Distances at sea are measured in nautical miles. A nautical mile is defined as 1 minute (1') of latitude (a minute is one-sixtieth of 1 degree of latitude). This distance is slightly greater at the poles than at the equator as the Earth is an imperfect sphere, but a standard nautical mile is internationally accepted as 6,076 ft (1,852 m). A cable is one-tenth of a nautical mile. Speed at sea is measured in knots (nautical miles per hour).
A position at sea is measured in degrees of latitude north or south of the equator and degrees of longitude east or west of the prime (or Greenwich) meridian. It is plotted by marking latitude on the side and longitude on the bottom of a chart. Horizontal and vertical lines drawn from these points will intersect at your position. For example, the position of Cape Cod Lighthouse, Massachusetts, is expressed (in degrees and minutes) as N 42° 02.3', W 70° 03.7'.
compass, you need to be able to convert between true and magnetic bearings. To convert from true to magnetic, add the westerly or subtract the easterly variation. To convert from magnetic to true, add the easterly or subtract the westerly variation.
Onboard magnetic and electromagnetic fields may cause the compass to deviate from magnetic north. Temporary deviation can be avoided by keeping everyday objects, such as beverage cans and MP3 players, well away from the compass. Permanent compass deviation is usually minor unless your yacht is steel-hulled,
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