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?

[DangerousPirate]

A mate of mine recently asked this and at first I thought "Well, obviously it's convenient for sailors. One knot equals one nautical mile. And a nautical mile is based on latitudes where 1/60 of one degree equals one nautical mile." but he kept saying "Okay, right. But why don't we just make the longitude into kilometres or normal miles? Is there any practical reason to stick to nautical miles?".

It's a bit of a rabbit hole because most explanations are circular and always point back to what it's based on but don't explain the reasons behind it.

Pretty sure if we'd just overthrow the current system, swapped all charts for ones where latitudes are based on kilometre and changed knot speeds to kmh, we'd be absolutely alright. Of course ignoring all the peripheral problems coming with this approach.

Only thing I could think of was the ease of calculations. Because 60 nautical miles are easier to calculate with than 111 kilometres, or 1 nautical mile and 1.85 kilometres. But to be fair; In the day and age of GPS, that's almost irrelevant now. I mean, who used a clock and a log to measure their speed really to know how far west or east they are?

 

[KompetentKrew]

Latitude is given as an angle that ranges from −90° at the south pole to 90° at the north pole, with 0° at the Equator.

One degree of latitude equals 60 nautical miles.

This has been the case forever.

You:

 

[KevinV]

A mate of mine recently asked this and at first I thought "Well, obviously it's convenient for sailors. One knot equals one nautical mile. And a nautical mile is based on latitudes where 1/60 of one degree equals one nautical mile." but he kept saying "Okay, right. But why don't we just make the longitude into kilometres or normal miles? Is there any practical reason to stick to nautical miles?".

It's a bit of a rabbit hole because most explanations are circular and always point back to what it's based on but don't explain the reasons behind it.

Pretty sure if we'd just overthrow the current system, swapped all charts for ones where latitudes are based on kilometre and changed knot speeds to kmh, we'd be absolutely alright. Of course ignoring all the peripheral problems coming with this approach.

Only thing I could think of was the ease of calculations. Because 60 nautical miles are easier to calculate with than 111 kilometres, or 1 nautical mile and 1.85 kilometres. But to be fair; In the day and age of GPS, that's almost irrelevant now. I mean, who used a clock and a log to measure their speed really to know how far west or east they are?

You're right of course that you can define any standard you want - degrees longitude being in hours, minutes and seconds would make a lot of sense to me for instance, but then you still have to agree where zero is.

Our current measuring standards are based on thousands of years of progress built on choices made for all kinds of reasons - other than Celcius being more logical than Fahrenheit I don't think it makes a jot of difference, and there's no reason to suppose any new standard would make any more sense than the old ones

 

[mil1194]

I get asked this all the time in the pub and I normally just bluff it, but it seems that there’s a technical reason now

 

[thinwater]

So nautical miles are based on a north-south circumference of 21,600 nautical miles (it's 21,602) and kilometers were originally based on a diameter of 10,000 kilometers (it's 12,700). Statue miles were sort of based on 5,000 Roman human feet, but actually pulled out of a king's ear.

All very logical.

 

[NBs]

Hi, I don't have the energy to twist the wire, read a good explanation on wikibedia , let's not reinvent the wheel again, thank you.

Nautical mile - Wikipedia

 

[mil1194]

Is it fair to say that theres not much difference for us day to day happy sailers/cruisers? I normally say that and that it’s just a sailers thing - or am I being disengeous

 

[Neeves]

You don't need to use nautical miles - you can use any conversion you like - as long as the people with whom you are communicating have a similar understanding to you.

I think you will find that outside of the UK, Oz, NZ and the US + colonies (add other countries as you wish) many people have no idea what a (statute) 'mile' is and even less understand a fathom, chain and only a mariner would understand a 'shackle'. So if you are talking to an Anglo Saxon mariner he will understand a nautical mile, a statute mile, a kilometre but might be puzzled at you choice of measuring distance. The man in the, British, street will understand miles, yards, inches and chains (the latter maybe) but actually might have no idea what a nautical mile is. And - we are in transition the older members will understand and be comfortable with miles and kilometres, the younger ones kilometres (and have increasingly less understanding of miles).

But the man on the omnibus actually has no need to know what a nautical mile might be - and is only now learning the mathematics to convert kilometres to miles or vice -versa.

As most people here in Oz don't know what a nautical mile is when defining a distance I'll convert, crudely, to kilometres. Fortunately location is easier - most destinations are either North or South of Sydney.

It does get complicated as when asking how far has your car travelled (what is the odometer reading) 'mileage' trips of the tongue with ease 'kilometerage' is a bit of a mouthful.

Jonathan

 

[onesea]

In this case we actually have one international standard.
Latitude, longitude speed distances.

Developed over century's, yes they're have been arguments about 0 longitude.

Yet multi nationally its a standard. Why would anyone consider a change?

 

[Refueler]

Km .. Feet ... Metres .... etc all have one thing in common - they are fixed standards of measurement and do not change wherever measured on earth / moon etc.
Nm though is not a fixed measurement but varies dependent on position on earth We do have a figure to use which is based on the average and to suit radio frequency propagation such as to to read distances on a radar. As we all know - the Nm is based on 1 minute of latitude ... originating from the 360 deg of the earth ... the terms Minute and Second being used because of the Time relationship to the earths rotation ....

Its been a long time since I sat down in College Nav classes .. but that is what I recall ...

 

[Sea Change]

Nm though is not a fixed measurement but varies dependent on position on earth

Really?

the terms Minute and Second being used because of the Time relationship to the earths rotation ....

I thought it was just a convenient way to subdivide a larger number.

 

[RobbieW]

Km .. Feet ... Metres .... etc all have one thing in common - they are fixed standards of measurement and do not change wherever measured on earth / moon etc.
Nm though is not a fixed measurement but varies dependent on position on earth We do have a figure to use which is based on the average and to suit radio frequency propagation such as to to read distances on a radar. As we all know - the Nm is based on 1 minute of latitude ... originating from the 360 deg of the earth ... the terms Minute and Second being used because of the Time relationship to the earths rotation ....

Its been a long time since I sat down in College Nav classes .. but that is what I recall ...

A Nm is 1 minute of a great circle iirc, making it a fixed length.

 

[ylop]

A mate of mine recently asked this and at first I thought "Well, obviously it's convenient for sailors. One knot equals one nautical mile. And a nautical mile is based on latitudes where 1/60 of one degree equals one nautical mile." but he kept saying "Okay, right. But why don't we just make the longitude into kilometres or normal miles? Is there any practical reason to stick to nautical miles?".

So the logical reply to that is:

Why would you. Yes calculators and GPS make it trivial now to report in other units if you want to by minutes of latitude is a convenience measure, universally recognised and understood by marine and aviation professionals the world over. If you were inventing global travel today you probably would use the km (except that without global travel the meter would never have achieved its prominence). But why would anyone consider converting to “normal” miles? The real issue is why anyone uses statute miles for anything - they are arbitrarily defined, recognised only in a handful of countries and a mathematically inconvenience unit for subdivision into feet/yards.

Perhaps he means why did anyone feel the need to invent nautical miles in the first place. I think he’ll find that the answer comes from how statute (land) miles were measured at the time. Trundle wheels or whatever the technology of the day was were a bit hard to use on water…

 

[Sandy]

A mate of mine recently asked this and at first I thought "Well, obviously it's convenient for sailors. One knot equals one nautical mile. And a nautical mile is based on latitudes where 1/60 of one degree equals one nautical mile." but he kept saying "Okay, right. But why don't we just make the longitude into kilometres or normal miles? Is there any practical reason to stick to nautical miles?".

I take it he has not heard of Sea Miles. The actual distance that is used on charts.

 

[PaulRainbow]

A Nm is 1 minute of a great circle iirc, making it a fixed length.

That's incorrect. A minute of lattitude is not constant, so a Nm based on that is also not constant.

However, from Wikepedia :

A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters.[2][3][4] Historically, it was defined as the meridian arc length corresponding to one minute (⁠1/60⁠ of a degree) of latitude at the equator, so that Earth's polar circumference is very near to 21,600 nautical miles (that is 60 minutes × 360 degrees). Today the international nautical mile is defined as 1,852 metres (about 6,076 ft; 1.151 mi).[5] The derived unit of speed is the knot, one nautical mile per hour.

 

[Chiara’s slave]

That's incorrect. A minute of lattitude is not constant, so a Nm based on that is also not constant.

However, from Wikepedia :

A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters.[2][3][4] Historically, it was defined as the meridian arc length corresponding to one minute (⁠1/60⁠ of a degree) of latitude at the equator, so that Earth's polar circumference is very near to 21,600 nautical miles (that is 60 minutes × 360 degrees). Today the international nautical mile is defined as 1,852 metres (about 6,076 ft; 1.151 mi).[5] The derived unit of speed is the knot, one nautical mile per hour.

The equator is a great circle. You seem to have misread Robbie.

 

[Dellquay13]

Hmmm, Kilometres as a measurement of distance over water on the surface of a sphere. If you really want that you should adopt a Decimal compass system, why not go the full hog and convert to a cuboid planet.

Bring back the French Revolutionary 10hr decimal day too…

 

[footsoldier]

A mate of mine recently asked

I would dump him - he is obviously a dangerous subversive who is out to overthrow the established order.

 

[RobbieW]

Taken from the Royal Museum Greenwich. 6080ft is the number drummed into me as an apprentice at BEA...​

A nautical mile​

The ‘standard’ nautical mile is taken as 6080 feet (1.151 statute miles or 1853 metres) and is the unit of length used in sea and air navigation.
A mile on land is equal to 5280 feet (1609.3 metres), while a sea or nautical mile is one-sixtieth of a degree of latitude, and varies from 6046 feet on the Equator, to 6092 feet at a latitude of 60°. This difference is due to the Earth not being a perfect sphere, but slightly flattened at the poles – an oblate spheroid. To avoid confusion, the standard nautical mile is accepted as 6080 feet.
Speed at sea is measured in knots, a knot being one nautical mile per hour.

 

[RunAgroundHard]

I measure distance in double decker buses. Planning a trip from the Canaries to Barbados is about 426396 double decker buses. It’s is much more intuitive than 1’ of arc.