A compass points to magnetic north, offset from true north by the variation (on the chart), and the boat's own iron adds a deviation that changes with heading.

Three norths: true, magnetic (shifted by the variation) and the compass's own (shifted again by the deviation of this boat's compass).

True bearing of the Sun: 112°
Magnetic bearing (true − variation): 124°
Compass bearing (magnetic − deviation): 121°
Compass error = variation + deviation: 9° W

Compass least, error east; compass best, error west. Knowing the true bearing (from the Sun) and reading the compass gives the error; take away the variation from the chart and what is left is the deviation.

Variation: 12° WDeviation: 3° E

Compass error = true bearing − compass bearing; deviation = compass error − variation

N 45°, Sun's declination N 20°, rising. sin A = sin 20° ÷ cos 45° = 0.484, so A = 28.9°: true bearing 090° − 28.9° = 061.1°.

By the steering compass it bears 068.0°: compass best, error west, 6.9° W.

The chart gives variation 12.0° W, so the deviation is −6.9 − (−12.0) = +5.1°: 5.1° E on this heading.

Compass error and deviation