Is a nautical mile the same as a ‘normal’ mile?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

09 June 2019

Reports about the Kenya – Somalia maritime boarder dispute have introduced a new unit of measuring distances into common speech. So, Simon Wainaina ask a straightforward question: What is a nautical mile and is it different is it from a normal mile?

The answer is yes: the nautical mile is different from the ordinary (or statute) mile. The latter has a boring definition: it is equal to 1,760 yards. One yard is three feet; one foot is twelve inches and one inch is defined by international agreement as 2.54 centimetres.

Multiplying all those numbers (2.54 x 12 x 3 x 1,760) reveals that one statute mile is equal to 160,934.4cm. Now since 100cm make a metre and 1,000m make one kilometre, it is easy to see (by division this time round) that one statute mile is equal to 1.609344km…or approximately 1.6km.

The nautical mile has a more interesting history. It was developed by navigators out of the appreciation that the earth is spherical in shape. Since navigational positions are determined from angles measured with respect to (stationary) stars in the sky, it follows that if you sailed on a straight path all the way around the planet, you would observe your position changing through 360 degrees.

Now, in order to conform to the subdivisions of time, navigators (and astronomers) subdivide degrees into minutes and seconds. Thus, one degree has 60 minutes of arc and one minute has 60 seconds of arc.

In this convention, the nautical mile was originally defined as the distance one travels when sailing along a straight path through an angle of one minute of arc. Thus, by this definition, the circumference of the earth is exactly 21,600 nautical miles (360 x 60).

Unfortunately, life is rather more complicated and such a definition has many challenges. First of all, how do we deal with mountains and valleys? Secondly, do we travel on the east-west path or a north-south one? The two circumferences are not equal.

Nevertheless, since the average circumference of the earth is about 40,000km, it turns out that one nautical mile is about 1.852km. Clearly, this is longer than the 1.6km of the statute mile.

In 1929, the International Hydrographic Organization agreed to fix the definition of the nautical mile to exactly 1,852m or 1.852km. Thus, the 200 nautical miles referred to in the Kenya – Somalia maritime boarder dispute works out to just over 370km (200 x 1.852).

A derived unit from the nautical mile is the knot. This the speed equal to one nautical mile per hour, in other words, one minute of arc on the earth’s surface per hour. This is equivalent to 1.852km/h. Knots are the normal units used in the shipping and aviation industries.

Astronomers have a similarly defined unit of measuring distances using angles. It is called the parsec and it works out to about 31 trillion kilometres or 3.26 light years. The details of how it is derived are a story for another day.