1. Junior Navigation Chapter 1
Hostile Vessel Operating in this area
Lighthouses in the Sky N E 5
Junior Navigation
Chapter 1

2. 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.

3. When body is at your zenith: Latitude = Dec and Longitude = GHA
Apparent Body
Body SD UL LL
GHA & Dec
Terms
Dec
Astronomical Refraction
Celestial Equator
Celestial Sphere
When body is at your zenith: Latitude = Dec and Longitude = GHA
Apparent Line of Sight
Parallax hs ha
Zenith
index correction (IC)
Observer’s Eye he
Sensible Horizon
dip
Ho (Altitude)
Geoidal Horizon DR N S GP
Latitude
COP
Celestial Horizon
Visible Horizon
Equator
Equator
Longitude
Terrestrial Refraction
Geometrical Horizon 3
Nadir 3

4. 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. 4 4

5. The Fundamental Idea
In AP, a Radar Fix was determined by plotting two LOPs taken from radar.

6. 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.

7. 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 º x 60nm = 5400nm
90º – 30º = 60º º x 60nm = 3600nm ?

8. COP and Altitude
At 1034 an observer in Galveston measures the sun’s altitude to be 77°41.5´. What is the radius of the COP?
90°00.0´
–77°41.5´
12°18.5´
12° x 60 nm/degree = nm
18.5´ x 1 nm/minute = nm
Total = nm (radius of COP)

9. COP and Altitude
At the same time observer in Los Angeles measures the sun’s altitude to be 57°34.1´. What is the radius of the COP?
90°00.0´
–57°34.1´
32°25.9´
32° x 60 nm/degree = nm
25.9´ x 1 nm/minute = nm
Total = nm (radius of COP) 9 9

10. Altitude-Intercept Method
At a sight of the sun is taken. Recorded altitude = 57°34.1’ COP 1,945.9nm
1034
1000

11. Altitude-Intercept Method
Calculate true bearing (azimuth) and altitude (Hc) from DR using Law of Cosines
to GP Zn
1034
1000 11 11

12. Altitude-Intercept Method
Difference between Ho (observed altitude) and Hc (calculated altitude) is the intercept
Ho 57º 34.1’
Hc 57º 24.1’
Diff ’ towards
Intercept 10.0nm
1034
10nm
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 12 12

13. Altitude-Intercept Method
Label with time of sight and name of body
1034
1034 Sun
1000 13 13

14. Altitude-Intercept Method
When you have only a single LOP, you obtain an estimated position (EP)
1034
1034 Sun
1000 14 14

15. 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.

16. Quiz
2. Polaris (the North Star) is located exactly over the earth's north geographic pole.
a. True
b. False 16 16

17. 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. 17 17

18. Quiz
4. A navigator determines the altitude of the sun to be 37°26.1'. What is the distance in nautical miles between the navigator's position and the GP of Sun?
Solution:
90 ° - 37°26.1´ = 52° 33.9´
52° x 60nm/° = nm
+ 33.9' x 1nm/' = nm
3,153.9nm 18 18

19. 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. 19 19

20. Quiz
6. The method used in plotting a celestial LOP is called “the altitude-intercept method”.
a. True
b. False 20 20

21. 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 21 21

22. End of
Lighthouses in the Sky
The Sextant
Junior Navigation
Chapter 2

23. 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 23 23

24. Parts of the Sextant ? Sight Tube 2X or 4X Telescope Handle Lanyard
Frame
Limb
Index Arm ?
Release Clamp
Arc (degrees)
Micrometer (min)
Whole
Split
Horizon
Vernier (tenths)
Horizon Glass
Index Mirror
Horizon Shades
Index Shades
Telescope 24 24

25. How a Sextant Works The sextant set to 0°00.0´
The horizon will appear as an unbroken line when the sextant is correctly adjusted
Split Field
Full Field 25 25

26. How a Sextant Works Telescope aimed at the horizon
Index arm adjusted to the appropriate angle
Split Field
Full Field 26 26

27. 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 27 27

28. Reading the Measured Angle
First read degrees from the arc
Then read minutes
from micrometer drum
Finally read tenths of minutes
from vernier 28 28

29. Reading a Measurement Read the Drum Read the Arc Read the Vernier
Index Mark
40°02.6’
40°02’
40° 29 29

30. Reading a Measurement 23.0’ 51.3’ 32.6’ 15 25 45 20 30 50 35 25 55 304 8 50 55 45 5 4 8 30 35 25 4 8 40 45
23.0’
51.3’
32.6’ 30 30

31. THIS INSTRUMENT IS FREE OF ERRORS FOR PRACTICAL USE
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 0°00.0´
Checking & adjustment procedures in Bowditch
Should only be made by experienced persons
Frequent adjustment might loosen screws
ASTRA IIIB
THIS INSTRUMENT IS FREE OF ERRORS FOR PRACTICAL USE 31 31

32. Index Error (IE) IE is common In good quality metal sextants
IE tends to remain fairly constant
In plastic sextants
Checking IE critical 32 32

33. Determining Index Error (IE)
Set sextant to 0°00.0´ and sight on horizon
If 2 images of horizon not superimposed OR
If horizon shows as broken line
IE present 33 33

34. 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 0°00.0´ is off the arc
If index mark is above 0°00.0´ is on the arc
Full Field
Split Field
OFF THE ARC
ON THE ARC 34 34

35. 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 it’s on, take it off
IE ‘off the arc’ requires positive IC
When it’s off, put it on
ON THE ARC
OFF THE ARC 35 35

36. 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 34°23.6´ 5 10 15 4 8
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 it’s on take it off 36 36

37. 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 34°23.6´ 55 5 4 8 50 10
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 it’s off put it on 37 37

38. 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 38 38

39. 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 39 39

40. Bring Down the Sun Set sextant to 00°00.0´
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 40 40

41. 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 41 41

42. Bring Down the Sun When sun’s image near horizon
Release clamp to reengage tangent screw
Bring sun to appear on the horizon, then 42 42

43. Swinging the Arc 43 43

44. 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 44 44

45. 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 45 45

46. Taking Sights at Sea
Taking sights at sea can be difficult, sometimes dangerous
Use a safety harness
Techniques: Hit and Run; Wait and See 46 46

47. Special Techniques Dip short of the horizon Acceptable for JN sights
Back sight
Artificial horizon
Not acceptable for JN sights
OK for practice sights
See Appendix A for details 47 47

48. 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 48 48

49. 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 49 49

50. Run of Sights Time Difference Altitude 14-16-43 38°06.2’ 58s –10.4´
37°55.8’
–10.1´
56s
37°45.7’
55s
+5.1´
37°50.8’
58s
–25.2´
37°25.6’ 50 50

51. Graphing a Run of Sights51 51

52. 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 52 52

53. JN Sight Requirements Arthur Mollica
Art Mollica (E066699)
St Paul
1 of 1
JN Sight Requirements
2012
Sun LOP, KP by GPS, DST, D1.3m fm chart 1
29 Jun
Sun LL
0-00 +6
35º 45.4’ E Ds
8.5
-0.3
44º 29.9’N
92º 18.7’W 2 “ “ “ “
35º 51.4’ “ “ “ “ “ “ “ 3
36º 02.8’ 4
36º 14.2’ 5
36º 24.8’
Sun LOP, KP by GPS, DST, D180yd fm chart 6
29 Jun
Sun UL
0-00 +6
47º 00.8’ W Ds
8.5
-.03
44º 29.9’N
92º 18.7’W 7
46º 55.8’ 8
46º 49.4’ 8 “ “ “ “
46º 42.6’ “ “ “ “ “ “ “ 8
46º 38.4’
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. 53 53

54. 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

55. Quiz
2. When timing sights, in what order are hours, minutes, and seconds read from the watch?
Seconds
Minutes
Hours
1st
2nd
3rd

56. 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'

57. 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'

58. Quiz
5. You need to use the shade glasses on the sextant when taking sights on the sun.
a. True
b. False

59. 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
° 01.4'
° 57.8'
° 58.7'
° 50.1'
° 50.0'
° 45.8'
Bad Sight
Bad Sight

60. Quiz hs
Bad Sight
Bad Sight WT

61. 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.

62. End of Junior Navigation Chapter 2
The Sextant
End of
Junior Navigation
Chapter 2 62 62