Magnetic Compass

A magnetic compass aligns its freely suspended magnetic element with the horizontal component of the earth's magnetic field, indicating magnetic north. As the oldest navigational instrument still in operational use, it remains the mandatory backup compass on all SOLAS vessels under SOLAS Chapter V, Regulation 19, precisely because it requires no power supply and no initialisation period. Its primary errors are variation—the angle between true and magnetic north, printed on navigational charts—and deviation, caused by the vessel's own magnetic field and recorded on the deviation card for each heading. The standard compass is sited in the binnacle on the monkey island or steering stand, away from steel structures and electrical equipment that would induce deviation. The OOW swings the ship periodically so that the deviation card, prepared by a qualified compass adjuster, remains current. Bowditch (American Practical Navigator) sets out the full correction sequence: compass to magnetic (apply deviation), magnetic to true (apply variation), using the memory aid CADET (Compass Add DEviaTion). In restricted visibility or power failure, the magnetic compass may be the sole heading reference, making familiarity with its use and its errors a non-negotiable watchkeeping competency.