On 11 January your vessel's 0655 zone time DR position is LAT 24°30'N, LONG 122°02'W, when an amplitude of the Sun is observed. The Sun's center is on the celestial horizon and bears 101.0° per standard compass. Variation in the area is 11.6°E. The chronometer reads 02h 52m 48s and is 02m 12s slow. What is the deviation of the standard compass?
• Using the amplitude formula to find the Sun’s true bearing at the moment it’s on the celestial horizon (sunrise/sunset) • The CDMVT relationship between Compass, Deviation, Magnetic, Variation, and True bearings • Correctly handling east/west signs when going between compass, magnetic, and true bearings
• Think about whether the Sun is rising or setting at 0655 local time at 122°W, and whether that puts it on the eastern or western horizon. That will tell you whether the true bearing should be around 090° or 270°. • Use the amplitude formula with your LAT and the Sun’s declination from the Nautical Almanac to get the true amplitude, then convert that amplitude into a true bearing of the Sun. • Compare the observed standard compass bearing of the Sun with the calculated true bearing to find the total compass error, then separate that into variation + deviation to solve for deviation.
• Make sure you apply the correct sign for the Sun’s declination (N or S) and for the latitude (N in this case) when using the amplitude formula. • When you go from true to compass (or compass to true), walk through C–D–M–V–T in order and check whether each correction is added or subtracted based on whether it is east or west. • After you find the deviation, double-check that the sense (E or W) you get is consistent with whether the compass is ahead of or behind true for that bearing.
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