On 17 April your vessel's position is LAT 21° 00' S, LONG 78° 30' W, when an amplitude of the Sun is observed. The Sun's center is on the celestial horizon and bears 082.7° per standard magnetic compass. Variation in the area is 2.0° W. The chronometer reads 10h 59m 24s and is 01m 24s fast. What is the deviation of the compass?
• Amplitude of the Sun as a method of finding compass error • Relationship between true bearing, variation, deviation, and compass bearing • How to use the date and latitude to determine Sun’s true amplitude
• How do you get the true amplitude of the Sun from the date (17 April) and your latitude 21° S? Think about the Sun’s declination and which hemisphere you are in. • Once you have the Sun’s true bearing at rising, how do you combine it with variation and deviation to arrive at the compass bearing? Set up the standard compass-error chain: True → Magnetic → Compass. • Compare the computed compass bearing with the observed standard magnetic compass bearing (082.7°). What does the difference tell you about the sign and magnitude of deviation?
• Be sure you are using the correct Sun’s declination for 17 April and applying the correct sign (N or S). • Confirm the correct amplitude formula for the Sun: it involves sine of declination and cosine of latitude. • When stepping through True → Magnetic → Compass, keep track of east/west signs: remember the common rule for converting (e.g., “True Virgins Make Dull Companions” or similar) and carefully apply W/E signs.
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