The Metrication of Navigation

Extract

THE following notes stem from Ronald Turner's somewhat controversial paper on the metrication of navigation.

The unique feature of the nautical or sea mile as a unit of distance is that it is directly related to a commonly-used unit of angular measure. To be more precise, the nautical mile may be defined as the length of a meridianal arc of the terrestrial sea-level surface, the geographical latitudes of the end points of which differ by one-sixtieth of a degree. In other words, the sea mile is a minute-mile, and the resulting convenience to mariners is one that is not too lightly to be sacrificed on the altar of the metric system.

The association of the seaman's unit of distance with the unit of angular measure and with the size of the spherical Earth, became a serious consideration soon after the initiation of the Age of Discovery by the Portuguese navigators sponsored by Prince Henry. Before this epoch, and indeed for some time after, seamen used landsmen's units of measure: units derived from homely, but certainly non-scientific, standards such as the length of a thumbjoint, forearm, foot and pace, and in no way related to the size of the Earth.

The inconveniences in trade and commerce caused by a wide variety of units of length and weight must always have prompted men to suggest standardizing these units.