is dependent upon theThe dip correction is dependent upon the
observer’s height of eye.observer’s height of eye.
 Values of the dip correction are tabulatedValues of the dip correction are tabulated
inside the front cover of theinside the front cover of the NauticalNautical
AlmanacAlmanac..

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Apparent AltitudeApparent Altitude
 Now, by applying the IC and the dipNow, by applying the IC and the dip
correction, we can determine the apparentcorrection, we can determine the apparent
altitude (ha).altitude (ha).
 Simply put,Simply put,
ha = hs + IC + dipha = hs + IC + dip

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Altitude CorrectionAltitude Correction
 The last major correction accounts for theThe last major correction accounts for the
refractive effects of the earth’srefractive effects of the earth’s
atmosphere.atmosphere.
 This correction is known as the altitudeThis correction is known as the altitude
correction and is tabulated inside the frontcorrection and is tabulated inside the front
cover of thecover of the Nautical AlmanacNautical Almanac..
 The next slide illustrates the need for thisThe next slide illustrates the need for this
correction...correction...

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Altitude CorrectionAltitude Correction

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Determination of HoDetermination of Ho
 The corrections needed to convert fromThe corrections needed to convert from
the sextant altitude (hs) to observedthe 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)

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Additional CorrectionsAdditional Corrections
 These corrections are all that are neededThese corrections are all that are needed
under normal circumstances to determineunder normal circumstances to determine
Ho of aHo of a starstar..
 An additional correction is required if theAn additional correction is required if the
observation is made under non-standardobservation is made under non-standard
conditions of temperature or pressure.conditions of temperature or pressure.

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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 morethe problem becomes a bit more
complicated.complicated.
 In addition to the corrections we alreadyIn 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 andsemidiameter of the body (sun and
moon)moon)
• augmentation (moon)augmentation (moon)

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Additional CorrectionsAdditional Corrections
 These additional corrections makeThese additional corrections make
determination of Ho for the sun, moon,determination of Ho for the sun, moon,
and planets generally more difficult thanand planets generally more difficult than
those for a star.those for a star.
 For simplicity’s sake, we’ll stick toFor simplicity’s sake, we’ll stick to
determination of Ho for a stardetermination of Ho for a star

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Use of a Strip ChartUse of a Strip Chart
 To walk us through the calculation, weTo 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 forAn example of a strip chart used for
calculating Ho of Dubhe is shown on thecalculating Ho of Dubhe is shown on the
next slide...next slide...

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