LittleSister
Well-known member
Only by the lay person.
The cognoscenti would distinguish between a 'fair mile', a 'goodly mile' and a 'long mile'. The differences can matter, especially if it's uphill, a hot day, and closing time is imminent!
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?
- Thread starter DangerousPirate
- Start date
Only by the lay person.
The cognoscenti would distinguish between a 'fair mile', a 'goodly mile' and a 'long mile'. The differences can matter, especially if it's uphill, a hot day, and closing time is imminent!
Malabar
Member
Definition of a nautical mile is a pillar of celestial navigation. I sense a degree of confusion when having to take into account the measurement of nautical miles takern from a mercator chart. Then, beacuse of the chart's projection then indeed the NM does vary in measured distance according to your latitude.
AntarcticPilot
Well-known member
It also varies slightly on the earth's surface because the earth is not a sphere, but is more closely described by an ellipsoid. However, changes in the length of a minute of arc in latitude are well below the precision of navigation!
The change is that the nautical mile is 19m longer at the pole than at the equator.
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B27
Well-known member
French Nautical mile, 19th century, 10,000km /(90x60)= 1851....m
UK admiralty mile 6080 feet
US Navy mile 6080.2 feet
International nautical mile 1852 meters. Exactly. Everywhere. Even when we've changed the definition of a metre
The polar and equatorial radii of the earth are only 1 part in 300 different, the circumferences are only 1 part in 588 different so it doesn't make much difference at sea level.
UK admiralty mile 6080 feet
US Navy mile 6080.2 feet
International nautical mile 1852 meters. Exactly. Everywhere. Even when we've changed the definition of a metre
The polar and equatorial radii of the earth are only 1 part in 300 different, the circumferences are only 1 part in 588 different so it doesn't make much difference at sea level.
AntarcticPilot
Well-known member
19m, as stated above.
Sandro
Active member
VERY SORRY! I only now notice that in post #115 I wrote "sex" instead of "six". I guarantee it was a tiping/spelling mistake.
rotrax
Well-known member
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.
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.
Refueler
Well-known member
mmmmm if it was based on "one second of a degree at the equator" .. that would mean equator was 1,296,000 nm circumference ....
Sandy
Well-known member
With all the ups and downs I've done I am sure they average out.
Momac
Well-known member
Not at all.
Whether going up hill or down the distance travelled is always greater than than the horizontal dimension .
DangerousPirate
Active member
I mention it again because it's not discussed enough: The argument is not "Where did NM come from?" but rather "Why is it still used?"
Refueler
Well-known member
Because no reasonable viable alternative has been put forward that all signatories / navigators / scientists et al would have to agree to.
It fulfills the function it was designated for then and still today.
DangerousPirate
Active member
Sure, but it's a little bothersome to change between litres per mile/km and litres per nautical mile among being able to feel distances out right.
There is no viable alternative yet, but are there any reasons for why it couldn't be replaced?
And I am not even advocating for it, I don't mind using this system. It's just a thought experiment.
penberth3
Well-known member
Litres per mile/km/NM is a simple conversion you can do in your head, it isn't a problem.
Already explained above, NM and Degrees Minutes Seconds are so well established change is just not practical. And what would be the benefit? Remember, most of the UK still struggles with the much simpler metric system after 60 years of trying.
Refueler
Well-known member
How can you change an 'elastic' measurement to km / statute mile etc ?? The NM is based on angular measurement of a body that is not a perfect sphere ... so it is 'elastic' ... it varies by a small amount depending on where on earths surface you are.
KM's .. Miles ... Metres ... etc are fixed standards and would then not always agree on the earths surface as all the nav is based on angular data.
KM's .. Miles ... Metres ... etc are fixed standards and would then not always agree on the earths surface as all the nav is based on angular data.
AntarcticPilot
Well-known member
Nautical miles have a direct connection with navigation; anything else would involve an additional conversion, and it would be necessary to take into consideration the change in scale with latitude of Mercator charts. Nautical miles being, for navigational purposes, equal to one minute of latitude, they can simply be scaled off the latitude graticule of a chart. But the change in scale is why you are (or should have been) warned always to use the latitude graticule at the same latitude as your measurement to step off distances.
I know some have thought my explanations of map projections and the figure of the earth are too detailed, but they are the fundamental reason why we use nautical miles.
AntarcticPilot
Well-known member
I actually relate fuel consumption to litres/hour; currents, waves and wind make anything else fairly arbitrary.
MontyMariner
Well-known member
Isn't that why we tend more to using ltrs/hr ?
penberth3
Well-known member
But only because Lat and Long are in Degrees Minutes Seconds. I think this is what started the OP's debate?
Refueler
Well-known member
mmmmm just reminding that you were not alone in talking projections and the shape of the earth ....,
OK ... Mercator charts ... lets be clear - the latitude scale is a distortion based on flattening out the earth to a sheet of paper ... the latitude scale will vary on the charts based on where in latitude you are - but the real measurement on the oblate spheroid earth is only slightly different dependent on location ...
Its interesting that the only time this variance of latitude scale is not adhered to - is with a Sight Plotting chart .... or if you don't have that - you can use paper sheets with gridlines. Basically the cm's variance of real nm's is ignored for practical purposes there.