On 22 April your 0344 zone time DR position is LAT 21° 16.0' N, LONG 107° 32.0' W. At that time, you observe Spica bearing 236° psc. The chronometer reads 10h 45m 16s, and the chronometer error is 00m 25s fast. The variation is 7.5° E. What is the deviation of the standard compass?
• Azimuth of a star to find compass error • Relationship between true bearing, magnetic bearing, and compass bearing (TVMDC table) • Applying chronometer error and zone description to get GHA and LHA for the star sight
• How do you convert the observed standard compass bearing of Spica into a true bearing using variation and an unknown deviation? What does that equation look like? • Once you compute the true azimuth of Spica from your DR position and time, how does it compare with the converted compass bearing, and what does their difference represent? • Are you correctly handling chronometer error (fast vs. slow) and converting zone time to GMT when entering the Nautical Almanac?
• Be sure you are using the standard compass (psc) as the starting point in your TVMDC work, not gyro or magnetic. • Double-check the sign of variation (7.5° E) and how it affects going from true to magnetic and magnetic to compass. • Confirm your conversion from chronometer time to GMT, and from GMT to LHA Aries/Spica, before taking the azimuth from sight reduction tables or calculator.
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