eli5: Why are nautical miles used to measure distance in the sea and not just kilo meters or miles?
In: 9827
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?
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