On 23 June in DR position LAT 21°39.0'S, LONG 106°28.0'W, you observe an amplitude of the Sun. The Sun's center is on the celestial horizon and bears 078°psc. The chronometer reads 02h 14m 39s and is 01m 43s slow. Variation in the area is 9°W. What is the deviation of the standard magnetic compass?
• Amplitude of the Sun and how it relates to true bearing at the horizon • Relationship between true bearing, magnetic bearing, variation, and deviation • How date, time, and DR position give you the Sun’s declination for the amplitude calculation
• From the DR latitude and the Sun’s declination on 23 June, what is the computed true amplitude (east or west, north or south of east/west)? • Once you have the true bearing of the Sun, how do you step-by-step get from true to compass or from compass back to true using variation and deviation? • Given the Sun’s observed compass bearing 078°psc, and the local variation 9°W, what must the deviation be so that the corrected bearing matches your computed true bearing?
• Be sure you are using the correct sign for western variation when converting between true and magnetic. • Confirm you are treating the Sun as on the celestial horizon (use the amplitude formula, not a normal azimuth sight reduction). • Double-check whether deviation is named east or west based on whether the compass bearing is greater or less than the magnetic bearing.
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