On 26 May your 0723 zone time DR position is LAT 24°50.0'N, LONG 38°11.0'W. At that time, you observe the Sun bearing 076.5°psc. The chronometer reads 10h 25m 43s, and the chronometer error is 02m 57s fast. The variation is 7°W. What is the deviation of the standard magnetic compass?
• Amplitude of the Sun to find compass error from an observed bearing • Converting chronometer time to zone time and then to local hour angle (LHA) of the Sun • The relationship: Compass error = True bearing – Compass bearing, and compass error = variation + deviation
• From the DR position, date, and corrected time, determine the Sun’s true bearing at the moment of observation. Think carefully about how to get GHA and declination from the Nautical Almanac, then LHA and true azimuth. • Once you have the Sun’s true bearing, compare it with the observed compass bearing (psc). What does the sign and size of that difference tell you about the total compass error (east or west)? • Use the known variation (7°W) and your calculated total compass error to back out the deviation. How do the signs combine in the formula compass error = variation + deviation?
• Be sure you have correctly applied the chronometer error (fast/slow) when converting to GMT (UT1) before entering the Almanac. • Confirm that you are using the correct hemisphere and date so your LHA and azimuth quadrant are right (N/S and E/W must match the DR). • Double‑check your sign convention: decide which direction (E or W) means the compass is reading higher or lower than true, and apply that consistently when solving for deviation.
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