On 8 August at 0545 ZT, morning stars were observed, and the vessel's position was determined to be LAT 26° 16.0' S, LONG 94° 16.0' E. Your vessel is steaming at 20.0 knots on a course of 346° T. A sextant observation of the Sun's lower limb is made at 0905 ZT. The chronometer reads 03h 02m 52s, and the sextant altitude (hs) is 38° 07.5'. The index error is 5.2' off the arc, and the chronometer error is 2m 17s slow. Your height of eye on the bridge is 72 feet (22.0 meters). What is the observed altitude (Ho) and azimuth (Zn) of this sight using the assumed position?
• Sextant corrections: index error, height of eye (dip), and converting hs to Ho • Time conversions: applying chronometer error, converting to GMT/UT, and then to GHA/Dec for the Sun • Assumed position and azimuth: using the assumed latitude and LHA to determine the Sun’s azimuth (Zn)
• First, think through each step to convert the sextant altitude (hs) to the observed altitude (Ho). What is the sign of each correction and how big are they likely to be? • Consider whether the time difference between the DR position (from morning stars) and the Sun sight will significantly change your position for the assumed latitude and longitude. • Once you have an approximate Ho and computed Hc from the assumed position, which side of the vessel is the Sun on, and is it in the NE, SE, SW, or NW quadrant?
• Make sure you apply index error correctly: "off the arc" vs. "on the arc" changes the sign of the correction. • Verify the dip correction for a 72-foot (22 m) height of eye is reasonable in size and direction (it always reduces the altitude). • Check whether the azimuth you get should be less than or greater than 180° based on your latitude in the Southern Hemisphere and local time after sunrise.
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