On 9 May your vessel's 1809 ZT DR position is LAT 48° 13.7' N, LONG 168° 36.3' E, when an amplitude of the Sun is observed. The Sun's center is on the celestial horizon and bears 283.7° per standard magnetic compass. Variation in the area is 13.0° E. The chronometer reads 07h 13m 19s and is 02m 56s fast. What is the deviation of the standard compass?
• Amplitude of the Sun and how it relates to true bearing at the moment the Sun is on the celestial horizon • The sequence of converting compass bearing → magnetic bearing → true bearing (and the reverse) using variation and deviation • Using chronometer time and date to find which side (N or S) and whether the amplitude is east or west
• First, decide whether this is a sunrise or sunset amplitude and whether the amplitude will be north or south of east/west based on date and latitude. • Think through how to calculate the true amplitude of the Sun from latitude and declination, then convert that to a true bearing of the Sun when on the horizon. • Compare the calculated true bearing with the observed compass bearing, step-by-step applying variation and deviation, and solve for the unknown (deviation).
• Be sure you are using the correct declination for the Sun on 9 May at the right GMT and applying the correct sign (N or S). • Confirm you are applying variation (13.0° E) in the correct direction when converting between magnetic and true bearings. • Check whether the final deviation you compute is east or west, based on whether the compass is reading higher or lower than magnetic.
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