LESSON%2019:%20The%20Marine%20Sextant,%20and%20Determination%20of%20Observed%20Altitude

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LESSON 19The 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.

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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.

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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...

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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.

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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 eye
• adjustment to the equivalent reading at the
  center of the earth and the center of the body
• refractive effects of the earths atmosphere

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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

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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...

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Index Correction

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

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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.

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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.

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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.

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3. Altitude Correction

• The 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

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Altitude Correction
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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)

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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.

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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)

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Additional Corrections

• These 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.

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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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