What Is a Nautical Mile and Why Do We Have Them?

(Last Updated On: April 12, 2019)

What Is a Nautical Mile and Why Do We Have Them?

What Is a Nautical Mile?

You can probably guess that a nautical mile is a measurement of distance, and you might know that nautical miles are used in sea and airfare travel as a unit of distance. But frustratingly, a nautical mile (NM or nmi) isn’t actually equal to one mile. It’s actually 1.1508 miles (or 1.852 kilometers for those who prefer metric).

At the same time, you’re probably wondering what a nautical mile even means. A foot is the length of some dead king’s foot, after all. Well, a nautical mile is by definition, one minute of arc on the Earth.

If you take the Earth as a sphere and then its circumference about the equator, you end up with a ring that would fit perfectly around our planet. This ring is equal to 360 degrees, where each degree is then cut into minutes (or just fractions over 60) to get minutes of arc. If you’re up on your latitude and longitude, the distance between each degree of latitude is roughly 69 miles (111 kilometers). Cut that into minutes (divide by 60), and you get 1.15 miles per minute of arc, also known as one nautical mile.

For those wondering, nautical miles are closely tied with knots, equal to one nautical mile per hour.

Why Use Nautical Miles?

To start, it’s best to remember that the definition of a mile is as arbitrary as it gets. 5,280 feet doesn’t really mean anything, and the fact that one foot was standardized by some dead dude’s foot doesn’t help either. So in essence, the nautical mile was used to make the arbitrary less so. Sort of.

It might not seem very significant to round out nautical miles to either 1 mile or 2 kilometers (and then convert from there), and that’s because in short distances, it doesn’t matter all that much. But when you’re travelling vast distances where you have to deal with the curvature of the Earth, things change a bit.

The Earth is split into degrees of latitude and longitude, one parallel to the equator, the other perpendicular, and that’s the origin of those grids you see on maps. Because this has to be standardized based on a unit of no standard we created the nautical mile (yes it would have been easier to just have a standard unit, but the Romans and the Imperial system are an adventure). So in short, one nautical mile is a constant to link latitude and longitude together on a grid.

But What Is a Knot?

So what is a knot? Well, it’s when you take two strings and tie them together.

Joking aside, as stated above, a knot is also a measurement of rate, specifically nautical miles per hour. But you probably already knew that to some degree, and you’re likely wondering why we call them knots.

It’s not that far off from straight up tying things together. The term “knot” itself is a derivation from old seafaring practices. Way back when we didn’t have more advanced technology for gauging speed, sailors would use knotted ropes (referred to as “logs”) tied to to their ships. These logs would be thrown overboard to trail behind the moving boat, and the number of knots that fell into the water over time was a measurement of speed, in knots.

We’re more partial to GPS units, though.

Now that you can properly explain what a nautical mile is, test your nautical knowledge in the quiz below!

About the Author:

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Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.