Why do boats use nautical miles and why are they different than normal miles? Couldn't you just convert it to normal miles or km?

[Buck Turgidson]

I own a number of rifle scopes, some calibrated in MOA others in Mil Rad. They all work. If im ranging in Yards I use MOA if im ranging in Meters I use Mils. If you want to use degrees then Nautical Miles is the solution for navigation. I think the French tried and failed to convince the world to change.

 

[rotrax]

mmmmm if it was based on "one second of a degree at the equator" .. that would mean equator was 1,296,000 nm circumference ....

Well, you get the idea, and dividing the equator into 360 degrees and sub dividing made sense to me at the time - I was at School!

 

[DangerousPirate]

I own a number of rifle scopes, some calibrated in MOA others in Mil Rad. They all work. If im ranging in Yards I use MOA if im ranging in Meters I use Mils. If you want to use degrees then Nautical Miles is the solution for navigation. I think the French tried and failed to convince the world to change.

Don't they always

 

[AntarcticPilot]

I think the French tried and failed to convince the world to change.

The French did try to decimalize everything, and if their original plan to link the metre to the Earth's circumference hadn't fallen flat because the length of a degree of latitude changes by about 1% from the equator to the Pole because the Earth isn't a sphere, it would have worked. Grads (a hundredth of a right angle) are still used for some survey purposes in France, I believe. But if the French scheme had worked 1 grad would be equivalent to 100km in latitude, and it would all have worked fine for navigation; a centiGrad would be a kilometre. But as the metre was intended to be a fixed unit, unlike the nautical mile which, for navigational purposes is very slightly flexible, so they abandoned the original definition in favour of the distance between two marks on a bar of platinum at specified temperature and pressure. Things have changed even more since then, and it's now defined in terms of the wavelength of light produced by a particular subatomic transition, which means it is tied to the speed of light!

 

[Sandro]

rotrax said:
Seen lots of stuff on here, some of it a bit uneccessary if the way it was explained to me years ago is true. I have no reason to believe it was not, so what I was told is this :-

The circumference of the world at the equator was measured/calculated and subsequently divided into 360 equal lengths, each length then became one degree.

Each degree was subdivided into minutes and seconds.

The resulting size of one second of a degree at the equator became a Nautical Mile.

Seems pretty reasonable to me, and is the reason a land mile differs from a NM.

As only Refueler noticed one nautical mile is one minute of a degree, not one second.

 

[billskip]

As only Refueler noticed

Not necessarily......could be others that haven't commented.

 

[requiem]

Seen lots of stuff on here, some of it a bit uneccessary if the way it was explained to me years ago is true. I have no reason to believe it was not, so what I was told is this :-

The circumference of the world at the equator was measured/calculated and subsequently divided into 360 equal lengths, each length then became one degree.

I think many times new units arise from being "close enough". Long years ago when the Babylonians used a sexagesimal numbering system, it could be debated which came first: the system or the recognition that there were close to 360 days in a year. (Don't ask me, it's been far too long!)

Its worth noting that they didn't use a pure base-60 system; it was still base-10 at its core, but in any case it persisted for many things and in particularly those where circles and angles were involved. My money is on people observing, after using degrees and minutes to measure the planet, that a degree was very close to 60 miles in length and thus opted to use that as a convenient "close enough" reference.

It's important to remember that back then there wasn't really a "standard" mile; each country or region had its own idea of the size of a mile (or foot, inch, stadion, etc). It is only recently (in relative terms) that measurements have been standardized, and even so it has been barely two years since the US side formally retired one of the two different "foot" definitions in use.

But as the metre was intended to be a fixed unit, unlike the nautical mile which, for navigational purposes is very slightly flexible

This part I would quibble with; if you treat the mile as varying rather than fixed you introduce more problems for close work. On an angled course, would the east-west component similarly vary in length with the difference in latitude? E.g. is a 100 mile course due west at 80 °N of a different length compared to a 100 mile course due north that starts at 80 °N? Does one need to re-calibrate their log as they progress further towards the poles?

I know the differences are minor, but here I would argue that the nautical mile itself is not changing, but rather the use of the latitude scale is merely a "close enough" convenient shorthand for chartwork that has the additional benefit of correcting for chart distortion.