Why the magnetic compass is wrong
Denis Korablev
  • 27.09.2021
  • 247

Why the magnetic compass is wrong

The official history of the compass begins with a description in a Chinese philosophical treatise of an instrument for offshore orientation. This date dates back to the 3rd century, although indirect confirmation of the use of primitive compasses by sailors of antiquity can be found in the works of Herodotus.

The general principle of the compass is based on the implementation of the physical principle of the ability of the magnetic needle to orient itself along the magnetic lines of the Earth. The first, the simplest compasses, were a magnetic needle (most often in the form of a fish), which moved freely in a vessel with water. At the same time, the head of the fish pointed to the south. Wind protection of the magnetic needle was provided by a glass plate. This compass was replaced by a more improved version - a magnetic needle was attached to a thin needle located in the center of a circle divided into 16 points. Later designs of the compass, the same needle with an arrow, were fixed on a gimbal, which made it possible to compensate for the sharp movements of the compass when rolling.

The age-old practice of using a compass in shipping has shown that a magnetic compass has the ability to show an inaccurate direction to the north, and when approaching the magnetic poles of the Earth, this error only increases. Nevertheless, despite the abundance of modern instruments capable of determining the location of the boat with an accuracy of a meter, sailors are not going to refuse the services of a magnetic compass. The whole advantage of the compass consists of its simplicity in operation, complete disregard for radio interference, autonomous operation without power supply.

Yachts, like small vessels, having logs, lots, goniometric instruments and tools for working with a map in the navigation set, are necessarily equipped with a magnetic compass. Even a beginner yachtsman using charter yacht knows that without determining the cardinal points and direction of movement, laying a course is impossible.

But, it would seem, an absolutely simple and reliable device, sometimes it definitely starts to "lie", showing the cardinal directions with some deviations. It was found that the reason for this behavior of the navigation device was the problem of magnetic declination and deviation.

Magnetic declination

It became known for a long time that the geographic poles of the Earth do not coincide with the magnetic ones. At the beginning of the 19th century, the laws of earthly magnetism legalized the correction for declination on all maps. The planet's magnetic poles are over 700 miles from the geographic (North Pole), and 1,500 miles from the geographic (South Pole). The angular difference formed by the direction of the compass needle to the North Pole and its actual location is commonly referred to as "magnetic declination." There are western and eastern declinations.

East declination indicates that the earth's magnetic field is pointing from the east side of the North Pole. West declination, respectively, shows the same direction on the west side of the pole. There are places where the declination value is negligible, and it makes no sense to take it into account.

But there are territories on the planet where the magnetic declination can change, sometimes reaching 25 °. The magnetic declination in the Black Sea is 7 °, and they can increase when approaching deep bedding of rocks. The skipper must be aware that error deviates course clockwise. Modern maps have tooltips indicating the declination angle at any point in the route, allowing you to make the necessary corrections. When starting orienteering, you need to use information from a reliable source about the exact value of the local declination.

Constant fluctuation (movement) of the Earth's magnetic field (1 ° in 8 years), leads to a constant change in the declination angle. If the map is 20 years old, the correction should be based on how much the pole has moved during this time. For this reason, it is simply not recommended to use old maps.

Many modern compasses have a special additional scale, with which you can adjust the declination for each location. The procedure is available even for beginners: take a direction using a goniometer, and then rotate the compass until the direction coincides with the declination for the given area. It should be borne in mind that in addition to declination, there are other phenomena that can make the compass "lie". These can be deposits of magnetic minerals, iron ore, etc.


Deviation (from Latin deviatio - deviation) - deviation of a moving vessel from the course under the influence of any random external causes.

If the magnetic declination is a relatively constant value, then the deviation directly depends on the ship's magnetic field. The compass needle, responding to the presence of metal, deviates from normal readings. A striking example of deviation are the events described in the excellent book by J. Verne "The Fifteen-Year-Old Captain". An ordinary metal ax, installed by the intruders near the compass, led the inexperienced captain to unknown lands. By the nature of the occurrence, semicircular, quarter and roll deviation are distinguished.

Semicircular deviation is created by magnetically solid iron. Usually, this is the metal hull of the ship. The quarter deviation is created by soft iron (a yacht partially with structural metal). The origin of the heeling deviation is simpler - the pitching of the vessel.

The invention of the flindersbar, which is a magnet - destroyer or compensator, in a different way, helped to completely eliminate the deviation. With the correct combination and placement of expansion joints, the compass will be fine-tuned and show true heading.

A modern yacht, whether made entirely of wood, is also capable of creating deviation. This is facilitated by modern mechanisms and devices, which a modern yachtsman cannot do without. The binnacle where the magnetic compass is located may succumb to the deviation effect of a nearby electrical cable, compass errors may appear due to electromagnetic radiation from electronic devices, stray currents, the magnet of a nearby speaker. Deviation arises from the magnetism of the engine, from the moving parts of the gearbox of the outboard motor, the compass needle is able to react to metal things in the pockets of the helmsman or skipper. You can eliminate the deviation yourself, with the help of compensators, but it is preferable to call a professional tuner. Using the effective "Eri method", by performing certain maneuvers, it is possible to completely eliminate the deviation. After the destruction of the deviation, the ship's magnetic compasses have a residual deviation, which usually does not exceed 2-3 °.

The rules of technical operation provide for the mandatory destruction of the deviation of the magnetic compass at least once every six months. Depending on the performance of certain repair work on the ship (electric welding, for example), after loading certain cargo that is capable of changing the magnetic state of the vessel (cargo of metal pipes, rails, metal structures), it is necessary to carry out additional destruction of the deviation. This takes 2 to 4 hours and an experienced skipper in the mission will make reference to this time. In commercial terminology, "deviation" means the illegal deviation of the vessel from the voyage specified in the policy, and has nothing to do with the corrections for the discrepancy between the magnetic and geographic meridians.

The skipper must remember that the skillful use of a magnetic compass is a guarantee of a clear course, timely completion of the assigned task, and safe sailing. The experts 2yachts point out that the correct use of the compass, the ability to use the map and make the necessary corrections to the course, are guaranteed to bring the yacht to the desired port.