On 13 September your 1830 ZT DR position was LAT 23°03'S, LONG 105°16'E, when you observed a faint unidentifiable star through a hole in the clouds. The star bore 148.0°T at a sextant altitude (hs) of 32°24.3'. The chronometer read 11h 24m 39s and is 05m 08s slow. The index error is 1.0' off the arc, and the height of eye is 52 feet. What star did you observe?
• Intercept method of celestial navigation (Marcq St. Hilaire) to plot a position line from a single star sight • Using the Nautical Almanac to find GHA and Dec of Aries and stars at a given UTC, then entering the Star Tables (or Pub. 249/229) with assumed position • Converting sextant altitude (hs) to observed altitude (Ho) by applying index correction, dip, and altitude corrections
• From the DR position and approximate LHA Aries at the time of sight, which region of the star chart or star table are you likely in, and which of the listed stars would even be above your horizon? • After you compute Ho and the azimuth/intercept, does the bearing of the plotted position line agree with the observed true bearing (148.0°T) to within a reasonable amount? • Which candidate star’s declination and right ascension (or SHA) place it in the correct part of the sky (southern vs northern hemisphere, and roughly southeast vs southwest) at that date, time, and longitude?
• Carefully apply chronometer error (05m 08s slow) to get the correct UTC of the sight before entering the Almanac • Apply the index error correctly: “1.0' off the arc” means the correction is added, not subtracted (verify the sign convention you use) • Confirm that both the computed altitude (Hc) and the computed azimuth (Zn) for your assumed position are consistent with a star in the southern sky at latitude 23°S and bearing about 148°T
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