On 16 July at 2000 zone time, you take a sextant observation of Polaris. Your vessel's DR position is LAT 27°22.0'N, LONG 148°35.0'W, and your sextant reads 26°57.5'. Your chronometer reads 05h 59m 16s, and your chronometer error is 01m 28s slow. Your height of eye is 48 feet, and the index error for your sextant is 1.3' off the arc. What is the latitude of your vessel from your observation of Polaris?
• Polaris latitude method (using Polaris altitude to approximate latitude) • Correcting sextant altitude: index error, height of eye (dip), and refraction • Effect of chronometer time and longitude on using the Polaris tables in the Nautical Almanac
• First, think about how to turn the sextant reading into a true observed altitude (Ho). What are the steps, in order, and which corrections will increase vs. decrease the altitude? • Once you have Ho, consider how the Polaris tables or rules relate Ho to latitude. In mid‑northern latitudes, should the latitude be close to the Ho of Polaris, or very different? • Look at the DR latitude (27°22.0'N). Given your corrected Polaris altitude, should the latitude from Polaris be a few minutes different from DR, or many degrees? Use that to rule out unlikely answers.
• Be sure you apply index error with the correct sign ("off the arc" vs. "on the arc") • Confirm you used the correct dip correction for 48 ft of height of eye, and that it is always subtractive from the sextant altitude • Verify whether, for this type of exam problem, you actually need to use the exact chronometer time and longitude with Polaris tables, or if a simplified Polaris‑latitude relationship is intended at this latitude. This choice will help you avoid unnecessary steps.
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