At 1542 ZT on 23 October, in DR position LAT...

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Question 1 of 2066636eea28f7522a1c51aafe

At 1542 ZT on 23 October, in DR position LAT 37° 28.5' N, LONG 156° 17.3' E, you observe an amplitude of the Moon. The center of the Moon is on the visible horizon and bears 282.5° psc. The variation is 0.0°. What is the deviation?

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Question 1 of 20

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🔍 Key Concepts

• Amplitude of the Moon as a compass check (similar method to amplitude of the Sun) • Relationship between true bearing, variation, deviation, and compass bearing: T ± Var ± Dev = C • How to decide if deviation is E or W based on whether the compass reading is higher or lower than the calculated true bearing

💭 Think About

• First, from the DR latitude and the Moon’s declination (from the Nautical Almanac for the given date and time), what is the true amplitude of the Moon? Is it north or south of due west? • Convert that true amplitude into a true bearing of the Moon at the visible horizon. How does that compare to the observed compass bearing 282.5° psc? • With variation = 0.0°, think carefully about whether the compass is reading too high or too low compared to true, and what that means for East versus West deviation.

✅ Before You Answer

• Be sure you identify correctly whether the Moon is setting north or south of west at that time and latitude; that changes the sign of the amplitude. • Confirm your use of the amplitude formula and that you convert from amplitude (degrees north/south of west) to a true bearing in degrees from 000°–359°. • When you compare the true bearing with the compass bearing, double-check the direction: If compass > true, is that East or West deviation? Verify this from your standard compass-error formula before choosing.

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