You are on watch at 0300, approaching a headland. The GPS is showing a position, but you want to verify it with a terrestrial bearing. You reach for the azimuth mirror on the compass bowl and take a bearing of the lighthouse — but before you record it, you must understand exactly what you are measuring, and what errors may be embedded in that reading.
What Each Instrument Does
Azimuth mirror (azimuth circle): Fits over the compass bowl. A prism and sight vane allow you to observe a celestial body or terrestrial object and read its bearing directly off the compass card. Used for compass error checks (by celestial azimuth or amplitude) and for taking position-fixing bearings.
Pelorus (dumb compass): A bearing ring graduated 000°–360° with sight vanes, but containing no magnetic element. It is set to the vessel's heading and used to take relative or true bearings when the magnetic compass is inaccessible or unsuitable. The helmsman must hold a steady course at the instant of observation, and the reading must be applied to the ship's heading to derive a compass or true bearing.
Hand bearing compass: A self-contained magnetic compass with integral sight, used away from the binnacle. Subject to its own deviation and to the influence of nearby ferrous objects — the candidate must stand clear of metalwork, winches and deck fittings when using it.
Compass Error and Why It Matters
Any bearing taken through an azimuth mirror on the steering compass contains both variation and deviation. To plot the bearing as a true bearing you must apply the total compass error (variation ± deviation). This is why a current deviation card for each heading is essential, and why regular azimuth checks are used to verify that the deviation card remains valid.
Taking a Compass Error by Terrestrial Bearing
If a charted object has a known true bearing from your position (e.g., a transit, or a bearing derived from the chart), compare it to the compass bearing of the same object taken through the azimuth mirror. The difference is compass error. Resolve it into variation (from the chart) and deviation (the residual). This method cross-checks the deviation card and validates the compass.
Taking a Compass Error by Azimuth of a Celestial Body
Calculate the true azimuth of the sun (or other body) using sight reduction tables, a nautical almanac, or an approved calculator. Compare it to the compass bearing taken at the same instant through the azimuth mirror. The difference is compass error. This is the most common method at sea and is examined directly in the oral.
Key Practical Points
- Record the ship's head, time, and body/object for every compass error observation.
- An azimuth mirror reading is only as good as the compass it sits on — check the bowl is correctly levelled and the card is swinging freely.
- With a pelorus, the helmsman's course must be steadied before the bearing is called; a yawing vessel produces an unreliable result.
- Hand bearing compass readings should always be treated with caution if taken near the binnacle, engine room casing, or any ferrous structure.