Going the extra nautical mile


Sir, – It is an interesting reflection on our methods of education that your correspondent Brian Gillen (April 13th) had to learn by rote the conversions between miles, kilometres and nautical miles (nm). Useful indeed to know them by heart, but what if one cannot remember? What should have been taught was that a kilometre was originally defined as one ten-thousandth of the distance from the pole to the equator and that 60 nautical miles equated to one degree of latitude. Given that there are 90 degrees of latitude from the equator to the pole, the rest is simple arithmetic – 10,000km divided by 90 degrees (60nm) equals 111km approximately, so 1nm = 1,850 metres (1,852m by convention).

The nautical mile is closely related to the sea mile which varies slightly with latitude as the earth is not a perfect sphere, and the sea mile was later defined as 6,000ft or 1,000 fathoms.

Why 10,000km from equator to pole? This was in the 18th century when the Earth was being measured by scientific expeditions using astronomical methods to confirm the shape of the planet and to provide a foundation for all terrestrial topographic surveys, mapmaking and navigation.

The metre was later re-defined in terms of atomic constants.

By comparison, the mile was derived from 1,000 (“millia”) paces, ie two steps of about 88cm, said to be the standard marching pace of Roman soldiers. – Yours, etc,



Blessington, Co Wicklow.

Sir, – In closing the debate regarding the nature of a nautical mile, we should defer to the heroic Capt Jack Aubrey RN, who, on being asked this very question by his particular friend Dr Stephen Maturin, replied that in comparison to a statute mile, a nautical mile is a little longer and a good deal wetter. – Yours, etc,


Castlefield Woods,

Clonsilla, Dublin 15.