LESSON 19:The Marine Sextant, and Determination of Observed AltitudeLearning ObjectivesKnow the purpose of a marine sextant.Apply proper procedures to determine the observed altitude (Ho) of a celestial body.

The Marine SextantA marine sextant is nothing more than a device designed to measure, with a great deal of precision, the angle between two objects.In celestial navigation, these objects are a celestial body (star, sun, moon, or planet)the visible horizon.

Use of the SextantA 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...

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.

Determination of Observed Altitude (Ho)These corrections account for the following:index error (error in the sextant itself)difference between visible and celestial horizon, due to the observers height of eyeadjustment to the equivalent reading at the center of the earth and the center of the bodyrefractive effects of the earths atmosphere

Determination of HoThe corrections needed to convert from the sextant altitude (hs) to observed altitude (Ho) are1. Index Correction (IC) - sextant error2. Dip (D) - height of eye (fm Nautical Almanac)3. Altitude Correction (Alt Corr) -refractive effects of the atmosphere

1. Index Correction (IC)Error present in the sextant itself is known as index error (IC).This error is easily determined by setting the sextant to zero and observing the horizon; if there is no error, the view looks like that of the following slide...

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

Index CorrectionTo account for this sextant error, we apply an index correction (IC).This correction number is a function of the individual sextant itself.

2. Dip Correction (D)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 (D).Values of the dip correction are tabulated inside the front cover of the Nautical Almanac.

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

ha = hs + IC + D

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

3. Altitude CorrectionThe third correction accounts for the refractive effects of the earths atmosphere.Known as the altitude correction, it is tabulated inside the front cover of the Nautical Almanac.Ho = ha + Alt Corr

Altitude Correction

Determination of HoAgain, the corrections needed to convert from the sextant altitude (hs) to observed altitude (Ho) wereIC (index correction, from sextant error)D (dip, from height of eye)Alt Corr (altitude correction, from refractive effects)

Additional CorrectionsThese 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.

Additional CorrectionsIf 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 forhorizontal parallax (sun, moon, Venus, Mars)semidiameter of the body (sun and moon)augmentation (moon)

Additional CorrectionsThese additional corrections make determination of Ho for the sun, moon, and planets generally more difficult than those for a star.For simplicitys sake, well stick to determination of Ho for a star.

Use of a Strip ChartTo aid in making any calculations in celestial navigation, 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...