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Compass error by the sun

Check the compass against the sun: its true bearing at sunrise or sunset (amplitude) or at any time (azimuth), worked out for your position, against what the compass or gyro shows.

Method

Date and time of the bearing (UTC)

Ship’s position

Latitude

Longitude

Compass or gyro bearing of the sun

Variation (optional, for deviation)

Direction

Fill in the fields to see the result.

How it works

The sun's declination and Greenwich hour angle come from standard almanac formulas (to about 0.01°). With the ship's position they give the sun's azimuth: tan Z = −cos δ sin LHA / (sin δ cos φ − cos δ sin φ cos LHA). Best taken when the sun is low (below about 30°).

The amplitude is the bearing when the sun's centre is on the celestial horizon: cos Z = sin δ / cos φ. At sea that is when the lower limb is about half a diameter above the visible horizon. No exact time is needed, only the date and position.

Compass error = true bearing − compass (or gyro) bearing; deviation = compass error − variation. Record it in the compass observation book.

For training and cross-checking. Not a substitute for approved navigation equipment, nautical publications and the ship’s own procedures. Calculations run in your browser and nothing you type is stored; a PDF you download is made on our server and not kept.

How it works

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