The address of anywhere

Every point at sea has an address: latitudelatitudePosition north or south of the equator, in degrees and minutes. One minute = one nautical mile.full glossary → (degrees north or south of the equator) and longitudelongitudePosition east or west of the Greenwich meridian, in degrees and minutes.full glossary → (degrees east or west of Greenwich). Marine practice writes minutes with a decimal, never seconds: 50°48′.5N 001°06′.7W. Latitude always reads first, and always carries N or S; longitude E or W. Leave a hemisphere letter off and the address is on the wrong side of the planet.

The scales engraved on the chart border are not symmetric in use. The side scales are latitude — and because one minute of latitude is one nautical milenautical mile1,852 m — one minute of latitude. The sea’s unit of distance.full glossary →, by definition near enough, the side scale doubles as your distance ruler. Span a leg with dividers, lift the span to the latitude scale alongside the leg, read off miles. The top and bottom scales are longitude and are useless for distance: longitude minutes shrink towards the poles.

Two units you will use daily. A nautical mile is 1,852 m — and, near enough, one minute of latitude, which is exactly why the side scale doubles as your distance ruler. A cablecableA tenth of a nautical mile, about 185 m — handy for harbour distances.full glossary → is a tenth of a mile, about 185 m: the handy unit for harbour distances and clearances. Speed in knots is simply nautical miles per hour, so chart, log and tide table all speak the same numbers, with nothing to convert.