Navigation and Seamanship

1.2.11. Distance in Navigation

                                                                            

         The shortest distance (abbreviated as Dist., symbol D) between any two points on the surface of the earth is always along the great circle between them. The more closely the plane of a small circle approaches the center of the earth, the more closely will distance measured along it approach the shortest distance. The converse is also true, of course

          Speed (S) is rate of motion, or distance per unit of time. A knot (kn.), the unit of speed commonly used in navigation, is a rate of 1 nautical mile per hour.

         Since a great circle is the shortest distance between two points on the surface of a sphere, it might be supposed that it would always be the route selected unless there were intervening dangers, such as reefs or shoals. The practical objection to following a great-circle route is that the direction of a great circle is constantly changing; it makes a different angle with each meridian it crosses from starting point to destination. This means that the ship’s heading on a great-circle route would be subject to continuous alternations.

        Since constant heading changes are scarcely practical, it is customary to follow a rhumb line, or a series of rhumb lines, rather than to follow a great circle.

         In 1929, the international community agreed on the definition of 1 international nautical mile as 1852 meters, which is roughly the average length of one minute of latitude i.e. one minute of arc along a line of longitude (a meridian).
         Or to put it shortly: 1 nm = 1' or 1 ° of latitude = 60 nm