On 31 October your 1700 zone time DR position is LAT 27°17.0'N, LONG 116°10.0'W, when an amplitude of the Sun is observed. The Sun's center is on the visible horizon and bears 246.5° per standard magnetic compass. Variation in the area is 8.5°E. The chronometer reads 01h 01m 23s and the chronometer error is 01m 54s slow. What is the deviation of the standard compass?
• Sun’s amplitude calculation at sunset using latitude and declination • Relationship between true bearing, variation, deviation, and compass error • How to determine E/W sign of deviation from the relative directions of true and compass bearings
• First, from the date and your DR latitude, what is the Sun’s approximate declination, and is it north or south? How does that affect whether the amplitude is taken north or south of west at sunset? • Use the amplitude to find the Sun’s true bearing at the time of observation. Is this bearing north or south of due west (270°), and by how many degrees? • Compare the true bearing with the standard magnetic compass bearing (246.5°). From that comparison, what is the total compass error, and does that indicate an easterly or westerly error?
• Be sure you apply the correct sunrise vs. sunset amplitude formula and sign convention. • When converting between true, magnetic, and compass, keep track of the standard sequence and signs (e.g., "True ± Variation = Magnetic; Magnetic ± Deviation = Compass"). • Double-check whether a compass reading smaller or larger than the true bearing corresponds to an easterly or westerly deviation before choosing the answer.
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