On 9 November at 0426 ZT, your position was LAT 25° 17.0' S, LONG 154° 16.0' E. Your vessel is steaming at 14.0 knots on course 066° T. A sextant observation of the Sun's lower limb is made at 0837 ZT. The chronometer reads 10h 35m 21s, and the sextant altitude (hs) is 50° 26.9'. The index error is 1.5' on the arc, and the chronometer error is 01m 48s slow. Your height of eye on the bridge is 56.0 feet. What is the observed altitude (Ho) and azimuth (Zn) of this sight using the assumed position?
• Sight reduction process: convert sextant altitude (hs) to observed altitude (Ho) using index correction, dip, and main corrections from the Nautical Almanac • Time and longitude relationship: correctly convert from zone time (ZT) to GMT/UTC using the vessel’s longitude and then apply chronometer error • Azimuth (Zn) determination: understand how to use assumed position, declination, and LHA (Local Hour Angle) with sight reduction tables or calculator to get Zn
• First, focus on getting the correct UTC of the sight: how do you go from ZT to GMT and then account for the chronometer error? • Next, step carefully through each correction from hs to Ho: are you using the right sign for index error and dip, and are you applying the lower-limb main correction? • When you compute Zn from your sight reduction, think about which general quadrant the Sun should be in given your latitude, time of day, and course—does the azimuth choice make sense physically (east vs. west, north vs. south)?
• Confirm the sign of index correction for 1.5' on the arc and make sure you apply it correctly to hs. • Check that your dip correction is appropriate for a 56.0 ft height of eye and has the correct (negative) sign. • Verify that the Sun’s approximate bearing at that local morning time in the southern hemisphere is consistent with the Zn option you select (is it reasonable for it to be slightly north or south of east?).
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