Why Knot?

A knot is simply a measurement of speed, one nautical mile per hour. That was easy! But wait a minute, you say, what is a nautical mile? A nautical mile is 1.1508 land miles, or 1,852 meters, or 6,076.1 ft. So why is a nautical mile different from a land mile? In a nutshell, because you can’t walk on water.

The term “mile” comes from Latin word for one thousand (mille), as in one thousand paces (mille passus), so a mile could be measured more or less accurately as a thousand paces. Not possible at sea, where distances were estimated by eye. Skip to the 1500s where some clever scientists and mathematicians determined the circumference of the earth, and consequently the distance covered by one degree latitude, which finally provided a universal way to measure distance at sea. British mathematician Edmund Gunter is given the credit for proposing that one nautical mile be defined as one minute of one degree of latitude, which everyone can easily calculate on a map and while at sea. A minute of latitude is simply one-sixtieth of a degree of latitude: there are 360 degrees in a circle, and 60 minutes in each degree (and 60 seconds in each minute of latitude, not relevant here). So by dividing the circumference of the earth (24,024 land miles) by 360 degrees x 60 minutes, you get one nautical mile equals 1.1122 land miles. Case closed, everybody was happy.

Or were they? The earth isn’t precisely round, with its flattened poles, so a minute of latitude isn’t the same distance everywhere on earth. Depending on where you measure it, you get a slightly different value for one minute of latitude, ranging from 1861 meters at the poles to 1843 at the Equator. This, of course, led to general, global disagreement until 1929, when, thanks again to clever mathematicians, the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as precisely 1,852 meters. Case closed, everybody was happy.

Or were they? The UK and US with their Imperial measurement system used different underlying mathematical models to determine the length of a nautical mile. Both had (differing) measurements slightly over 1853 meters and didn’t feel the pressure to change them. The US didn’t adopt the international nautical mile until 1964, while the UK’s Imperial unit (called the Admiralty mile) held out until 1970 before capitulating.

Today, finally, we’re all happily on the same page. Case closed for good.

If asked, just remember that a nautical mile is one minute of a degree, which is a little longer than a land mile, and that a knot is slightly faster than a mile-per-hour.

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