On 13 September your 1830 ZT DR position was LAT 23°03'S, LONG 105°16'E when you observe a faint unidentifiable star through a hole in the clouds. The star bore 132.3°T at a sextant altitude (hs) of 29°34.6'. The chronometer read 11h 24m 39s and is 5m 08s slow. The index error is 1.0' off the arc, and the height of eye is 52 feet. What star did you observe?
• Local hour angle (LHA) and how it is computed from GHA and longitude • Using sextant altitude (hs) to get observed altitude (Ho) by applying index error, height of eye, and dip corrections • Comparing your assumed position (AP) and calculated intercept with star data in the Nautical Almanac or star tables
• How do you correct the chronometer time (given its error) to get the correct UTC at the instant of observation, and then find GHA Aries for that time? • Once you have LHA Aries and your DR latitude, how do you determine which stars are in a suitable position (altitude and azimuth) for observation near your bearing 132.3°T and altitude about 29.5°? • When you compute the computed altitude (Hc) and azimuth (Zn) for each candidate star, which one matches most closely the observed Ho and the measured bearing?
• Be sure you apply the chronometer correction in the right direction (slow vs fast) before entering the Nautical Almanac. • Verify the sign of your index error (off the arc vs on the arc) when correcting hs to get Ho. • Check that the final star you select has a Zn close to 132.3°T and an Hc within a few minutes of arc of your corrected Ho from the sight reduction.
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