Nautical mile vs mile: let's see how they differ, and what do knots have to do with it

If you have been yachting for a long time, you already know what a nautical mile is and how it differs from an ordinary mile. For those who are a beginner in this outdoor activity or just thinking about trying it, this article will help to understand one of the most important sailing concepts about why there are two types of miles.

What is a nautical mile?

A nautical mile is a unit of measurement derived from the ordinary land mile, which is used to measure distance primarily at sea, but also in the air.

It is defined as 1.151 miles or 6,076 feet or 1,852 meters according to international standards adopted at the International Hydrographic Conference in Monaco in 1929. A nautical mile is abbreviated as M in shipping, NM in aviation, or nmi in other activities. As you can see, a nautical mile is 0.151 more than an ordinary mile.

In turn, the common mile as a concept and unit of measurement first appeared in ancient Rome. So the Romans measured every 1,000 steps taken with the left foot, that is, every second step or two steps with both feet. Translated into meters, the value of the ancient Roman mile (mille passus, lit. "thousand paces") was 1.482 meters.

In addition to the Roman Empire, the mile was used in antiquity and Middle Ages in some other countries. The exact values of the Roman mile were also adjusted and depended on the country. Miles varied from 0.58 km in Egypt to 11.3 km in ancient Norway. In all, between the 15th and 18th centuries, there were 46 different units of measurement in Europe called miles.

Miles were used before the metric system of measures was introduced. Countries with a non-metric system of measures still use the mile, but only the U.S. and a couple of smaller countries.

Why can’t ordinary miles be used at sea? The answer is simple: historically, ships at sea have used the same unified coordinate system for navigation as they do on land, consisting of longitude and latitude. But then sailors, who were also excellent geographers and explorers, noticed that a nautical mile is a minute of arc in the latitude. That is, it is simply very convenient.

Center of Rome, Italy. Gabriella Clare Marino / Unsplash

How exactly is a nautical mile counted, and what does it have to do with knots?

The nautical mile is used not only by yachtsmen but also in many other water activities, such as kitesurfing. I asked the founders of one kitesurfing school, "Kite Club Gals", to explain exactly how a nautical mile is counted:

"It’s very simple. The distance from the pole to the pole of the Earth in circumference is about 20,000 kilometers. If we divide this number by 180 degrees (from the equator to each pole is 90 degrees), then we get 111,111... km per degree of the meridian. Then we divide the resulting distance by 60 minutes (one degree) and get 1.851851851... km or ~1.852 m. It turns out that one mile is equal to one minute of the meridian of the Earth.

In navigation, it is very easy to operate with such categories and measure distances on the map. Let’s not forget to mention one-tenth of a mile, a cable length, equal to 185.2 meters.

There are also four types of cable lengths:

International cable length equals 1/10 nautical mile or 6 arc seconds of meridian or 185.2 meters
Ordinary cable length equals 100 nautical fathoms or 600 feet or 182.88 meters
British cable length equals 608 feet or 185.3184 meters
Old U.S. cable length equals 120 nautical fathoms or 720 feet or 219.456 meters

The concept of nautical mile is closely related to the concept of the knot. A knot is a unit of measurement for the speed of any type of vehicle in marine navigation and sailing, as well as in aviation, where it also has moved from the former. Another application of knots is meteorology.

So one knot equals one nautical mile per hour, which is equal to 0.514 m/s or 1.852 km/h or 1.151 mph when converted into the land speed units. Learn more about knots.

Knot. Miguel a Amutio / Unsplash

To summarize: the prevalence of the knot as a unit of measurement is connected with the considerable convenience of its application in navigational calculations: a vessel going at a speed of 1 knot along the meridian for one hour passes one angular minute of geographical latitude.

Nautical miles and knots are so convenient that they are used not only in navigation at sea but also in aviation, where the same longitude-latitude coordinate system applies. Pilots have adopted the nautical system and often use nautical miles and knots to indicate distance and speed instead of miles or kilometers per hour, although other units of measurement are sometimes encountered.

These units are used in all navigational instruments where distance determination is required: radars, GPS, etc.

Where to use nautical miles and knots?

In the Windy.app you can use nautical miles and knots to navigate at sea when yachting, marine fishing, kitesurfing, and other water sports by selecting this unit of measurement in the app's settings. You can also use them for aviation. For this purpose, the app has several special weather profiles with a ready set of weather parameters for these and other water and air sports.

Nautical miles and knots in the Settings section of the Windy.app for iOS

Read our advice on how to select the measurement units in the Support section of the site. Read about all existing weather units of measurement in a separate article.

Text: Ivan Kuznetsov, an outdoor journalist, editor and writer from the Dolomites, Italy, and Karelia, Finland, with 10 years of professional experience. His favorite sports are cycling, hiking and sauna. Read his other articles

Cover photo: Rolex Middle Sea Race 2019 in Grand Harbour, Malta. Mike Nahlii / Unsplash