On 16 January at 1804 zone time, you take a sextant observation of Polaris. Your vessel's DR position is LAT 36°12' N, LONG 124°36' W, and your sextant reads (hs) 37°16.4'. Your chronometer reads 02h 02m 12s, and is 01m 36s slow. Your height of eye is 60 feet, and the index error is 1.5' on the arc. From your observation of Polaris, what is the latitude of your vessel?
• Polaris (North Star) altitude and latitude relationship in mid-northern latitudes • How to correctly reduce a sextant altitude: apply index correction, dip (height of eye), and small corrections for refraction if needed • Difference between observed latitude from Polaris and DR latitude, and how to compare them
• First, carefully reduce hs = 37°16.4' using index error and dip for 60 ft. What approximate value do you get for Ho (observed altitude)? • For Polaris, how does Ho relate to your latitude at about 36°N? Should they be almost equal, or is there a small constant correction involved? In which direction? • After you find the latitude from Polaris, compare it with the DR latitude 36°12' N. Is the result slightly north or south of DR, and by about how many minutes?
• Be sure you apply index error with the correct sign for 'on the arc' (+ or -?) • Verify the dip correction for a 60-foot height of eye using your sight reduction tables or formula; confirm whether it is subtracted or added • After all corrections, check that your final latitude is reasonable and close to both your Ho and your DR latitude (around 36°N)
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