On 2 October your 1845 DR position is 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 11°37.6' bearing 066°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 0m 51s fast. What star did you observe?
• Sextant altitude corrections: index error, height of eye (dip), and main altitude correction tables to find the observed altitude (Ho). • Greenwich Hour Angle (GHA) and Local Hour Angle (LHA): using chronometer time, its correction, and your DR longitude to find LHA of Aries and then the star. • Star Identification: using Sidereal Hour Angle (SHA) and declination to match your intercept position and azimuth with one of the candidate stars.
• After applying all sextant corrections, what is your final Ho, and how does this compare to the Hc (computed altitude) from a sight reduction table for each possible star? • How do you turn chronometer time (with its error) into Greenwich Hour Angle of Aries, and then into Local Hour Angle at your DR longitude? • When you test each star’s SHA and declination, which one produces an azimuth close to 066°T and a small intercept from your DR position?
• Be sure your index error sign is applied correctly: "off the arc" vs "on the arc" changes whether you add or subtract it. • Confirm you apply chronometer error in the right direction: if the chronometer is fast, should you add or subtract the error to get UTC? • Verify that the resulting LHA, declination, and Ho/Hc difference for the chosen star give a reasonable intercept (not many tens of miles off) and an azimuth close to 066°T.
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