We all know that a mile is just a measure of distance that you use in your car. Navigators also use the nautical mile to measure distances at sea. You may not realise, however, that the mile we use at sea is different to the mile used on land.
A nautical mile is the measure of distance that navigators use at sea. It is 1852m, or 1 minute of the distance along the Earth’s equator. It is especially useful when navigating on a global scale because it is mathematically linked to the circumference of the Earth.
How does the Nautical Mile Compare with Land Distances?
The nautical mile is 1852m, which longer than the statute mile that we commonly use on land. A statute mile is 1609m, which in turn is longer than a kilometre at 1000m.
Nautical Mile = 1852 m
Statute Mile = 1609 m
Kilometre = 1000 m
At 1852m, it may seem like some arbitrary distance, but it is not. It’s mathematically linked to the size of the globe. It is 1/60th of a degree of longitude along the equator.
Derivation of the Nautical Mile
Imagine slicing the world through the equator. You are left with two hemispheres: the northern hemisphere; and the southern hemisphere. The base of each hemisphere would be a circle.
You can measure a circle in a few different ways. Either you can measure the circumference and get a distance, or you can measure mathematically and get 360°.
The physical distance around the equator is roughly 40,000 km or 40,000,000 m.
Linking the physical distance to the mathematical distance is relatively simple. You divide by 360, which tells you that each 1° around the equator is around111 km.
As 111 km is still quite a large distance to relate to, we can break it down further. Each 1° is made up of 60 minutes, so we can divide by 60 to find 1 minute equates to 1852 m. 1852 m is a manageable distance, so has been renamed as the Nautical Mile.
How does that Actually help?
To realise the full potential of equating the nautical mile with a minute on the equator, you have to look at the shape of the Earth.
The Earth is actually an oblate spheroid, but we can approximate it as a sphere. We need to use spherical trigonometry to successfully navigate on the surface of a sphere.
Spherical trigonometry effectively gives you courses and distances in degrees and minutes. Linking the nautical mile to degrees and minutes means it is simple to turn it back into a distance we can understand.
Why not use a Statute Mile?
There is nothing stopping you from using the statute mile in your calculations. You would just be adding an extra layer of complexity and introducing a potential source of error.
Using a nautical mile means that you can directly transpose 1 minute of distance = 1 nautical mile.
To use a statute mile instead, you would just have to transpose 1 minute of distance = 1.151 statute miles.
Of course, if you start transposing your calculations to be in statute miles, you also need to transpose all your equipment to give statute miles as well. Historically we have always used the nautical mile, so that is how everything is set up.
Nautical Mile and Speed
Speed is a measure of how far you travel in a given time.
On land, we use either miles per hour or km per hour. Given that a different unit of distance us used at sea, the speed measurement also changes. It simply becomes nautical miles per hour, or knots.
In the past, sailors would lower a wooden log over the stern of their ships to measure speed. The log was tied to a rope with knots tied along it. They would count the number of knots passing through their hands in a given time to get their speed. “Knots” became the unit for a ship’s speed, and the “log” became the instrument for measuring the speed.
If you look at charts, you will sometimes spot areas labelled “measured mile”.
They measure the official speed of vessels. A measured mile is just a timed run between two points that are exactly 1 nautical mile apart.
Measured miles are still marked in many places around the UK. They are visible, with a transit marking the beginning and the end.
Of course, the advent of GPS means that measured miles are becoming obsolete.
Who uses the Nautical Mile now?
The maritime industry isn’t the only one that uses the nautical mile.
The aviation and the space industry both also use nautical miles as their measure of distance. When you look a the similarities between those, and the maritime industry it is clear why. All three navigate on a global scale. Courses and distances follow spherical paths referenced to the earth’s surface.
Planes and space craft also use knots for measuring their speed. Just like with the maritime version, knots just measure nautical miles per hour.
Even though it is not longer in the nautical environment, the navigation techniques are still the same.