On 23 October your vessel's 1722 zone time DR position is LAT 27°36'S, LONG 96°16'W, when an amplitude of the Sun is observed. The Sun's lower limb is about 20 minutes of arc above the visible horizon and bears 246° per standard compass. Variation in the area is 14.0°E. The chronometer reads 11h 24m 19s and is 01m 43s fast. What is the deviation of the standard compass?
• Amplitude of the Sun and how it’s used to find compass error • Difference between true bearing, magnetic bearing, and compass bearing • How to apply variation and then separate deviation from total compass error
• From the DR position, determine the approximate true azimuth of the Sun at the time of the amplitude observation (using date, time, and longitude). How will this compare to the bearing you see on the compass? • Once you have the true bearing of the Sun, think about the sequence of corrections: do you apply variation first or deviation first when going between true and compass? • After you find the total compass error (difference between true and compass), how do you decide which part is due to variation and which part is due to deviation, and what sign (E or W) the deviation must have?
• Be sure you use corrected UTC time for the Sun’s data (account for zone description and chronometer error carefully). • Confirm you are using the proper amplitude formula/sign conventions for latitude South and Sun declination on that date. • When you compare bearings, double‑check which way an E or W error will move a compass reading relative to true bearing before deciding between E or W deviation.
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