On 15 October your 0325 zone time DR position is LAT 26° 51.0' N, LONG 138° 17.0' W. At that time, you observe Canopus bearing 167° pgc. The chronometer reads 00h 25m 36s, and the chronometer error is 00m 20s slow. The variation is 2° E. What is the gyro error?
• Gyro error = difference between true bearing and gyro bearing • Conversion steps: gyro → pgc (per gyro), then apply variation to get magnetic, then deviation (if any) to get true – or reverse that logic for a star bearing • Using a calculated true azimuth of a star (from sight reduction tables or Pub. 249/229) to compare with observed bearing
• From the DR position, date, and time (corrected for chronometer error), what are you actually able to compute for Canopus using sight reduction tables – an altitude, an azimuth, or both? Which quantity is relevant to gyro error? • Once you know the true azimuth of Canopus, how do you relate it to your observed 167° pgc? Think: which side of the equation is true, which side is gyro, and what sign convention defines east vs west error? • How does the given 2° E variation enter (if at all) when comparing a true star azimuth to a gyro reading? Are you going through magnetic, or can you go directly from true to gyro?
• Be clear on the sign convention: if the gyro bearing is greater than the true bearing, is the error east or west? (and vice versa) • Confirm you have corrected the chronometer time using the chronometer error before entering the sight reduction tables; an uncorrected time will give the wrong azimuth. • Make sure you are comparing like with like: a true azimuth from computation must be compared to a bearing that has been converted to true, or vice versa; double‑check whether variation should actually be applied in this specific comparison.
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