Compass error from a celestial body
1The lesson in plain words
Calculate a body's true bearing for the moment you take its compass bearing. The difference is your compass error, anywhere in the ocean.
The idea
Far from land, a celestial body gives you a reference bearing. Its true azimuth is its bearing clockwise from true north. Calculate it for your position and the observation time. Compare it with the compass bearing taken at that moment. Compass error = true − compass. Add or subtract 360° if needed to keep the result between −180° and +180°. Positive is east; negative is west.
Two methods
An azimuth observation can be made whenever the body is low enough to bear accurately over the compass; you calculate its true azimuth for the exact time. An amplitude observation is made as the body rises or sets; its true bearing then needs only latitude and declination. Polaris gives a third check at night.
Why the Sun
The Sun is the usual body: bright, easy to bear and up every day. Use the compass sight's solar filter, and take a bearing of the centre of the disc. Bright planets and stars work as well, low in twilight.
How often
On an ocean passage, check the steering compass at least once a day, and after a large change of heading or when new gear is stowed near it. Log every check: date, UT, heading, true bearing, compass bearing, error, variation and deviation.
Ships too
A ship's gyrocompass has no variation or deviation but can still have a small error. The same observation finds it: gyro error = true − gyro bearing. The magnetic standby compass is checked the same way.
2Key data
- Compass error
- True − compass; + is east, − is west
- Azimuth method
- Any time the body is low; true azimuth for the exact UT
- Amplitude method
- At rising or setting; needs only latitude and declination
- Frequency at sea
- At least daily, and after big heading changes
- Gyro error
- True − gyro bearing, by the same observations
- Spanish terms
- Corrección total (Ct) por azimut o por amplitud
3Key points
- 1
The sky gives a true bearing anywhere: compass error = true − compass.
- 2
Azimuth: any time the body is low, timed exactly. Amplitude: at rising or setting.
- 3
Check the steering compass at least daily on passage, and log every check.
- 4
The same method finds a ship's gyro error.
4Frequent mistakes
Wrong: Compass error = compass bearing − true bearing.
Right: Subtract compass from true. For 005°T and 355°C, add 360° to −350°: the error is 10°E.
Wrong: Once the deviation card is made, the compass needs no more checks at sea.
Right: Deviation changes with heading and with gear moved near the compass. Check it daily on passage.
5Practice case & exam keys
Two heading displays disagree by 6°. You have a reliable DR position, accurate UT and a low afternoon Sun. Sunrise visibility looks poor. Which compass check do you make now, and what do you log?
Show the answer
Make an azimuth check now. Take the Sun's compass bearing on a steady heading and note the exact UT. Calculate its true bearing, then find compass error and, with variation, deviation. Log the date, UT, heading, true and compass bearings, error, variation and deviation. Use the corrected steering compass to check the other display.
Exam keys
- Compass error (corrección total) = true bearing − compass bearing.
- Azimuth: body low, exact UT. Amplitude: body rising or setting, latitude and declination only.
- Check daily at sea; log heading, UT, bearings, error, variation and deviation.
- Gyro error is found the same way.
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