On 28 November your vessel's 0652 DR position is LAT 37°30'N, LONG 124°12'W, when an amplitude of the Sun is observed. The Sun's center is on the visible horizon and bears 103° per standard magnetic compass. Variation in the area is 16.3°E. The chronometer reads 02h 54m 18s and is 02m 06s fast. What is the deviation of the compass?
• Amplitude of the Sun and how it relates to true bearing at sunrise/sunset • Relationship between true bearing, variation, magnetic bearing, compass bearing, and deviation • Using the chronometer error and date to get the Sun’s declination from the Nautical Almanac
• First, think about how to get the Sun’s true amplitude/true bearing from your latitude and the Sun’s declination on 28 November at the given time. • Once you have the true bearing, consider how to apply variation to find the magnetic bearing and then compare that with the compass bearing 103° to determine deviation. • Be careful with the east/west signs: how does east variation affect the relationship between true and magnetic, and how do you decide if the deviation is east or west?
• Make sure you correctly determine the Sun’s declination for the correct GMT (adjusting the chronometer time for its error). • Verify your use of the amplitude formula, especially whether the amplitude is measured from east or west and how you convert it to a true bearing. • Double-check your sign convention: (T \leftrightarrow M \leftrightarrow C) steps and whether east or west deviation will make the compass bearing larger or smaller than magnetic.
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