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

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.

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

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

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

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.

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 ?

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)

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 9

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

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

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 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

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

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

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.

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

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

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 18

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

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

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

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

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

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

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

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

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

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

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

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

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 2000 4 24 31 31

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

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

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

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

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

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

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

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

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

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

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

Swinging the Arc 43 43

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

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

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

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

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

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

Run of Sights Time Difference Altitude 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 50

Graphing a Run of Sights 51 51

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

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 09-05-40 35º 45.4’ E Ds 8.5 -0.3 44º 29.9’N 92º 18.7’W 2 “ “ “ “ 09-06-40 35º 51.4’ “ “ “ “ “ “ “ 3 09-07-40 36º 02.8’ 4 09-08-40 36º 14.2’ 5 09-09-40 36º 24.8’ Sun LOP, KP by GPS, DST, D180yd fm chart 6 29 Jun Sun UL 0-00 +6 17-12-45 47º 00.8’ W Ds 8.5 -.03 44º 29.9’N 92º 18.7’W 7 17-12-45 46º 55.8’ 8 17-13-45 46º 49.4’ 8 “ “ “ “ 17-14-45 46º 42.6’ “ “ “ “ “ “ “ 8 17-15-50 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

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

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

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'

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'

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

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

Quiz hs Bad Sight Bad Sight WT

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.

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