On 23 September while taking stars for an evening fix, an unidentified star is observed bearing 261° T at an observed altitude of 61° 35'. Your 1836 zone time DR position is LAT 25° 18' S, LONG 162° 36' E. The chronometer reads 07h 34m 12s, and the chronometer error is 01m 54s slow. Your vessel is steaming on a course of 230° T at a speed of 18 knots. What star did you observe?
• Local hour angle (LHA) and how it is found from GHA and longitude • Using the chronometer time and chronometer error to find accurate GMT at the time of observation • Comparing the computed altitude (Hc) and azimuth (Zn) of candidate stars with the observed altitude and bearing
• How do you convert the chronometer reading and its error into the correct UTC, and then into the Greenwich hour angle (GHA) and declination (Dec) of candidate stars for that exact date and time? • Once you have your DR position and the star’s assumed position (from the star data), how do you determine which candidate produces an altitude and azimuth that best match 61° 35' and 261° T? • Given that you are in the Southern Hemisphere at about 25° S, which of the listed stars are normally prominent and well-placed in the evening sky in late September near that altitude and azimuth?
• Make sure you apply the chronometer error (slow/fast correctly) when finding actual UTC; a sign mistake will shift all your GHAs. • Verify you are using the correct date (23 September) and cycle of Aries for evening stars, not mixing morning/evening data. • Check that the star you choose yields both a computed Zn close to 261° T and an Hc close to 61° 35' from your DR position; both must be consistent.
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