Pancyprian Compass & Instruments key terms

The angle between the direction of the Earth's magnetic field at a place and the direction of true north, measured in degrees east or west of true north and shown on charts for a stated year.

The angle between the direction the compass needle actually points and magnetic north at the ship's position, caused by the vessel's own magnetic fields and changing with the ship's heading.

The total angle between the compass needle direction and true north on a given heading; it equals variation plus deviation, with east and west signs combined algebraically.

A table or curve, posted near the compass, giving the residual deviation for each compass heading or ship's head, prepared after a swing and used to correct courses and bearings.

The procedure of turning the vessel through a full circle, or a series of headings, comparing compass bearings with known true or magnetic directions so that deviation on each heading can be measured and tabulated.

The process of placing and tuning corrector magnets and soft iron spheres or Flinders bar near a magnetic compass so that induced deviation is reduced to small, stable residual values.

Pairs of soft iron spheres or plates mounted on the binnacle that neutralise deviation varying with twice the heading, produced mainly by the vessel's induced magnetism in horizontal soft iron.

Fore-and-aft and athwartships permanent magnets in the binnacle used to cancel deviation that changes sign every half turn, arising from the ship's own permanent magnetism.

A soft iron vertical bar fixed to the binnacle to compensate for deviation caused by the ship's vertical soft iron magnetism, particularly the change in deviation when sailing between the northern and southern magnetic hemispheres.

Deviation that appears when a vessel rolls or heels, caused by vertical magnetic force acting on the compass when the ship is inclined; largest on north and south headings and corrected by a vertical magnet and soft iron.

The stand housing a magnetic compass, fitted with corrector magnets, soft iron spheres and a Flinders bar, and often including a light, gimbals and a viewing hood.

A pair of perpendicular pivoted rings that keeps a compass card or instrument horizontal despite the vessel's roll and pitch, allowing the card to float freely.

The circular disc of a magnetic compass, marked in degrees and sometimes points, that rotates with the needles and is read against a lubber line to give heading.

The reference mark inside a compass bowl, aligned with the vessel's fore-and-aft line, against which the compass card is read to obtain the ship's heading.

A direction instrument whose magnetised needles align with the local magnetic field, indicating magnetic north and, after applying variation and deviation, giving true direction.

A north-seeking instrument using a rapidly spinning rotor and gravity or earth-rate control to align its axis with true north independently of the Earth's magnetism.

The angle between the direction indicated by a gyrocompass and true north on a given heading and latitude, determined by comparison with a known bearing and applied to gyro readings.

The heading at which a gyrocompass rotor axis comes to rest, oscillating about true north after the instrument is started and has had time to settle.

The means by which oscillations of a compass or gyro are reduced so that it settles at its indication instead of swinging indefinitely, for example by fluid or by an offset weight.

The direction of the Earth's geographic North Pole along a meridian at the observer's position, the reference against which variation, deviation and compass error are measured.

A line on a chart joining places of equal magnetic variation, used with the annual change note to estimate variation for the date of the voyage.

The particular isogonic line along which magnetic variation is zero, so that magnetic and true north coincide for an observer on that line.

The true bearing of a celestial body or terrestrial object measured clockwise from north, used to check compass error by comparing observed and calculated directions.

The bearing of a celestial body at rising or setting, measured from east or west toward north or south, used with its declination and the observer's latitude to find compass error.

A dumb compass consisting of a graduated ring and sighting vanes mounted on a stand, used to take bearings of objects relative to the ship's head for compass checks and pilotage.

The direction of an object measured clockwise from the vessel's heading rather than from north, obtained by pelorus or bearing compass and converted to a true bearing by adding heading.

The direction in which the vessel's fore-and-aft line points, expressed as a compass, magnetic or true direction and read from the compass against the lubber line.

The intended or actual direction of travel of a vessel over the ground or through the water, distinguished from heading when current or leeway causes the track to differ.

The angle at which the Earth's magnetic field dips below or rises above the horizontal at a place, greatest near the magnetic poles and zero near the magnetic equator.

The liquid-filled container holding the compass card and needles, sealed with a glass top and fitted with expansion bellows so the liquid can change volume with temperature.

The fluid filling a magnetic compass bowl, usually a refined spirit mixture chosen for low freezing point, low viscosity and damping of card oscillation.

A magnetic compass whose card movements are sensed electronically or optically and repeated at remote displays, allowing heading to be shown on the bridge and to other instruments.

A bearing measured relative to magnetic north, obtained directly from a magnetic compass or by converting a true bearing using the local variation.

The minimum distance from a magnetic compass at which magnetic material or electrical equipment may be placed without causing unacceptable deviation, stated on the equipment or in the manual.

Two reference paths used with compass directions: a great circle is the shortest route on the sphere, while a rhumb line crosses all meridians at the same angle and plots as a straight line on a Mercator chart.