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Celestial navigation

Compass check – compass error and deviation by the sun

Where the sun lies true can be calculated for any moment. Take a compass bearing of it and the difference gives the compass error – and after subtracting the variation, the deviation on the current heading. NauticTools2 works this as an azimuth bearing or at true sunrise or sunset (amplitude), for magnetic and gyro compasses.

• Azimuth bearing
• Amplitude
• Compass error
• Deviation
• Variation from IGRF
• Gyro compass

The principle

The compass does not point true. Variation depends on the place, deviation on the ship and the current heading. Together they make up the compass error. From time and position NauticTools2 calculates the sun's true bearing; comparing it with the compass bearing gives the compass error, and with the variation from the IGRF model the deviation remains.

Two methods

• Azimuth bearing: The sun is taken at any time. NauticTools2 calculates its azimuth from latitude, declination and local hour angle. Most accurate with a low sun – above 30° altitude the app warns you.
• True sunrise or sunset (amplitude): The sun is taken when its centre is on the rational horizon – to the eye the lower limb is then about half a sun's diameter above the horizon. A bearing on the horizon is particularly accurate, and the calculation only needs the declination.

When do you need it?

• Checking the deviation card: Does the deviation on the current heading still match the card? NauticTools2 shows the card value for comparison if you enter the compass heading.
• On long passages: Without landmarks, the sun is the only reference for the compass.
• Gyro compass: The gyro error can be found the same way.

Formulas and working

1. Sun's position

   GHA and declination of the sun for the time of the bearing, and with the longitude the local hour angle.

2. True bearing

   Azimuth bearing: the azimuth from latitude, declination and LHA. Amplitude: sin A = sin δ / cos φ, counted from east (sunrise) or west (sunset) towards the side of the declination; TB = 90° − A at sunrise, 270° + A at sunset (A north positive).

3. Compass error

   Error = TB − CB: if the true bearing is larger, the error is east (+), otherwise west (−).

4. Deviation

   Dev = Error − Var, valid for the heading at the time of the bearing. Variation comes from the IGRF model and can be overwritten, e.g. with the value from the chart.

5. Gyro compass

   gyro bearing + gyro error + speed error = TB, with the speed error from speed, gyro heading and latitude.

Worked examples

Things to watch out for

• Time to the second in UT: The sun moves about 1° in four minutes – a time error goes straight into the compass error.
• Note the heading: The deviation only applies to the heading at the time of the bearing. For the deviation card, check several headings.
• True, not visible sunset: For the amplitude take the moment when the lower limb is about half a diameter above the horizon – not when the sun disappears.
• Plausibility: The app flags unusually large compass errors – usually time, position or bearing is wrong.

How it works in NauticTools2

1. Choose method and compass

   Azimuth bearing or true sunrise/sunset, magnetic or gyro compass.

2. Time and position

   Date and time of the bearing in UT, latitude and longitude.

3. Enter the bearing

   Compass bearing of the sun, optionally the compass heading for comparison with the deviation card. Variation is filled in from the IGRF model and can be changed.

4. Result

   True bearing, compass error and deviation with the working; for the gyro, speed error and gyro error. Inputs can be saved and the result printed.

Frequently asked questions

How do you check the compass by the sun?

Take a compass bearing of the sun and note the exact time. Time and position give the true bearing; the difference from the compass bearing is the compass error, and after subtracting the variation the deviation remains.

What is the amplitude?

The angle between east or west and the direction of the sun at true sunrise or sunset: sin A = sin δ / cos φ.

When is the compass error east?

When the true bearing is greater than the compass bearing. The error is then positive, otherwise negative (west).

Related tools

• Magnetic variation – variation from the IGRF model for place and date
• Bearing conversion – compass, magnetic and true bearings
• Rising and setting times – when true sunset is