On 4 June at 0630 ZT, morning stars were observed, and the vessel's position was determined to be LAT 26° 15' S, LONG 121° 20' W. Your vessel is steaming at 13.0 knots on a course of 246° T. A sextant observation of the Sun's lower limb is made at 0915 ZT. The chronometer reads 05h 14m 27s, and the sextant altitude is 25° 57.8'. The index error is 2.1' off the arc, and the chronometer error is 0m 53s slow. Your height of eye is 39.0 feet. What is the azimuth (Zn) and intercept (a) of this sight using the assumed position method?
• Assumed position method for sight reduction (using closest whole degree of latitude and LHA) • Correcting sextant altitude to get Ho (index error, dip, and main altitude corrections) • Using Zn (true azimuth) and intercept (a) relationship: "toward" if Ho > Hc, "away" if Ho < Hc
• First, convert the sextant time to correct UTC/GMT using the chronometer reading and its error. From there, determine GHA and declination for the Sun at that time and form your assumed position. • Carefully correct the sextant altitude (Hs) step-by-step to obtain Ho. Then, using your assumed position, compute Hc and Zn from the sight reduction tables or formulas. • Compare Ho and Hc to decide whether the intercept is toward or away, and use the relative bearing of the Sun to decide which Zn quadrant (northeast vs southeast) is reasonable for a morning Sun in your latitude and longitude.
• Be sure your chronometer error (slow vs fast) is applied in the correct sense to get the right UTC. • Check that your assumed latitude and LHA are rounded correctly to whole degrees as required by the assumed position method. • Confirm that the Sun’s approximate bearing at that date, time, and position matches the Zn quadrant you select; eliminate any option with an impossible Sun bearing for that morning situation.
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