LESSON 19: The Marine Sextant, and Determination of Observed Altitude Learning ObjectivesLearning Objectives –Know the purpose of a marine sextant. –Apply proper procedures to determine the observed altitude (Ho) of a celestial body.

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

Use of the Sextant A sextant is used to determine the sextant altitude (hs) of a celestial body.A 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.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...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.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:These corrections account for the following: –index error (error in the sextant itself) –difference between visible and celestial horizon, due to the observer’s height of eye –adjustment to the equivalent reading at the center of the earth and the center of the body –refractive effects of the earth’s atmosphere

Determination of Ho The corrections needed to convert from the sextant altitude (hs) to observed altitude (Ho) areThe corrections needed to convert from the sextant altitude (hs) to observed altitude (Ho) are 1. Index Correction (IC) - sextant error 2. Dip (D) - height of eye 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).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...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 Correction Often, however, the sextant has a slight error. In this case, the view is as follows:Often, however, the sextant has a slight error. In this case, the view is as follows:

Index Correction To account for this sextant error, we apply an index correction (IC).To account for this sextant error, we apply an index correction (IC). This correction number is a function of the individual sextant itself.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.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).This is known as the dip correction (D). Values of the dip correction are tabulated inside the front cover of the Nautical Almanac.Values of the dip correction are tabulated inside the front cover of the Nautical Almanac.

Apparent Altitude Now, by applying the index correction (IC) and the dip correction (D), we can determine the apparent altitude (ha).Now, 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.Note that this is not yet the observed altitude (Ho) required for our calculations.

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

Altitude Correction

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

Additional Corrections These corrections are all that are needed under normal circumstances to determine Ho of a star.These 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.An additional correction is required if the observation is made under non- standard conditions of temperature or pressure.

Additional Corrections If we are using the sun, moon, or planets, the problem becomes a bit more complicated.If 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 forIn addition to the corrections we already mentioned, we must also accout for –horizontal parallax (sun, moon, Venus, Mars) –semidiameter of the body (sun and moon) –augmentation (moon)

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

Use of a Strip Chart To aid in making any calculations in celestial navigation, we normally use a form called a strip chart.To 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...An example of a strip chart used for calculating Ho of Dubhe is shown on the next slide...