Lesson 19: The Marine Sextant, and Determination of Observed Altitude Learning Objectives

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LESSON 19: The Marine Sextant, and Determination of Observed Altitude

  • Learning 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.

  • In 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.

  • 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 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) 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).

  • 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:

Index Correction

  • To 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)

Apparent Altitude

  • 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.

3. Altitude Correction

  • 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.

  • Ho = ha + Alt Corr

Altitude Correction

Determination of Ho

  • Again, 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.

  • 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.

  • In 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.

  • 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.

  • An example of a strip chart used for calculating Ho of Dubhe is shown on the next slide...

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