How did the tradition start to have different units when measuring speed at sea vs measuring speed on land?
As to why nautical miles and statute miles are different lengths: I don’t know.
Knots (nautical miles per hour) probably were used before statute (land) miles per hour. Although miles themselves existed, speeds weren’t usually calculated. Any distances over a days travel was expressed in the number days to travel.
Normal speeds for horse-drawn travel are in the 10-20 mph range (galloping/running don’t count, since you’d rarely have your horse do this for a long stretch of time); walking is 1-4 mph, depending on the company. These numbers are too small to be of note (who would care that they’re walking 3.4 mph?).
However, sailing ships did go pretty fast. Plus, travel time varied going from port to port, depending on the weather and its winds. So knowing their instantaneous speed became important.
It wasn’t until the advent of steam engine propulsion and trains that instantaneous speeds on land were needed to be known, so as to correctly construct bends and inclines in the rails. This need carried over to automobiles, for the roads that were being built to carry them.
To measure speed with sail, they tossed a float off the stern and counted the knots tied in a cord that fed out. Perhaps it is a function of the space between the knots.
Ah, here we are:
The length of the nautical mile is very close to the mean value of the length of 1 minute of latitude, which varies from approximately 1,843 metres at the equator to 1,861.6 metres at the pole.
However, land miles varied from country to country. At one time, Italy’s was 4060 ft. (aka geometric mile), Roman (old Roman that is) was 4860 ft., and English was 5280 ft.
So the nautical mile had an international base (the size of the planet, also the [initial] base for the meter), but land miles were defined by the country you were in.
A nautical mile is one minute of an arc. It makes calculations based on latitude and longitude much easier. A degree of arc is then 60 nautical miles instead of some odd figure.
Slower than that. The point of having a wagon or carriage wasn’t that it was that much quicker than walking. It was that it saved you from walking and allowed baggage to be carried. The Overland stage averaged 8 mph on good terrain, and they switched horse often to keep them fresh:
http://www.pittsford.monroe.edu/programs/instruction/socialstudies/thewest/transport.htm
It wasn’t that ships travelled that much faster, either. The importance of measuring speed on a ship was that it was required for navigation, since there are no landmarks at sea. Better navigational instruments like sextants came along eventually, but early sailing vessels had to use dead reckoning.
The culmination of commercial sail-driven speed before the age of steam was the clipper ships. They topped out at 20 knots:
http://www.schoonerman.com/clipper.htm
Earlier ships were lucky to make 5. Which makes me wonder why a knot isn’t a smaller unit. For accurate results, sailors must have had to calculate in fractional knots.
AWB:
You kids today with your newfangled “meters.” When I was in the Navy back in '89-'93, we learned the nautical mile as 2000 yards which, according to my measurement conversion application, is about 1828 meters. But 2000 yards is easier to keep in your head.
Each degree of latitude is 60 nm apart, and there are 90 degrees from equator to pole.
more like 6076 feet but traditionally 6080 was the number used.
The nautical mile was defined for a long time as one minute of mean meridian arc and better measurements of the circumference of the Earth resulted in changes in the length on the NM.
Also a minute of arc is not constant because the Earth is not a sphere. So the NM was redefined as 1852 m which is pretty close to one minute of arc.
Check out http://www.unc.edu/~rowlett/units/dictN.html#nautical_mile
Measuring speed in Knots also makes more sense for vehicles which tend to travel in a straight line. Knowing that a nautical mile is one second of arc is useful when you’re flying or sailing a long distance and need to calculate waypoints and arrival times on a map. It’s pretty much useless in a car or for a race, which is typically shorter in distance and in random directions. So ships and aircraft use Nautical Miles, and the rest of the world just keeps using whatever convention they already have, because there is no advantage in converting.