Compass has a half-life of only about 12 years, so a compass that contains 120 mCi of tritium when new will contain only 60 when it is 12 years old, 30 when it is 24 years old, and so on. Consequently, the illumination of the display will fade. Mariners' compasses can have two or more magnets permanently attached to a compass card, which moves freely on a pivot. A "lubber line", which can be a marking on the compass bowl or a small fixed needle, indicates the ship's heading on the compass card. Traditionally the card is divided into thirty-two points
Compass secular variation. The effect of this means a map with the latest declination information should be used. Some magnetic compasses include means to manually compensate for the magnetic declination, so that the compass shows true directions. There are other ways to find north than the use of magnetism, and from a navigational point of view a total of seven possible ways exist (where magnetism is one of the seven). Two sensors that utilize two of the remaining six principles are often also called compasses, i.e. gyrocompass and GPS-compass. A "gyrocompass" is similar to a gyroscope. It is a non-magnetic compass