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
Thumb compass city and park race orienteering. Thumb compass A thumb compass is a type of compass commonly used in orienteering, a sport in which map reading and terrain association are paramount. In cases of homogenous terrain with few distinct features, a bearing between 2 known points on the map may be used. Consequently, most thumb compasses have minimal or no degree markings at all, and are normally used only to take bearings directly from a map, and to orient a map to magnetic north. Thumb compasses are also often transparent so that an orienteer can hold a map in the hand