Sextant: The Basics 2 |
(III) Obtaining the Tabulated Altitude 1.Calculate GMT Deck Watch Time (DWT) at ddmmyy -/+ Deck Watch Error (DWE) Greenwich Mean Time (GMT) 2.Find the HB's GP at the moment the sight is taken Ø Refer to Nautical Almanac i. Find Greenwich Hour Angle (GHA): similar to terrestial longitude but measured from Greenwich only in Westerly direction (0°-360°) ii. Find declination: similar to terrestial latitude |
Ø LHA = GHA + E Longitude or W Longitude 3. Chosen Position (CP) Ø DR usually shown in degrees and minutes e.g. 48° 10' N, 5° 35' W. We can use an alternate position CP so that a whole degree of latitude and longitude can be used. Ø For CP's latitude choose round degree of latitude nearest the DR latitude Ø For CP's longitude, choose one that will result in a whole degree of LHA when subtracted from or added to GHA o i.e. For W longitude, CP longitude's minutes are made the same as GHA's minutes e.g. DR long 21° 34'.0 W GHA 93° 45'.0 CP long W - 21° 45'.0 LHA 72° o i.e. For E longitude, CP longitude's minutes are made the complement of GHA's minutes e.g. DR long 33° 18'.0 E GHA 284° 45'.0 CP long E +33° 14'.0 LHA 318° o Note that the CP's longitude must be within 30' of DR longitude, thus it may necessary to change the degree at times. 4. Use Sight Reduction Tables (Appendix 3 Extracts from Sight Reduction Tables for Air Navigation AP 3270 Vols 1-3) Ø Need: Whole degree of CP latitude, whole degree of LHA, Declination Ø Find tables for CP's latitude in AP 3270 e.g. lat 50° Ø If Dec and CP latitude both N or both S, use same table, otherwise use contrary table. Use the one that contains Declination, either the 0°-14° or 15°-29° table (Appendix 3 Figs. A3.1-A3.7) Ø Look under column with degree of Dec e.g. 3°: Get Hc, d and Z. Ø Refer to formula at top left-hand corner of table: If LHA > 180°, Zn (azimuth)=Z. If LHA < 180°, Zn=360-Z. Ø Look at Table 5 Correction to Tabulated Altitude for Minutes of Declination (Appendix 3 Fig A3.8): Find Dec's minutes and d and make correction to Hc Example 3b: A complete sight (Use extracts in Appendix A) On the morning of 19 May at 07h 52m 05s DWT in DR position 50° 15'N 9° 40'W, a sight was taken of the sun's lower limb giving a sextant altitude of 27° 01'.4. Index error 2'.0 on the arc, height of eye 2.0metres. The DWE was 10s slow. |
Fig 3.3 Sight Reduction Form |
Fig 3.1 At A, GHA is 40° and Declination is N20° |
Fig 3.2 Local Hour Angle (LHA) |
(IV) Plotting the Sight If true altitude > tabulated altitude, intercept = true altitude tabulated altitude. Plot line of azimuth towards GP (Zn/azimuth). If tabulated altitude > true altitude, intercept = tabulated altitude true altitude. Plot line of azimuth away from GP. To Plot 1. Mark CP 2. From CP, draw line in direction of Zn, either towards or away from it. Mark the number of miles (intercept) 3. Draw position line perpendicular to intercept 4. This is part of the true position circle on which your boat lies! Example 4.1: Plotting a sight Chosen position 50°N, 2°15'W. Intercept 15M towards; azimuth 095°T. |
Fig 4.1 Step 1 Plot the CP. |
Fig 4.3 Step 3 Draw position line perpendicular to line of intercept |
Fig 4.2 Step 2 Draw line in Direction of azimuth |
Example 4b: Plotting the sight for Example 3b (Refer to Sight Reduction Table from previous chapter) |
Fig 4.4 Plotting the sight for Example 3b |
Fig 5.1 Plot from 4 star sights |
(V) Incorporating multiple sights 1. Transferred position line/running fix/sun-run-sun sigh Ø Relies on accuracy of: o Course steered o Distance run o Assessment of tidal stream and current o Estimation of leeway o Accuracy of both sights Ø Thus it is better to take simultaneous sights of a few heavenly bodies 2. Simultaneous Sights (taken within few minutes of one another must record the exact GMT) Ø During the day: sun and moon Ø Dawn and Dusk: Stars, planets, moon Ø Dark: Stars planets, moon It is best to observe 4 or more planets/stars: all resulting position lines should cross at nearly the same point (see Fig 5.1). |
Planets Ø Similar as sun sight except: o v factor applied to GHA o For true altitude/sextant sight, altitude correction table used to correct apparent altitude (N.B. Venus, Mars additional correction) (Appendix 2 Fig A2.1) Moon Ø Similar to sun sight except: o separate altitude correction table for sextant sight (Appendix 2 Fig A2.2) o Additional HP factor, v and d tabulated hourly for corrections to Dec and GHA Stars o Fund GHA of Aries for your GMT: Get LHA Aries o Use sight reduction table AP3270 Volume 1 (Appendix 3 Fig A3.1) to find altitude and azimuth of your star, no corrections needed o Correct sextant sight by using altitude corrections table (Appendix 2 Fig A2.1) under star column |
iii. Find d factor (hourly rate of change) iv. Note whether declination is North or South v. Note whether declination is increasing or decreasing Ø Example 3a: Find the GHA and Dec for 19 May at 07h 52m 15s GMT. i. From Nautical Almanac, find the GHA and Declination for the hour of GMT. The GHA and Dec for 07h is 285° 54'.1 and N19° 38'.9 respectively (see table A2.4 in appendix ). ii. Note that the d factor (found at the bottom of the table under the sun column) is 0.5, and that Dec is North and increasing. iii. From the Increments and Corrections table in the Nautical Almanac (see table A2.5-2.6 in appendix), find the table corresponding to the minutes of GMT (i.e. 52m), then look for the seconds of GMT (15s) in the column under 52m, and note the correction to be made for 52m 15s GMT to the right of 15s. This correction is 13° 03'.8. iv. From the same Increments and Corrections table, look for the correction to declination under the column v or d corrn. The figure to the right of the d factor found previously (0.5) will be the correction to Declination (0.4). v. Make corrections to GHA and Dec o For GHA, add correction because GHA increases throughout the day o For Dec, add if Dec is increasing and subtract if Dec is decreasing GHA Dec 07h 285° 54'.1 N19° 38'.9 52m 15s +13° 03'.8 d-factor 0.5 +0'.4 07h 52m 15s 298° 57'.9 N19° 39'.3 2. Get Local Hour Angle (LHA): angle at either pole between the meridien passing through the Chosen Position (or CP) and meridien passing through the sun's GP |
Another Sextant |
Text file is 7mb may take a long time to load for dial-up internet users. Please refer to Ocean Navigator, Kenneth Wilkes, 7th ed, Adlard Coles Nautical ( London ) for the Appendix |
Another John Hadley Sextant |
