eli5: Why are nautical miles used to measure distance in the sea and not just kilo meters or miles?
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A nautical mile is a easily divisible unit of measurement into the 360 degrees around the earth, with of latitudes providing 180*
It is more a consequence of the fact that we measure a full circle in terms of 360. Which is a “close enough” unit of measurement the Babylonians came up with to approximate a calendar year.
A nautical mile is equal to one minute of latitude (overall average), which approximates as 1.151 statute miles. A knot is one nautical mile per hour. It makes more sense to me to ask why a statute mile is not equivalent to a nautical mile.
The measurements came from different sources and people feel no reason to change once they have gotten used to something. That is why there are different units in different places.
Nautical miles came from measurements of the earth which simplified calculations with a sextant.
Land miles came from ancient Roman measurements.
Kilometres came from standardizing and decimalization of units and, fun fact, uses the same base measurement as the nautical mile. It is no coincidence that a nautical mile is 1.81 km. It comes from it being 180 degrees in a half planet, but measuring things slightly differently.
Just wanted to add that Nautical miles are also used in aviation for the same reason they’re used in sea travel. Because nautical miles line up with latitude they’re more useful for the greater distances of travel you’re dealing with on boats and planes.
The earth turns once around its own axis (360 degrees) every (approximately) 24 hours.
That means it has an angular speed of 15 degrees/hour. These are timezones!
Each of these 15 degrees can be further divided into 60 small parts – these small parts are called arc minutes. They are each 1/60 of a degree.
And how long is 1 arc-minute at the equator?
We know that there are (60 * 15 * 24) 21600 of them around the earth.
What is the circumference of the earth at equator? 40.075km = 40.075.000 meters.
40.075.000 / 21600 = 1855 which is fairly close to 1852 meters (there are various small errors in the above.
So!
1 nautical mile = 1852 meter (at the equator) = 1 arc minute = 1/21600 of the circumference of earth!
Bonus question: How do you calculate the length of 1 arc minute anywhere else than the equator?
By using cos(latitude)*1852!
Examples:
Equator = cos(0) = 1 and 1 * 1852 = 1852m
Poles = cos(90) = 0 and 0 * 1852 = 0m (it is a point, not a circle!)
Halfway between the two = cos(45) = 0,71 and 0,71 * 1852 = 1310m.
So at 45 degrees latitude, 1 arc minute is 1310 meters.
Knowing that, can you now calculate the circumference of the earth at 32 degrees lattitude?
A wheel rolls on the ground. Let’s make the circumference of the wheel 1 meter. 1000 rotations is 1 km. Very easy to measure.
A wheel does not roll in water. So now we have to use a different tool to measure. Let’s take a rope and tie knots at regular intervals. If we tie a float to one end of the rope and toss it overboard the float with remain where we dropped it as the boat moves forward. The number of knots that spool out over one minute is our speed in knots.
This unit, knots, in now used to describe all distances over a body of water. All our charts, logs, and routes use this unit. If we want to describe larger distances, we simply increase the time of measurement from 1 minute to one hour. One knot equals one nautical mile per hour. So now when I calculate my current speed, say 5 knots, I instantly know how much distance this represents. Remember, all my charts and maps are keyed to this unit. So it remains the standard because it is easy to use and makes sense to me as a sailor.
The Earth can be segmented into a Northern and Southern Hemisphere. Those can each be segmented into 90 degrees (times 4 equals 360, a circle). If we choose to divide each of those degrees by 60, we get a Nautical Mile or minute North or South latitude.
The Earth is an oblate spheroid. That means it’s fatter near the equator and therefore when measuring East to West (longitude) the distance is farther per minute near the Equator. The lines also converge at the poles, making them very close together at high latitudes. Due to the inconsistency in longitude, a Nautical mile is a minute of latitude.
When navigating across an ocean, the only tools available hundreds of years ago were the Sun, Stars, and Math. When plotting a position on a chart using celestial navigation and dividers, the consistent minute of latitude is already on the chart. If you travel half a degree of latitude in 2 hours, you travel 30 nautical miles. If you timed that travel, you can predict where you will be in another 2 hours pretty easily if you maintain course and weather/sea conditions remain the same.
TL;DR; miles and kilometers are arbitrary measurements on a chart of Earth, where nautical miles are the Earth divided into 360 degrees all the way around North and South, then each degree is divided by 60 minutes North and South Latitude.
Source: Have navigated with celestial navigation in an airplane for fun.
My dad as a joke told me nautical miles were longer than regular miles because you have waves that make you go up and down…
I believed this til I was 17.🤦🏼♀️
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