Compasses

The compass is an instrument is used for navigation; it generally has a magnetic needle that points towards the Earth’s magnetic North Pole. This is without doubt the most important of all instruments aboard even the most modern vessel, and it is probably the most reliable. The earliest compasses were most likely invented by the Chinese in around 1050 BC. They were created first for the purposes of spiritual life or developing a feng shui environment and then later used for navigation. It was used for maritime navigation by 1117. The use of a compass is recorded in Western Europe between 1187 and 1202 and in Persia in 1232.

There are two widely used and radically different types of compass:

The magnetic compass. The magnetic compass, the most simple and common type of compass, contains a magnet that interacts with the Earth’s magnetic field and aligns itself to point to the magnetic poles. This compass points to the Earth’s magnetic North Pole.

The compass rose. The compass rose is a depiction of orientation and directional that is placed on compasses, maps and charts. Thirty-two points are depicted around a circle in equal intervals, making the four cardinal directions (N, E, S, W), the four intercardinal directions (NE, SE, SW, NW), and the other sixteen secondary intercardinal directions (NE by N, N by E, etc.) Most modern compasses use the 360-degree system of indicating direction on compass. Though the use of degrees, navigation is more accurate than through the use of the compass rose.

Why do we need great compass accuracy at sea? The marine magnetic compass at sea is mainly used for steering a course. The compass reading must be accurate, especially on long sea passages, because a error of one or two degrees in a long course can make a difference of thousands of nautical miles in reaching your destination after many days.

The compass is also used to take bearings of terrestrial and celestial objects for navigation, and errors must be minimized for this purpose. The compass reading at sea must be therefore corrected as accurately as possible.

Compass error is divided into two parts, namely magnetic variation and magnetic deviation.

Compasses are used to determine the direction of true North. However, the compass reading must be corrected for two effects. The first is magnetic declination, the angular difference between magnetic North (the local direction of the Earth’s magnetic field) and true North. The second is magnetic deviation, the angular difference between magnetic North and the compass needle due to nearby sources of iron. Magnetic deviation occurs when a large iron or steel mass warps the magnetic field near the compass. This can be caused by the ship itself or by the cargo if it made up of steel or steel containers.

A gyrocompass is a type of non-magnetic compass which has a needle that spins in relation to the rotation of the Earth to automatically find geographical direction. Gyrocompasses are widely used for navigation on ships, because they have two significant advantages over magnetic compasses:

- they find true north as determined by Earth’s rotation, which is different from, and navigationally more useful than, magnetic north;

- they are unaffected by ferromagnetic materials, such as ship’s steel hull, which change the magnetic field.