On 16 September your vessel's 0736 zone time DR position is LAT 27° 34' S, LONG 174° 49' E, when an azimuth of the Sun is observed. The chronometer time of the sight is 07h 38m 11s, and the Sun is bearing 079.8° per gyrocompass. The chronometer error is 01m 46s fast, and the variation in the area is 11.0° W. At the time of the sight, the helmsman reports that he was heading 252° pgc and 258° per magnetic compass. What is the deviation of the magnetic compass?
• Gyro error and how to get from gyro bearing to true bearing • Relationship between true, variation, deviation, and compass (T-V-M-D-C) • Using an observed azimuth of the Sun to determine gyro error or compass error
• From the Sun’s observed azimuth and known true bearing, how can you determine the gyro error? Once you know gyro error, how do you convert the reported gyro heading to a true heading? • You are given both a gyro heading and a magnetic compass heading at the same moment. Once you know the true heading, how do you back out the deviation of the magnetic compass using the T-V-M-D-C chain? • Pay attention to the signs (East vs West) for variation, gyro error, and deviation. How do these signs affect whether you add or subtract each correction?
• Confirm what 079.8° per gyro actually represents: a bearing of the Sun by gyro, not a heading. • Carefully apply the chronometer error and zone time only to get the correct Greenwich time for the sight; that time drives the Sun’s true azimuth from the tables. • When converting between true and compass values, always move through the full sequence: True → Variation → Magnetic → Deviation → Compass, watching East/West signs.
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