On 22 February your 1857 ZT DR position is LAT 23° 46.0' S, LONG 93° 16.5' E. You observe an unidentified star bearing 159° T, at an observed altitude (Ho) of 34° 30.0'. The chronometer reads 01h 00m 35s and is 03m 25s fast. What star did you observe?
• DR position and ZT (Zone Time) to find GMT and then GHA/Dec of Aries and stars for the time of observation • Using chronometer correction (fast/slow) to get accurate UTC for sight reduction • Relating true bearing (Zn) and altitude (Ho) from DR position to pick the most likely star from a short list
• How do you convert the given ZT and chronometer reading (with a fast chronometer) into UTC/GMT to use with sight reduction tables or a nautical almanac? • Once you know the approximate Local Hour Angle (LHA) and latitude, which of the candidate stars would be expected to appear at a true bearing close to 159° and an altitude around 34° at that date and time? • Think about which of these stars is typically visible from a latitude about 24° South and roughly east-southeast to south-southeast on a February night.
• Make sure you correctly adjust the chronometer time for the stated 03m 25s fast condition before using it. • Verify you are using the correct zone description (ZD) for longitude 93° 16.5' E to get from ZT to UTC. • From your DR position and the computed star position, confirm that the calculated azimuth (Zn) and altitude are consistent with 159° T and 34° 30.0' before choosing the star.
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