On 12 June your 1845 DR position is LAT 21° 47' N, LONG 46° 52' W when you observe a faint unidentifiable star through a break in the clouds. The star bears 313° T at a sextant altitude (hs) of 14° 56.3'. The index error is 0.5' on the arc, and the height of eye is 45 feet. The chronometer reads 09h 43m 27s, and the chronometer error is 1m 46s slow. What star did you observe?
• Sextant corrections: Applying index error (on the arc), height of eye, and main altitude corrections to convert hs → ho • Time conversion for sights: Using chronometer reading and chronometer error (slow) to find correct UTC (Greenwich Mean Time) of observation • Star identification by intercept method: Using your DR position, calculated azimuth (Zn), and true bearing to match the observed star with a candidate from the star list
• First convert the sextant altitude (hs) to observed altitude (ho). Are you applying the index error in the correct direction for an error that is on the arc? • Convert the chronometer reading to correct UTC using the given error. Once you have UTC, what is the corresponding LMT or GHA Aries you need to enter the star tables? • From your DR position and the time, determine which candidate star would have an azimuth close to 313° T and an altitude close to your corrected ho. Which option best matches both altitude AND bearing?
• Be sure the index correction sign is applied properly: "on the arc" vs "off the arc" • Confirm that a chronometer that is slow means the actual time is later than the reading (how do you apply that to the sight time?) • Verify that the star you choose matches both computed altitude vs. observed altitude (small intercept) and computed azimuth vs. observed bearing (close agreement)
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