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Advanced Navigation
Sextant
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Objectives
1. Describe the parts of a marine sextant. 2.
Demonstrate how to read a sextant. 3. Describe
how to observe celestial bodies with a
sextant.
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Davis Quadrant circa 1775
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The parts of a sextant

• FRAME
• Made of strong material and is the structure on
  which all other parts are mounted.

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The parts of a sextant
HANDLE Place to hold on to the sextant so as
not to interfere with the movements of other
parts.
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The parts of a sextant

• TELESCOPE Magnifies both objects being viewed.
  Greatly increases accuracy and makes stars and
  planets easier to see.

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The parts of a sextant
INDEX MIRROR Reflects the image of
the celestial body from the sky to the
horizon glass.

• LIMB Lower part of the frame, on which the arc
  of degrees is inscribed.

ARC Altitude graduations of arc
inscribed on the limb.
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The parts of a sextant

• INDEX ARM Pivots at the top of the frame.
  Mounted on the top is the index mirror. The
  bottom of the arm has a pointer to indicate the
  number of
• degrees of separation between objects.

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The parts of a sextant

• HORIZON GLASS Permanently mounted circular
  piece of glass about the size of a half dollar.
  The left half of the glass is clear. The right
  half of the glass is mirrored. When observing
  a celestial body, the horizon is seen
• through the left side, while
• the body is viewed on
• the right side in the mirror
• as reflected down from the
• index mirror.

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The parts of a sextant

• INDEX and HORIZON SHADES Similar to sunglasses,
  these are used to reduce the intensity of bright
  objects, such as the sun and the moon or the
  glare of the the sun on the water.

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The parts of a sextant

• TANGENT SCREW Screw gear on the end of the
  micrometer drum shaft. It engages
• the teeth of the
• limb.

MICROMETER DRUM Allows readings to a minute of
arc
(between whole degrees on the arc).
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The parts of a sextant

• VERNIER Adjacent to the micrometer drum and
  fixed to the index arm. Allows readings to a
  tenth of a
• minute of arc

RELEASE LEVERS By compressing the release
levers the tangent screw can be disengaged,
allowing movement of the index arm along
the arc.
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Reading a sextant

• Degrees are measured by noting the position of
  the arrow on the index arm in relation to the
  arc. Read the whole number of degrees - DO NOT
  ROUND UP.

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Reading a sextant

• Minutes are measured on the micrometer drum by
  noting the position of the 0 on the vernier (or
  longer mark, signifying zero) in relation to the
  graduations on the drum. Again, read the whole
  number of minutes - DO NOT ROUND UP.

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Reading a sextant

• Tenths (or sometimes fifths) of a minute are
  measured by noting which mark on the vernier
  lines up with a mark on the micrometer drum -
  CHOOSE THE BEST FIT.

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Reading a sextant
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Hs 29º 42.5
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Determining Error

• Before a sextant can be used to sight bodies, we
  must find Index Error (IE).
• Set the index arm and micrometer drum for 0º,
  and observe the horizon.....

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Determining Error

• If the image in the horizon glass and horizon
  mirror are not a straight line,
• Adjust the micrometer drum to achieve the
  effect.....

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Determining Error

• Read the IE off the micrometer.....

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5
0
1
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What is the Index Error?
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IE 39.8
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Observing celestial bodies

• After IE is known, determine the bodies available
  for observation from a starfinder or CAPN.
• Record the Hc (altitude) and Zn (true bearing)
  for at least five or six different first
  magnitude stars. Like visual land navaids,
  optimum spread in bearing is 60º.

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Observing celestial bodies

• For the first star, set the sextant to the
  altitude and look on the true bearing (using your
  gyrocompass).
• Once the star is sighted, adjust the micrometer
  drum (or release levers, if necessary) to bring
  the star into view.

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Observing celestial bodies

• With the star in view, adjust the micrometer
  drum in order to bring the body down to the
  horizon and fine tune your sighted altitude
  (Hs) reading.

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Questions?
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Compliments of Curtis Senior High School NJROTC