Lighthouses in the Sky

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Lighthouses in the Sky
Hostile Vessel Operating in this area

• Junior Navigation
• Chapter 1

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Learning Objectives

• Define terms altitude, circle of position,
  geographical position, intercept, and celestial
  line of position.
• Given altitude, determine the radius of a circle
  of position and vice versa.
• State why accurate time is important in celestial
  navigation.
• Describe the altitude-intercept method of
  plotting a celestial line of position.

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Terms
GHA Dec
Celestial Sphere
Celestial Equator
When body is at your zenith Latitude Dec and
Longitude GHA
Apparent Line of Sight
Zenith
index correction (IC)
Observers Eye
Sensible Horizon
Ho (Altitude)
Geoidal Horizon
DR
COP
Visible Horizon
Equator
Equator
Longitude
Terrestrial Refraction
Geometrical Horizon
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Nadir
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Law of Cosines
(cos LHA x cos Lat x cos Dec) (sin Lat x sin
Dec) sin Hc convert sin Hc to Hc (calculated
sextant height) Difference between Hc and Ho
provides distance from your DR to COP. sin Dec
(sin Lat x sin Hc) / (cos Lat x cos Hc) cos
Z convert cos Z to Zn provides direction
(azimuth) to GP.
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The Fundamental Idea

• In AP, a Radar Fix was determined by plotting two
  LOPs taken from radar.

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Plotting a Circle of Position

• You need to know
• Direction from observer to the GP of the body
  and
• Distance from observer to the GP of the body but
  first
• You need to know how to convert altitude
  difference to nautical miles.

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Angular Distance

• Radius of a circle of position is equal to 90
  minus the altitude (1º latitude 60nm).

?
90º 90º 0º 0º x 60nm 0nm
90º 0º 90º 90º x 60nm 5400nm
90º 30º 60º 60º x 60nm 3600nm
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COP and Altitude

• At 1034 an observer in Galveston measures the
  suns altitude to be 7741.5. What is the
  radius of the COP?

9000.0 7741.5 1218.5 12 x 60
nm/degree 720.0 nm 18.5 x 1 nm/minute
18.5 nm Total 738.5 nm (radius of COP)
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COP and Altitude

• At the same time observer in Los Angeles measures
  the suns altitude to be 5734.1. What is the
  radius of the COP?

9000.0 5734.1 3225.9 32 x 60
nm/degree 1920.0 nm 25.9 x 1 nm/minute
25.9 nm Total 1945.9 nm (radius of COP)
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Altitude-Intercept Method

• At 10-34-15 a sight of the sun is taken.
  Recorded altitude 5734.1 COP 1,945.9nm

1034
1000
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Altitude-Intercept Method

• Calculate true bearing (azimuth) and altitude
  (Hc) from DR using Law of Cosines

Zn
1034
1000
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Altitude-Intercept Method

• Difference between Ho (observed altitude) and Hc
  (calculated altitude) is the intercept

Ho 57º 34.1 Hc 57º 24.1 Diff 10.0
towards Intercept 10.0nm
1034
When Hc is greater than Ho, your azimuth is the
reciprocal of computed azimuth.
1000

• COP drawn as a straight line perpendicular to the
  azimuth

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Altitude-Intercept Method

• Label with time of sight and name of body

1034
1034 Sun
1000
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Altitude-Intercept Method

• When you have only a single LOP, you obtain an
  estimated position (EP)

1034
1034 Sun
1000
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Quiz
1. If two observers at different DR positions
measure the altitude of the same celestial body
at the same time, a. the observer closer to the
GP of the body measures the larger altitude. b.
the observer closer to the GP of the body
measures the smaller altitude. c. both observers
measure the same altitude. d. the positions of
the observers relative to the GP cannot be
determined because the azimuths from each
observer are not given.
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Quiz
2. Polaris (the North Star) is located exactly
over the earth's north geographic pole. a.
True b. False
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Quiz
3. The vertical angle measured with a sextant
between a celestial body and the horizon is
called a. azimuth. b. intercept. c.
altitude. d. zenith.
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Quiz
4. A navigator determines the altitude of the sun
to be 3726.1'. What is the distance in nautical
miles between the navigator's position and the GP
of Sun?
Solution 90 - 3726.1 52 33.9 52 x
60nm/ 3120.0nm 33.9' x 1nm/'
33.9nm 3,153.9nm
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Quiz
5. The difference between the calculated altitude
(Hc) and the observed altitude (Ho) is
called a. azimuth. b. co-altitude. c.
altitude. d. intercept.
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Quiz
6. The method used in plotting a celestial LOP is
called the altitude-intercept method. a. True
b. False
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Quiz
7. The geographical position (GP) of a body is
defined as the point on the surface of the earth
directly beneath the center of the body. a. True
b. False
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Lighthouses in the Sky

• End of

The Sextant
Junior Navigation Chapter 2
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Learning Objectives

• Identify the parts of a sextant and understand
  how a sextant works
• Determine index error index correction
• Describe how to handle, maintain stow a sextant
• Describe techniques for taking Sun sights
• Describe safety procedures for taking sights on a
  boat
• Record the time of a sight
• Identify the ideal practical accuracy limits
• Identify erroneous sights in a run of sights
• Describe the sight requirements for JN

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Parts of the Sextant
Handle
Lanyard
Frame
Limb
Index Arm
?
Release Clamp
Arc (degrees)
Micrometer (min)
Vernier (tenths)
Horizon Glass
Index Mirror
Horizon Shades
Index Shades
Telescope
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How a Sextant Works

• The sextant set to 000.0
• The horizon will appear as an unbroken line when
  the sextant is correctly adjusted

Split Field
Full Field
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How a Sextant Works

• Telescope aimed at the horizon
• Index arm adjusted to the appropriate angle

Full Field
Split Field
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Reading a Sextant

• Accurate reading is necessary
• 0.1 of arc equals 0.1 nm
• Full turn of micro drum
• moves index arm one degree
• Vernier - auxiliary scale to interpolate the
  minute scale of micrometer drum

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Reading the Measured Angle

• First read degrees from the arc
• Then read minutes
• from micrometer drum
• Finally read tenths of minutes
• from vernier

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Reading a Measurement
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4002
4002.6
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Reading a Measurement
32.6
51.3
23.0
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Sextant Error

• Non-adjustable error
• Adjustable error
• Telescope axis - not parallel to frame
• Index mirror - not perpendicular to frame
• Horizon glass - not perpendicular to frame
• Index mirror and horizon glass are not parallel
  when sextant set to 000.0
• Checking adjustment procedures in Bowditch
• Should only be made by experienced persons
• Frequent adjustment might loosen screws

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Index Error (IE)

• IE is common
• In good quality metal sextants
• IE tends to remain fairly constant
• In plastic sextants
• Checking IE critical

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Determining Index Error (IE)

• Set sextant to 000.0 and sight on horizon
• If 2 images of horizon not superimposed OR
• If horizon shows as broken line
• IE present

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Determining Index Error (IE)

• To determine value of IE.
• Adjust micro until horizon appears as straight
  line.
• IE is the sextant reading
• If index mark is below 000.0 is off the arc
• If index mark is above 000.0 is on the arc

Split Field
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Index Correction (IC)

• IC - value applied to the altitude measured to
  correct for IE
• IC - always opposite to the sign of IE
• IE on the arc requires negative IC
• When its on, take it off
• IE off the arc requires positive IC
• When its off, put it on

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Index Error

• When the horizon line is continuous, the index
  mark is between 0 and 1 and the micrometer/
  vernier reads 4.5
• The sextant altitude (hs) is 3423.6

• On or Off the Arc?
• What is the IE?
• What is the IC?
• What is ha?

ON the Arc 4.5 4.5 34º 19.1
when its on take it off
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Index Error

• When the horizon line is continuous, the index
  mark is between 0 and 1 and the micrometer/
  vernier reads 56.3
• The sextant altitude (hs) is 3423.6

• On or Off the Arc?
• What is the IE?
• What is the IC?
• What is ha?

OFF the Arc 3.7 3.7 34º 27.3
when its off put it on
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Caring/Cleaning for a Sextant

• Delicate precision instruments
• Handle sextant by grasping its frame or handle -
  never by its limb, index arm, or telescope
• Avoid touching mirrors except to clean them
• Set sextant down on its legs - never mirror side
• Never put sextant where it can fall
• Stow sextant in its case in a secure spot

• Clean mirrors with lens paper or soft lint-free
  cloth
• Remove salt spray with fresh water
• Lubricate with light coat of fine instrument oil

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Sight-taking Supplies

• Sextant (obviously)
• Watch with second hand
• Notebook/pencil record sight data
• Chart of the area
• Tape measure
• THEN

• Familiarize yourself with your sextant
• Practice taking sights at a beach or pier
• Natural horizon vs. dip short of the horizon
• When comfortable, take sights from a boat

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Bring Down the Sun

• Set sextant to 0000.0
• Move all horizon shades into position
• Aim it up at the sun
• Sweep sky to find sun
• If sun not visible, remove shades, one at a time
• When visible, select index shades of same density
  

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Bring Down the Sun

• When the Sun is caught
• Release and slowly move index arm forward while
  rotating sextant downward
• Keep sun in view in telescope constantly
• Continue until you are near the horizon
• Adjust horizon shades, if needed
• Sun also seen near horizon

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Bring Down the Sun

• When suns image near horizon
• Release clamp to reengage tangent screw
• Bring sun to appear on the horizon, then

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Swinging the Arc
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Recording Sextant Altitude

• Call out Stand by to Recorder
• Recorder responds Ready
• Adjust micrometer drum to place sun on horizon
• When sun on horizon, call Mark
• Recorder notes time
• Seconds, minutes, hour in that order
• Read angle from sextant for Recorder
• Repeat steps for a run of sights

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Alternate Method

• To take sights at predetermined intervals
• Call out Stand by to Recorder
• Recorder responds Ready in xx seconds and
  begins countdown
• During countdown, adjust micrometer drum to keep
  sun on horizon
• Recorder calls Mark when countdown complete
• Recorder notes time
• Seconds, minutes, hour in that order
• Read angle from sextant for Recorder

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Taking Sights at Sea

• Taking sights at sea can be difficult, sometimes
  dangerous
• Use a safety harness
• Techniques Hit and Run Wait and See

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Special Techniques

• Dip short of the horizon
• Acceptable for JN sights
• Back sight
• Acceptable for JN sights
• Artificial horizon
• Not acceptable for JN sights
• OK for practice sights
• See Appendix A for details

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Accuracy of Sights

• Modern marine sextant - readable to 0.1
• Nautical Almanac data are given to 0.1
• Sights timed to nearest second
• Error of 1 second in time lead to error of 0.25
  of arc

• Practical Accuracy limited by
• Skill of Observer
• Quality of Sextant
• Stability of observing platform
• Visibility Atmospheric Conditions

Practice Practice Practice
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Runs of Sights

• Taking several sights on a body improves accuracy
• Corresponding altitude changes should be
  proportionately constant
• Positive direction for rising bodies
• Negative direction for setting bodies

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Run of Sights
Time Difference Difference Altitude









14-16-43
3806.2
58s
10.4
14-17-41
3755.8
10.1
56s
14-18-37
3745.7
55s
5.1
14-19-32
3750.8
58s
25.2
14-20-30
3725.6
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Graphing a Run of Sights
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JN Sight Requirements

• Two Sun sights simulating RFix
• One upper and one lower limb sight
• Acceptable accuracy of all sights is 5nm
• Qualified run of sights
• Sights with altitude greater than 75 are
  discouraged
• Recording your sights
• USPS Sight Log Form
• Sight Folder must be completed before you can
  take exam
• Details in Appendix D

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JN Sight Requirements
Art Mollica (E066699)
St Paul
1 of 1
2012
Sun LOP, KP by GPS, DST, D1.3m fm chart
1
29 Jun
Sun LL
0-00
6
09-05-40
35º 45.4
E
Ds
8.5
-0.3
44º 29.9N
92º 18.7W
09-06-40
35º 51.4
2
09-07-40
36º 02.8
3
09-08-40
36º 14.2
4
36º 24.8
09-09-40
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Sun LOP, KP by GPS, DST, D180yd fm chart
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29 Jun
Sun UL
0-00
6
17-12-45
47º 00.8
W
Ds
8.5
-.03
44º 29.9N
92º 18.7W
46º 55.8
7
17-12-45
8
46º 49.4
17-13-45
8
46º 42.6
17-14-45
8
46º 38.4
17-15-50
Arthur Mollica
Sight Folder will also contain USPS SIGHT
REDUCTION FORM (SR96a) for each of the selected
sights and a USPS CLSSAPS (Form CLS86) plotting
the resulting RFix of these selected sights.
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Quiz
1. When reading sextant altitude, in what order
are degrees, minutes, and tenths of minutes read?
Degrees Minutes Tenth of Minutes
1st
2nd
3rd
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Quiz
2. When timing sights, in what order are hours,
minutes, and seconds read from the watch?
Seconds Minutes Hours
1st
2nd
3rd
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Quiz
3. Before taking sights on the Sun, you sight the
horizon and align the direct and reflected images
of the horizon. Your Sextant reading is 1.8' on
the arc.

• What is the IE?
• b. What is the IC?

1.8' -1.8'
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Quiz
4. Before taking sights on the Sun, you sight the
horizon and align the direct and reflected images
of the horizon. Your Sextant reading is 58.2'
off the arc.

• What is the IE?
• b. What is the IC?

-1.8' 1.8'
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Quiz
5. You need to use the shade glasses on the
sextant when taking sights on the sun. a.
True b. False
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Quiz
6. You take a run of sights on the Sun with the
following times of sights and sextant altitudes.
Which of the sights are probably erroneous?
WT hs 17-42-33 27 01.4' 17-43-12
26 57.8' 17-43-52 26 58.7' 17-44-27
26 50.1' 17-44-49 26 50.0' 17-45-17
26 45.8'
Bad Sight
Bad Sight
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Quiz
hs
Bad Sight
Bad Sight
WT
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Quiz
7. What is the purpose of "swinging the arc" when
taking a sight with a sextant? a. To help
obtain a clear view of the horizon. b. To be
sure that the sextant is horizontal at the time
of the sight. c. To help focus the body in the
sextant telescope. d. To be sure that the
sextant is vertical at the time of the sight.
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The Sextant

• End of
• Junior Navigation
• Chapter 2

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