What is a nautical mile?

For instance, in the US and the UK, distance is still measured in the archaic imperial miles, whereas we’re a little more familiar with the metric system.

However, if you’re on the water (or in the air, and even in space), the main unit of length is the nautical mile. So, how long is it, and where does the measure come from?

First, we need a bit of a history lesson, going back a few centuries to when navigators could use the sun to tell how far around the Earth they had travelled, but not how far up or down it.

Nautical miles are based on the circumference of the Earth. To help with navigation, the Earth was split into 360 equal slices – a full circle is 360 degrees – vertically and horizontally.

The vertical slices are known as longitude, measured in degrees. Each longitude is further divided up into 60 smaller sections called minutes.

One of these smaller “minute” sections is one nautical mile long.

How long is this in language we can understand? If you are familiar with the imperial system, one nautical mile is the equivalent of 1.15 miles. If you’re more metric-minded, it’s the equivalent of 1.85 kilometres.

In theory, then, a nautical mile is almost twice as long as a kilometre. What’s that old joke about a farmer living closer to town under the imperial system than under the metric system?

Nautical miles are also the basis on which boat speed is measured. A boat that travels one nautical mile in an hour is doing the equivalent of one knot.

A boat travelling at 10 knots, then, can travel 10 nautical miles in an hour.

So where does “knot’ come from? A knot is a measure of speed, a function of distance over time.

Before the age of modern electronics, ships used a timber wedge shaped like a quarter of a pie and tied to a length of rope to measure how fast they were going.

The board was thrown overboard and remained fairly stationary in the water compared with the moving ship. The person measuring the boat’s speed would then count how many equally spaced knots tied into the rope at about 14-metre intervals passed through their hands as it paid out, over a set amount of time determined by an hourglass.

Crunch the maths and out have the boat’s speed.

Why do we still use nautical miles? Well, it’s probably for the same reason we still use “mayday” to call for help – it’s too deeply rooted in tradition for anyone to change it.