The sun stands high and breathes a halo, a path of glints runs from it to the observer, and each glint flares in its own rhythm, so the water never stands still for a second. Three trains of waves run against each other at different speeds, a ship crosses the plate above the horizon, and along the meridian arc, dotted from the equator to the pole, an amber marker creeps — that very minute of arc from which the mile was born. Along the graduated line at the bottom the log marker runs, counting off the distance made good.
Unit passport
Non-SI · accepted for navigation sheet 1/1 · rev. 2026-09Nautical mile M · nmi
Non-SI unit · quantity: length · exactly 1852 metres
A unit you can see in the sky
Land measures of length were always brought from somewhere: the cubit, the foot, the royal standard in a cellar. At sea there was nothing to bring, but overhead hung a ready-made ruler — the sky with its angles — and the navigator solved the problem elegantly: let the unit of distance be the arc along the Earth’s surface that corresponds to one minute of angle. Then distance and coordinate are one and the same, and by measuring the altitude of a heavenly body with a sextant the mariner gets not only his position but at once the distance run, with no conversion tables at all.
Hence a simple rule still in use today: one degree of latitude is sixty miles, and the side border of any nautical chart, where the minutes are marked, serves as a scale. Take a distance between two points with dividers, lay it against the side border at the same height — and you have miles, without looking at any scale bar, because nature herself has drawn it there.
The trouble is that the Earth is not a sphere but a slightly flattened ellipsoid, and a minute of arc at the equator is shorter than at the pole by almost nineteen metres. For three centuries every maritime power settled this in its own way, until in 1929 in Monaco they agreed once and for all: the mile equals one thousand eight hundred and fifty-two metres, exactly, with no proviso about latitude.
The International Organization for Standardization and the maritime conventions write the symbol as a single capital Latin M, aviation and meteorology prefer three lower-case letters — nmi — and older English documents use NM, which is easily confused with the nanometre. In English the word takes a plural, the symbol does not; a non-breaking space goes after the number, and dotted abbreviations such as “n. m.” do not exist.
The nautical mile is longer than the statute land mile by almost fifteen per cent, and substituting one for the other in passage planning gives an error that is noticed only on the approach to the coast. The knot is not a distance but a speed, one mile per hour, so “knots per hour” is wrong; the geographical mile measures a minute of arc on the equator, not on the meridian, and is therefore slightly longer.
01 · Definition
minute of arc · flattening · the 1929 agreementThe nautical mile is defined as one thousand eight hundred and fifty-two metres, and the definition is final: there is no reference to the Earth in it any more. Its historical origin, however, has not gone anywhere — the number was chosen to match the length of one minute of meridian arc, taken at roughly middle latitude.
The match is approximate by necessity. The Earth is flattened at the poles, so near the pole the surface is less curved, and the same minute of angle takes a longer arc: at the equator it comes to about one thousand eight hundred and forty-three metres, at the pole almost one thousand eight hundred and sixty-two. A spread of nineteen metres looks trivial, but over an ocean passage it grows into kilometres, and so the unit had to be assigned rather than measured.
Practice only gained from this. As long as the mile depended on latitude, every chart needed a proviso, and comparing the logs of two ships was a torment; now the mile is constant, and all the curvature of the Earth is built into the chart projection, that is, into the way the chart itself stretches the high latitudes. The navigator still takes distances off the side border, and the error of this method stays within limits that do not matter for navigation.
Inseparable from the mile is the knot — a speed of one mile per hour. The name comes from an instrument: a line with knots tied at equal intervals was paid out astern, and one counted how many ran into the water while the sand ran through a glass. The interval was not chosen at random but so that the count of knots gave the speed in miles per hour at once, and this arithmetic, invented in the sixteenth century, sets the figure on the bridge of every container ship today.
The mile on the chart and the mile under the keel
On the left the ship makes her way under a high sun, and below her runs a graduated line marked every hundred miles, along which the marker of distance run creeps; the faster the chosen ship, the quicker it runs, and next to it you see how long the passage will take. On the right the same passage is shown from the other side: your latitude is laid off along the meridian arc, and the curve beneath it tells you the true length of a minute of arc there against the agreed one thousand eight hundred and fifty-two metres. Move the latitude towards the pole — and you will see how the difference, negligible for one mile, adds up to kilometres over a long passage, and you will understand why they argued so long in 1929.
02 · Conversion
lengths · speeds · national milesOne mile in three conversations
The first tab breaks the entered distance down into nautical and land measures. The second turns it into speeds and passage times. The third shows what the same distance would have come to in the years before the Monaco agreement, had different navies measured it with their own miles.
A rule of thumb: a mile is just under two kilometres, and a knot is about half a metre per second, and both estimates will do as long as fuel is not at stake.
The English abbreviation NM is read now as nautical mile, now as nanometre, and in mixed documents this has led to oddities and once to a serious inquiry. Aviation rules therefore prescribe nmi, and the maritime conventions a single capital M; in running text the safest course is to write the word out.
| Quantity | Value | Note |
|---|---|---|
| {k} | {v} | {note} |
{text}
04 · Measuring instruments
sextant · chip log · chronometer · satelliteFour ways to know how far you have come
The instrument the unit grew out of
The sextant brings two images into the field of view — the horizon and the heavenly body — and lets you read the angle between them on the arc to the nearest minute. It was this very precision that set the value of a division in all of marine navigation: since less than a minute cannot be measured, it is natural to count distance in miles, each answering to one minute. A good observer with a good instrument fixed his position to about a mile, and so it stayed for a century and a half, until radio engineering came into play.
A speed counted by hand
A board on a line was thrown astern, and the knots running through the hand were counted while the sand ran through the glass. The distance between the knots and the time of the glass were matched in such proportion that the number of knots at once equalled the speed in miles per hour, and so the count went without any arithmetic — something worth a great deal on a wet deck at night. Hence the name of the unit of speed, which has outlived the instrument itself: the log has long since become electromagnetic or Doppler, but the knots on the bridge remain.
Longitude that had to be bought with clocks
The sextant gives latitude at once, but longitude requires knowing the time at the reference point, and so it was taken from clocks carried on board. An error of four minutes shifted the position by a whole degree, that is, by sixty miles at the equator — a distance quite enough to put a ship on the rocks. The British Parliament offered a prize for solving the problem, and it was solved not by an astronomer but by the clockmaker John Harrison, whose marine chronometer lost only a handful of seconds on a voyage to the West Indies.
Precision grew, the unit stayed
The satellite receiver computes its position in metres and on the ellipsoid; it needs no minutes of arc at all, yet on the screen it still shows miles and knots. The reason is not inertia but the fact that all charts, sailing directions, collision regulations and the boundaries of areas of responsibility are written in these units, and converting everything at once would cost more than any gain. Besides, the mile keeps its main convenience: it still equals a minute of latitude, and so can be read straight off the chart border.
05 · Writing rules
M and nmi · knot — speed · cableA letter sets the nautical mile apart from the land mile
In ship’s documents the cost of an error in notation is measured not in style but in miles, so the rules here are strict and few.
The first mistake is the most tenacious: the knot already contains the hour, and adding another is like saying “kilometres per hour per hour”. The second is dangerous in mixed documents where optics and navigation sit side by side. The third and fourth put the land mile or an old national mile in place of the international one, and over a passage of a thousand miles the first gives a miss of two hundred and forty kilometres, the second one of a little over a kilometre. The fifth mangles a symbol that simply does not exist. The sixth extends a particular rule to everyone: a tenth of a mile and a hundred fathoms are close but not equal, and in British practice the cable was slightly different, which is worth remembering when reading old sailing directions.
06 · Neighbouring units
knot · cable · fathomA small family has grown up around the mile, and all its members are its fractions or derivatives: the fathom for depth, the cable for the harbour, the knot for speed. They hold together because they let you reckon in your head without going over to decimal fractions.
The only quantity derived from the mile that entered the international rules on a par with it: one mile per hour, that is, a little over half a metre per second.
The measure of short distances: the length of an anchor cable, the passing distance, the interval in a line of ships. Exactly one tenth of a mile, that is, one hundred and eighty-five metres.
The span of a sailor’s arms hauling in the lead line: a measure of depth, handy because it is counted off with the body rather than with an instrument. On old charts depths are marked in precisely this unit.
Among the Simetrium passports, next door stand the parsec and light year: they are built exactly like the mile — distance expressed through an angle, except that instead of a minute of arc of the Earth’s meridian they take a second of annual parallax.
07 · Historical section
archive · from a minute of arc to an assigned numberHow every navy had its own mile
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The spread between the national miles did not exceed three and a half metres, but it surfaced constantly whenever charts and soundings were compared, and with the arrival of radio direction finding it became simply intolerable.
Why the mile survived the metric reform
The metric system displaced almost all the old measures, but at sea it ran up against a fact that does not depend on anyone’s consent: the mariner’s chart is built on a grid of degrees, and distance on it is naturally measured in fractions of a degree. The kilometre is not tied to this grid at all, whereas the mile is tied to it by definition, so a switch to kilometres would have required either tables or a new grid — that is, remaking all of cartography at once.
Add to this the Mercator projection, in which the chart stretches towards the poles and the scale therefore changes from the top edge of the sheet to the bottom. The only ruler that stays true at any latitude is the side border with its minutes, read at the same height as the segment being measured. The navigator takes the distance with dividers and lays it against the border alongside; with a metric scale one would have to keep a correction in mind, and on the bridge in bad weather that is one more chance to make a mistake.
And so the unit was kept but freed of its former instability: since 1929 the mile no longer depends on latitude or on the adopted figure of the Earth, but equals an assigned number of metres. The link with the minute of arc left the definition and remained in practice, and the difference between them is so small that there is nothing on the chart to notice it with. The same device was applied to the light year and the parsec — first the quantity is born of observation, then it is fixed by a number, so as to stop recalculating tables with every refinement.
Catalogue of quantities
length, distance and navigationSI base units
the metre and the second are highlighted — the mile is expressed in metres, the knot adds timePassages and what a mile means on them
kilometres and time computed from the same formula| Passage | Miles | Kilometres | Minute of arc at latitude | |
|---|---|---|---|---|
| {name} | {nm} | {km} | {time} | {arc} |
The last column shows the very difference they argued about in Monaco: on tropical routes the true minute of arc is shorter than the agreed mile, on northern ones longer, and over an ocean passage the total comes to one to three kilometres. For navigation this is immaterial, since the dead-reckoning error from wind and current is an order of magnitude larger, but for surveying the coast and marking out economic zones it matters a great deal, and so boundaries are computed not in miles on the chart but in coordinates on the ellipsoid.