At 1524 ZT on 14 June, in DR position LAT...

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Question 1 of 2066636eea28f7522a1c51aafa

At 1524 ZT on 14 June, in DR position LAT 30° 51' N, LONG 30° 02' W, you observe an amplitude of the Moon. The center of the Moon is on the visible horizon and bears 103.9° pgc. The variation is 10° W. What is the gyro error?

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Question 1 of 20

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🔍 Key Concepts

• Amplitude of the Moon to determine error of the gyrocompass • Relationship between pgc (per gyro compass), true bearing, variation, and gyro error • Use of Nautical Almanac data (declination and semi-diameter) to find the Moon’s true amplitude

💭 Think About

• From the DR position, date, and time, determine the Moon’s declination from the Nautical Almanac. How does declination and latitude give you the true amplitude at the visible horizon? • Once you have the Moon’s true amplitude, convert that to a true bearing. Then, how do you use the given variation (10° W) to find the corresponding magnetic or true reference needed to compare with 103.9° pgc? • How is gyro error defined in sign and sense? Think: gyro reading vs. true direction – if the gyro heading/bearing is larger or smaller than the true value, is the error E or W?

✅ Before You Answer

• Be sure you are using the Moon’s center on the visible horizon, not upper or lower limb – check if you must correct for semi-diameter and parallax when using amplitude tables. • Confirm the sign convention: when gyro bearing minus true bearing is positive or negative, which corresponds to east and which to west gyro error? • Double‑check that you applied variation 10° W in the correct direction when converting between true, magnetic, and gyro – mixing the sign here will flip the error direction.

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