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 Apply proper procedures to determine the observed altitude (Ho) of a celestial the observed altitude (Ho) of a celestial body.body.

The Marine SextantThe Marine Sextant

• 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 planet)a celestial body (star, sun, moon, or 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 eyehorizon, due to the observer’s height of eye– adjustment to equivalent reading at the adjustment to equivalent reading at the

center of the earth and the center of the bodycenter of the earth and the center of the body– refractive effects of the earth’s atmosphererefractive effects of the earth’s atmosphere

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 sextant itself; this is known as index error.error.

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

Index CorrectionIndex Correction

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

Index CorrectionIndex Correction

• 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 correction, we can determine the apparent altitude (ha).apparent altitude (ha).

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

Altitude CorrectionAltitude Correction

• The last major correction accounts for The last major correction accounts for the refractive effects of the earth’s the 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 correction and is tabulated inside the front cover of the front 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 under normal circumstances to determine Ho of a determine Ho of a starstar..

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

Additional CorrectionsAdditional Corrections

• If we are using the sun, moon, or planets, the If we are using the sun, moon, or planets, the problem becomes a bit more complicated.problem becomes a bit more 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, and horizontal parallax (sun, moon, Venus, and

Mars)Mars)– semidiameter of the body (sun and moon)semidiameter of the body (sun and 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...