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Lighthouses in the Sky

N. E. 5. Hostile Vessel Operating in this area. Lighthouses in the Sky. Junior Navigation Chapter 1. Learning Objectives. Define terms: altitude, circle of position, geographical position, intercept, and celestial line of position.

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Lighthouses in the Sky

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  1. N E 5 Hostile Vessel Operating in this area Lighthouses in the Sky 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. Terms Apparent Body SD UL Body LL Astronomical Refraction Dec Parallax ha hs he dip GP N Latitude Latitude Celestial Horizon S 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) Observer’s Eye Sensible Horizon Ho (Altitude) Geoidal Horizon DR COP Visible Horizon Equator Equator Longitude Terrestrial Refraction Geometrical Horizon 3 Nadir

  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

  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 90º x 60nm = 5400nm 90º – 30º = 60º 60º 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 = 720.0 nm 18.5´ x 1 nm/minute = 18.5 nm Total = 738.5 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 = 1920.0 nm 25.9´ x 1 nm/minute = 25.9 nm Total = 1945.9 nm (radius of COP) 9

  10. Altitude-Intercept Method • At 10-34-15 a sight of the sun is taken. Recorded altitude = 57°34.1’ COP 1,945.9nm 1034 1000

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

  12. 10nm 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 12

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

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

  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

  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/° = 3120.0nm + 33.9' x 1nm/' = +33.9nm 3,153.9nm 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

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

  22. Lighthouses in the Sky The Sextant End of 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

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

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

  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

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

  30. Reading a Measurement 15 25 45 20 30 50 0 0 4 4 25 35 55 8 8 0 30 4 40 0 8 45 35 5 32.6’ 23.0’ 51.3’ 30

  31. 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 ASTRA IIIB THIS INSTRUMENT IS FREE OF ERRORS FOR PRACTICAL USE ASTRA IIIB 2000 4 24 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

  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

  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 0 0 OFF THE ARC ON THE ARC Split Field 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 0 0 OFF THE ARC ON THE ARC 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´ On or Off the Arc? What is the IE? What is the IC? What is ‘ha’? 0 0 4 8 5 10 15 0 ON the Arc + 4.5’ – 4.5’ 34º 19.1’ when it’s on take it off 36

  37. 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´ 50 0 55 4 8 0 5 10 0 Index Error • 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

  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

  39. Sight-taking Supplies 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 • Sextant (obviously) • Watch with second hand • Notebook/pencil – record sight data • Chart of the area • Tape measure THEN 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

  41. 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 Bring Down the Sun 41

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

  43. Swinging the Arc 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

  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

  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

  47. 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 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

  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

  50. Run of Sights 14-16-43 38°06.2’ 58s –10.4´ 14-17-41 37°55.8’ –10.1´ 56s 14-18-37 37°45.7’ 55s +5.1´ 14-19-32 37°50.8’ 58s –25.2´ 14-20-30 37°25.6’ 50

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