On 2 October your 1845 DR position was LAT 28°09.2'S, LONG 167°48.1'E. You observe a faint star through a hole in the clouds at a sextant altitude (hs) of 20°45.6' T, bearing 201.5°T. The index error is 1.3' off the arc, and the height of eye is 42 feet. The chronometer reads 07h 46m 19s and is 00m 51s fast. What star did you observe?
• Intercept (Marcq St. Hilaire) method for sight reduction • Using chronometer time and GHA/Dec to identify a star • Relating true bearing from the DR position to the star’s azimuth and approximate location in the sky
• After applying index error and dip, what is the apparent altitude (ha) and then the observed altitude (Ho) of the star? • Given the date, time (with chronometer correction), and DR longitude, what is the approximate local hour angle and declination of the body you must have seen? • Which of the candidate stars lies in the correct part of the sky (south vs north, east vs west) and has an altitude consistent with your DR latitude and the computed Ho?
• Be sure to apply index error correctly: remember that "off the arc" and "on the arc" affect the sign differently. • Convert chronometer time to GMT, then to local hour angle using your DR longitude before consulting star data. • Check that the chosen star’s declination hemisphere (N/S) and approximate magnitude match a star that could be faint and located on a 201.5°T bearing from your position at that time.
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