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The Marine Sextant

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... procedures to determine the observed altitude (Ho) of a celestial body. ... a celestial body (star, sun, moon, or planet) the visible horizon. Use of the Sextant ... – PowerPoint PPT presentation

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Title: The Marine Sextant


1
The Marine Sextant
  • Learning Objectives
  • Know the purpose of a marine sextant.
  • Apply proper procedures to determine the observed
    altitude (Ho) of a celestial body.

2
The Marine Sextant
  • A marine sextant is nothing more than a device
    designed to measure the angle between two objects
    with a great deal of precision.
  • In celestial navigation, these two objects are
  • a celestial body (star, sun, moon, or planet)
  • the visible horizon

3
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4
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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6
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.

7
Determination of Observed Altitude (Ho)
  • These corrections account for
  • index error (error in the sextant itself)
  • difference between visible and celestial horizon,
    due to the observers height of eye
  • adjustment to equivalent reading at the center of
    the earth and the center of the body
  • refractive effects of the earths atmosphere

8
Index Correction
  • There may be some error present in the sextant
    itself this is known as index error.
  • This is easily determined by setting the sextant
    to zero and observing the horizon if there is no
    error, the view looks like the next slide...

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10
Index Correction
  • However, often there is a slight error. In this
    case, the view looks a little different

11
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12
Index Correction
  • To account for this error, we apply an index
    correction (IC).

13
Dip Correction
  • 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.
  • The need for this correction is illustrated on
    the next slide...

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15
Dip Correction
  • The dip correction is dependent upon the
    observers height of eye.
  • Values of the dip correction are tabulated inside
    the front cover of the Nautical Almanac.

16
Apparent Altitude
  • Now, by applying the IC and the dip correction,
    we can determine the apparent altitude (ha).
  • Simply put,
  • ha hs IC dip

17
Altitude Correction
  • The last major correction accounts for the
    refractive effects of the earths atmosphere.
  • This correction is known as the altitude
    correction and is tabulated inside the front
    cover of the Nautical Almanac.
  • The next slide illustrates the need for this
    correction...

18
Altitude Correction
19
Determination of Ho
  • The corrections needed to convert from the
    sextant altitude (hs) to observed altitude (Ho)
    are
  • IC (sextant error)
  • Dip (height of eye)
  • Altitude (refractive effects)

20
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.

21
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, and Mars)
  • semidiameter of the body (sun and moon)
  • augmentation (moon)

22
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

23
Use of a Strip Chart
  • To walk us through the calculation, 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...

24
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