LESSON 19:LESSON 19:The Marine Sextant, and The Marine Sextant, and

Determination of Observed AltitudeDetermination of Observed Altitude

• 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, with a device designed to measure, with a great deal of precision, the angle great deal of precision, the angle between two objects.between two objects.

• In celestial navigation, these objects are In celestial navigation, these objects are – a celestial body (star, sun, moon, or planet)a celestial body (star, sun, moon, or planet)– the visible horizon.the visible horizon.

Use of the SextantUse of the Sextant

• A sextant is used to determine the A sextant is used to determine the sextant altitude (hs) of a celestial body.sextant 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 We must make some corrections to hs to come up with the Ho, which we hs to come up with the Ho, which we need to use the altitude-intercept need to use the altitude-intercept method. method.

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

• These corrections account for the These corrections account for the following:following:– 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 the equivalent reading at the adjustment to the 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

Determination of HoDetermination of Ho

• The corrections needed to convert The corrections needed to convert from the sextant altitude (hs) to from the sextant altitude (hs) to observed altitude (Ho) areobserved altitude (Ho) are1. Index Correction (IC) - sextant error1. Index Correction (IC) - sextant error

2. Dip (D) - height of eye2. Dip (D) - height of eye

3. Altitude Correction (Alt Corr) -3. Altitude Correction (Alt Corr) -refractive effects of the atmosphererefractive effects of the atmosphere

1. Index Correction (IC)1. Index Correction (IC)

• Error present in the sextant itself is Error present in the sextant itself is known as index error (IC).known as index error (IC).

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

Index CorrectionIndex Correction

• Often, however, the sextant has a Often, however, the sextant has a slight error. In this case, the view is slight error. In this case, the view is as follows:as follows:

Index CorrectionIndex Correction

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

• This correction number is a function This correction number is a function of the individual sextant itself.of the individual sextant itself.

2. Dip Correction (D)2. Dip Correction (D)

• 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 (D).This is known as the dip correction (D).

• Values of the dip correction are Values of the dip correction are tabulated inside the front cover of the tabulated inside the front cover of the Nautical AlmanacNautical Almanac..

Apparent AltitudeApparent Altitude

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

ha = hs + IC + Dha = hs + IC + D

• Note that this is not yet the observed Note that this is not yet the observed altitude (Ho) required for our calculations.altitude (Ho) required for our calculations.

3. Altitude Correction3. Altitude Correction

• The third correction accounts for the The third correction accounts for the refractive effects of the earth’s refractive effects of the earth’s atmosphere.atmosphere.

• Known as the altitude correction, it is Known as the altitude correction, it is tabulated inside the front cover of the tabulated inside the front cover of the Nautical AlmanacNautical Almanac..

Ho = ha + Alt CorrHo = ha + Alt Corr

Altitude CorrectionAltitude Correction

Determination of HoDetermination of Ho

• Again, the corrections needed to Again, the corrections needed to convert from the sextant altitude (hs) convert from the sextant altitude (hs) to observed altitude (Ho) wereto observed altitude (Ho) were– IC (index correction, from sextant error)IC (index correction, from sextant error)– D (dip, from height of eye)D (dip, from height of eye)– Alt Corr (altitude correction, from Alt Corr (altitude correction, from

refractive effects)refractive effects)

Additional CorrectionsAdditional Corrections

• These corrections are all that are These corrections are all that are needed under normal circumstances needed under normal circumstances to determine Ho of a to 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 standard conditions of temperature or pressure.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, Mars)horizontal parallax (sun, moon, Venus, 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, determination of Ho for the sun, moon, and planets generally more moon, and planets generally more difficult than those for a star.difficult than 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 star.determination of Ho for a star.

Use of a Strip ChartUse of a Strip Chart

• To aid in making any calculations in To aid in making any calculations in celestial navigation, we normally use celestial navigation, we normally use a form called a strip chart.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 calculating Ho of Dubhe is shown on the next slide...the next slide...