Source: Mariners Learning System, by Captain Bob Figular & americanboating.org

The magnetic compass, even though it has been around for a long time, is still very important for safely navigating a boat. Whether steering a course out of sight of landmarks or in poor visibility, the magnetic compass is the primary tool for guiding the boat to its destination.

The magnetic compass is standard equipment on all boats. Mechanically, it is a simple piece of equipment. The magnetic compass is used to determine the boat’s heading. A prudent mariner will check its accuracy frequently realizing that the magnetic compass is influenced, not only by the earth’s magnetic field (Variation), but also by fields radiating from magnetic materials aboard the boat (Deviation). It is also subject to error caused by violent movement as might be encountered in heavy weather.

The arc of the compass card is divided into 360 degrees (°) and is numbered all the way around the card from 000° through 359° in a clockwise direction. Attached to the compass card is a magnet that aligns itself with the magnetic field around it. The zero (north) on the compass card is in line with the magnet or needle attached to the card. When the boat turns, the needle continues to align itself with the magnetic field. This means the compass card stays stationary and the boat turns around it.

The lubber’s line is a line or mark scribed on the compass housing to indicate the direction in which the boat is heading. The compass is mounted in the boat with the lubber’s line on the boat’s center line and parallel to its keel.

Direction is measured clockwise from 000° to 359°. When speaking of degrees in giving course or heading, three digits should always be used, such as 270° or 057°. The heading of 360° is always referred to or spoken as 000°.

Directions measured on a chart are in true degrees or magnetic degrees as follows:

• True direction uses the North Pole as a reference point.
• Magnetic direction uses the magnetic North Pole as a reference point.
• True direction differs from magnetic direction by variation.

Directions steered on the compass by the boat are magnetic degrees.