Knots and Nautical Miles: Why Ships and Planes Use Different Speed Units Than Land

In 1929, hydrographers — the experts who survey the shape and depth of the sea — from a number of countries gathered in a conference hall in Monaco. There was exactly one item on the agenda: how many meters, precisely, is one nautical mile? That might sound like an odd thing to spend an international conference on. It means that even as the 20th century was well underway, humanity — after millennia of sailing the oceans — still hadn’t agreed on the basic unit for measuring a ship’s speed. And that was, in fact, exactly the case. The United States defined a nautical mile as 1,853.248 meters, Britain as 1,853.184 meters, while France’s navy had already been using 1,852 meters since 1906.^[1] A unit that shared the same name meant something slightly different from country to country.

The number this conference agreed on — and everything that followed as ships, aircraft, and cars each ended up with different speed units — hides a different logic in each case. One was a compromise forced by the shape of the Earth. One was a piece of navigational history preserved in the knots of an old rope. One was inertia created by politics and custom. And the last was pure physics.

How One Minute of Latitude Became the Nautical Mile

The nautical mile began as a way of measuring the Earth itself. Draw a circle around the Earth through the North and South Poles — a meridian — and that circle divides into 360 degrees, each degree splitting further into 60 minutes. The length of the arc corresponding to one minute of latitude is, by definition, one nautical mile.^[1][2]

Why choose this particular definition? The answer is practicality. Nautical charts carry a scale of latitude marks running down their vertical edges. A sailor who wants to measure the distance between two points doesn’t need to reach for a ruler — the latitude scale on the chart already is one. Since one minute of latitude equals one nautical mile, counting off a few tick marks gives the distance directly. Coordinates for marking position (latitude and longitude) and the ruler for measuring distance were, in effect, the same scale.^[2]

That’s where the trouble starts. The Earth isn’t a perfect sphere. Because it spins, it bulges slightly at the equator and flattens slightly at the poles — like an orange gently squeezed from top and bottom. On a shape like that, the actual distance covered by one minute of latitude changes depending on where you measure it: roughly 1,843 meters near the equator, and roughly 1,862 meters near the poles, a gap of nearly 20 meters.^[2][3] A definition that looked geometrically elegant on paper fell apart the moment it met the Earth’s actual, irregular shape.

Each country solved this differently. The United States, working from a new survey of the Earth’s shape calculated in 1866 by the British geodesist Alexander Ross Clarke, set its nautical mile at 6,080.20 feet (about 1,853.25 meters). Britain used 6,080 feet (about 1,853.18 meters). The reason the values diverged is simple: since the Earth isn’t a perfect sphere, the “standard” length of one minute of latitude was bound to shift slightly depending on which latitude you used as a reference and which survey data you trusted.^[3]

So, in 1929, the International Extraordinary Hydrographic Conference held in Monaco reached a verdict. It fixed the nautical mile at exactly 1,852 meters.^[1][2] The number wasn’t picked arbitrarily — it sat close to the average of the values then in use, and it matched what the French navy had already been using for more than twenty years. Where geometry had failed, international negotiation stepped in to patch things up. Rather than keep arguing over the Earth’s exact shape, everyone agreed to settle on one clean number and follow it.

What’s striking is that the agreement wasn’t followed everywhere right away. The United States didn’t drop its old value and adopt the international standard of 1,852 meters until July 1, 1954. Britain followed even later, in 1970.^[2][3] That’s a 41-year gap. Unifying a single unit for measuring a ship’s speed took that much time and inertia to overcome — a theme that connects directly to the story of the mile, later in this piece.

What a Knotted Rope Reveals About How Big Old Sailors Thought the Earth Was

The word “knot” itself comes from a much older and much humbler tool: a knot tied in a rope.

Before the 17th century, sailors already measured a ship’s speed with a device called a chip log. A fan-shaped piece of wood was tied to a rope, and the rope itself was knotted at regular intervals. Thrown overboard, the wood would resist the water and stay more or less in place while the ship sailed forward, letting the rope play out through a sailor’s hands. Using an hourglass to time a fixed interval, the crew counted how many knots slipped past during that interval. That count was the ship’s speed. If seven knots went by, the ship was making “7 knots.”^[4][5]

The first printed record of this method appeared in 1574, in a navigation manual written by the Englishman William Bourne.^[5] But back then, the spacing between knots was different from today. The rope was knotted every 7 fathoms (a fathom being roughly the span of a person’s outstretched arms, about 1.8 meters) — 42 feet — and timed with a 30-second hourglass.^[4][5]

Working the numbers back reveals something interesting. Covering 42 feet in 30 seconds, converted to an hourly rate, comes out to 5,040 feet per hour (about 1,536 meters) — far short of today’s precise nautical mile of about 6,076 feet (1,852 meters). That’s not a coincidence. In 16th-century England, a “mile” was reckoned at 5,000 feet, and sailors set their knot spacing by calculating how far a ship would travel in 30 seconds at that old mile’s implied speed. (Converting 5,000 feet per hour down to a 30-second interval gives about 41.7 feet, but since rope was far easier to measure in fathoms — an arm-span unit — than in fractions of a foot, sailors rounded to the nearest whole number of fathoms: 7 fathoms, or 42 feet.)^[6] In other words, the spacing between knots wasn’t just an arbitrary tool specification — it was a record of how big people of that era believed the Earth to be.

This error didn’t stay uncorrected for long. In the 1630s, the English surveyor Richard Norwood walked the distance between London and York himself, measuring it directly. Combining that with observations of the sun’s angle at both locations, he calculated the actual distance covered by one minute of latitude. His results, published in 1637, showed that the old 5,000-foot mile had significantly underestimated the true size of the Earth.^[7][8] As surveying techniques improved and the nautical mile’s value grew more precise, the spacing between knots was adjusted to match. The standard that survives today calls for a knot every 47 feet 3 inches (about 14.4 meters) of rope, timed with a 28-second hourglass.^[4][5]

This number, too, is worth checking. Take the nautical mile at exactly 1,852 meters (about 6,076 feet): the distance covered in 28 seconds works out to 6,076 feet × (28 seconds ÷ 3,600 seconds) ≈ 47.26 feet — 47 feet 3 inches. It lines up exactly, with no error. In the end, the rope’s knots were reworked twice: first, roughly, to match an inaccurate estimate of the Earth’s size; later, precisely, to match a properly surveyed one. Some sources claim the 47-foot-3-inch spacing was already in use “before the 17th century” — but that’s a factual error the math doesn’t support. That spacing could only have emerged once the value of the nautical mile itself had become far more precise.

The Sky Inherited the Sea’s Coordinates

When aircraft appeared in the early 20th century, aviation didn’t need to build an entirely new navigation system from scratch. It could simply inherit the latitude-longitude coordinate system and charting conventions that maritime navigation had already spent centuries refining. The aeronautical charts pilots use to plot ocean crossings carry the same latitude scale down their edges that nautical charts do, and one tick on that scale is still one nautical mile. A pilot or navigator laying a ruler across the chart and counting tick marks gets the distance the same way a navigator in the age of sail did.^[9]

As seen in how the directional terms port and starboard carried over from ships to aircraft, aviation inherited maritime convention in more ways than one. But in the case of speed and distance units, the channel of inheritance is unusually direct. This wasn’t just a vague continuation of custom — aviation adopted the concrete technical framework of latitude-longitude coordinates and charts wholesale, and the nautical mile and knot came along as part of that same framework.

That’s why Annex 5 of the International Civil Aviation Organization (ICAO), adopted in 1948, made the International System of Units (SI) the default for aviation while carving out an explicit exception for the nautical mile, the knot, and feet for altitude.^[10] The reasoning was practical: airlines, air traffic controllers, and pilots worldwide were already fluent in these units, and, more importantly, the units meshed naturally with latitude-longitude-based navigation. The reason today’s cruising aircraft speed is reported as “500 knots” rather than “900 kilometers per hour” traces directly back to this inheritance.

There’s also a practical reason layered on top of that. Pilots, air traffic controllers, and search-and-rescue authorities often have to track the positions of ships and aircraft on the very same coordinate system. When a distress call comes in at sea, nearby vessels and rescue aircraft need to exchange position and distance using the same chart, the same latitude and longitude, and the same nautical-mile unit in order to respond quickly. If aircraft used kilometers while ships used nautical miles, an unnecessary conversion step would creep into every exchange. Sea and sky sharing the same unit, then, isn’t just tradition — it reflects the practical reality that the two domains genuinely overlap on a regular basis.

The Countries That Couldn’t Let Go of the Mile

On land, meanwhile, an entirely different story played out. American and British roads still measure distance in miles today. The mile traces back to the Roman legion’s 1,000-pace march (mille passus), redefined in Elizabethan England in 1593 as 5,280 feet. Its detailed origins are already covered in the history of distance measurement, so here it’s worth focusing on the more interesting question: most of the world switched to the metric system long ago, so why have the United States and Britain never quite let go of the mile?

It’s not that the United States never tried. In 1975, President Gerald Ford signed the Metric Conversion Act, declaring the metric system “the preferred system of weights and measures for United States trade and commerce.”^[11] But the law had a critical flaw: it left conversion voluntary rather than mandatory. No deadline, no penalty. Businesses and consumers had little reason to give up the inches and miles they already knew. Combined with public pushback, the U.S. Metric Board that had been created to oversee the effort was disbanded just seven years later, in 1982, under the Reagan administration.^[11] The kilometer markings that still linger on some signs along Interstate 19 in Arizona are a relic of that short-lived push.

Britain’s path was more tangled. The country officially committed to switching to the metric system in 1965 and planned to convert its road signs by 1973. But a change of government in 1970 put the plan on indefinite hold, and it was never revived over the following half-century.^[12] For a time, European Union rules stipulated that Britain’s use of miles and yards on road signs would be permitted only “until a date set by the government” — but in 2010, the British government successfully negotiated to have that deadline clause removed entirely. As a result, Britain today is free to decide for itself whether to keep the mile.^[12]

Both countries’ failures share a common calculation. Replacing every road sign, reworking speedometers and driver training, and getting an entire population to think in a new sense of distance all cost more, and cause more disruption, than the benefit was judged to be worth. The cultural identity wrapped up in the mile played a role too. Phrases like “go the extra mile” or a “65 mile-per-hour speed limit” are woven deep into everyday language, not just numbers on a sign. In the end, what preserved the mile wasn’t logic — it was inertia, cost, and a lack of political will.

There’s an irony buried in all this, though. As we saw earlier, those same two countries — the United States and Britain — did switch the nautical mile to the international standard of 1,852 meters, in 1954 and 1970 respectively. They never gave up the mile on land, yet they ultimately unified the nautical mile at sea around a metric number. The gap comes down to scale: repainting every road sign is an entirely different order of magnitude, in terms of the public and infrastructure it touches, from changing one reference value on a nautical chart.

For American and British readers, this stalemate probably shows up in daily life in ways this article hasn’t spelled out yet. Neither country ever fully committed either way, so units sit side by side in odd combinations: US drivers measure the trip in miles, but they’ve been buying “2-liter” bottles of soda for decades, and track meets run the 1,500 meters on the same campus where the football team measures yardage. British drivers still navigate by miles, but the pump has charged them by the litre since the mid-1990s, when regulations phased out gallon-calibrated fuel dispensers.^[15] It’s also worth noting that not every English-speaking country got stuck the way the US and UK did. Canada converted every speed-limit sign in the country from mph to km/h over a single Labour Day weekend in September 1977, and Australia switched its entire road network to kilometers on July 1, 1974 — a changeover known locally as “M-day,” largely completed within a month.^[16] The shorthand “English-speaking countries use miles” really describes two holdouts, not a linguistic bloc, and that becomes obvious the moment you look at their closest neighbors.

Mach: A Unit of a Different Kind

The nautical mile, the knot, and the mile covered so far are all units built up from historical accident and custom. But there’s another unit that comes up constantly when talking about aircraft speed — Mach — and it belongs to a fundamentally different category.

Mach isn’t a unit at all, strictly speaking. It’s a ratio comparing an aircraft’s speed to the speed of sound. Mach 2, for instance, means “flying at twice the speed of sound” — it doesn’t point to a fixed number of kilometers per hour. The key is that it’s a ratio, not a unit.

Why measure against the speed of sound instead of an absolute speed? Because as an aircraft approaches the speed of sound, the behavior of the air around it changes. Under normal conditions, air parts smoothly around an approaching aircraft. But as speed nears that of sound, the air can no longer get out of the way in time — it piles up and compresses instead. That compressed layer of air produces a shock wave, and in that instant, the drag, buffeting, and control response the aircraft experiences change completely. The violent shaking and unresponsive controls that fighter pilots experienced near certain speeds in the closing years of World War II were exactly this compressibility problem at work. At the time, without understanding the cause, pilots simply called it the “sound barrier.”

But the speed of sound itself isn’t a fixed value — it slows as altitude increases and temperature drops. So knowing only that an aircraft is “flying at 1,000 kilometers per hour” tells you nothing about how close it actually is to dangerous territory at that particular altitude and temperature. “Flying at Mach 0.9,” on the other hand, always means the same thing regardless of altitude or temperature: “90 percent of the speed of sound, just below the danger zone.” For a pilot, that ratio is far more useful information than an absolute speed.

The unit takes its name from the Austrian physicist Ernst Mach. Working with the photographer Peter Salcher, Mach ran a series of experiments between 1886 and 1888 using special photographic techniques to capture bullets traveling faster than sound, publishing the results in a paper in 1887. The photograph below, taken in 1888 during this same series of experiments, was humanity’s first visual proof that an object moving faster than sound produces a cone-shaped shock wave ahead of it.^[13]

As it happens, the name “Mach number” wasn’t coined by Mach himself. Mach died in 1916, and it wasn’t until thirteen years later, in 1929, that Jakob Ackeret, an aeronautical engineer at ETH Zurich, proposed naming the ratio after Mach during a lecture.^[14] It was, in effect, a tribute paid to a scholar who was no longer alive to receive it. The term first appeared in English-language literature three years after that, in 1932.

Where There Is No Air

The fact that Mach is a ratio carries an interesting consequence. The denominator of that ratio — the speed of sound — only exists where there is a medium such as air, because sound is a wave that travels by shaking matter. In a vacuum there is no speed of sound, and therefore no Mach number to speak of. “Mach 5” is not a meaningful statement about a craft in open space.^[17]

That doesn’t mean spacecraft have nothing to do with Mach. On the way up through the atmosphere and on the way back down, the number still applies: a vehicle returning from low Earth orbit re-enters at roughly 17,500 mph, which works out to a Mach number approaching 25.^[18] The unit simply stops meaning anything once the air thins out to nothing.

Beyond the atmosphere, speed gets described differently. The baseline is ordinary metric: meters or kilometers per second. But in orbital work the number that actually matters is rarely “how fast is it going right now.” It is “how much can it still change its velocity.” That quantity is called delta-v (Δv) — change in velocity — and it, too, is quoted in meters per second.^[19] Raising an orbit, changing direction, coming home: every one of those maneuvers is an act of changing velocity, and every one of them spends propellant. On a spacecraft, in other words, the unit that measures speed doubles as the unit that measures remaining capability.

One more thing is worth noticing. At speeds approaching that of light, the convention is to quote a percentage of light speed. Where Mach is a ratio against the speed of sound, this is a ratio against the speed of light. Only the reference has changed; the underlying instinct — describe a speed by comparing it to the limiting speed of the environment rather than by an absolute figure — is exactly the same.

Four Different Kinds of Logic

The four speed and distance units covered here were each shaped by a different force. The nautical mile started out rooted in the geometry of the Earth, but once that geometry ran into the reality of an imperfect sphere, it was ultimately settled by international negotiation. The name “knot” survives as a record: every knot tied into that rope preserved, in physical form, how big people of a given era believed the Earth to be. The mile began in Rome, was redefined under the Tudors, and survived on American and British roads purely through inertia and the political cost of change. And Mach belongs to an entirely different world from the other three — it’s a ratio born of physical law, not history.

That a ship measures speed in knots, an aircraft uses both knots and Mach, and a car measures speed in kilometers or miles isn’t something a single logic can explain. Each unit is a trace of what the people who first needed it were wrestling with. For sailors, the tick marks on a chart mattered. For drivers, a familiar number mattered. For a supersonic pilot, the instant the air begins to compress mattered. That something as simple as measuring speed splintered this many ways, in the end, just reflects how different the needs of the people who first invented these units really were.