The Naughty Mile

[Sailfree]

Encouraging some new sailors this weekend going through competent crew practical and some day skipper theory I was asked about the Nautical Mile.
I explained that it was the distance at the circumference of 1 second but this person then asked "When and how did they know that?" followed by why is the land mile 10% less.

Can anyone out there explain or point me in the direction of a information source so that I know in future and can go back to sleeping at night instead of wondering how I would establish the length of the Nm by either theory or with basic measuring equipment?

Teaching idiots is easier than those with an enquiring mind!!

 

[snowleopard]

A statute mile is a multiple of the yard which is reputed to be defined by Henry VIII as the distance from the tip of his nose to his outstretched fingers. The fact that it shares its name with the nautical equivalent is due only to the fact that it is similar in size.

The nautical mile actually varies in size at different latitudes owning to the earth not being spherical, it averages 6080 feet. A nautical mile is defined as a minute of latitude, Equating it to a distance is secondary and probably had considerable inaccuracies in the early days of measurement.

 

[Sailfree]

I understand the definition but when and how was the length determined? How do you establish the diameter of the earth or more importantly how did they?

 

[jrt]

I think what you have described is the Sea Mile which was 1 minute of latitude, 6076 ft with the navy standardising on 6080 ft.

The nautical mile is now defined as 1852m /forums/images/graemlins/wink.gif

 

[Keen_Ed]

Try here: http://en.wikipedia.org/wiki/Mile
and http://en.wikipedia.org/wiki/Nautical_mile

 

[snowleopard]

For navigation puposes, the general measure is the minute of latitude. When using charts covering a large area one should always take measurements using the latitude scale on the same latitude as the distance being measured. For some purposes it is acceptable to use a standard value e.g. measuring speed.

 

[Sailfree]

Thanks but all these posts establish what the current "standards" are.

In the year "?" someone determined the diameter of the earth , any ideas how this was done and what the units of measurement were? From there someone determined a Natical Mile and when it was realised that the earth was not a sphere (it varied dependant on where you were )so various bodies standardised it.

So the question really revolves on how someone first "sized" the earth and then what "standard" was used to give that measurement a physical length.

Thanks for the posting of the Romans double steps, that is the sort of empirical history I would have expected to determine the first measured mile but an interesting piece of Trivai I never knew before.

 

[fireball]

[ QUOTE ]
The nautical mile actually varies in size at different latitudes

[/ QUOTE ]
So - do you sail faster at higher latitudes than at the equator?? /forums/images/graemlins/laugh.gif

 

[Superstrath]

Accepting all that about using a minute of latitude at your current latitude as a mile, IIRC the actual definition is a minute of longitude at the equator.
Also IIRC, the circumference of the earth was first calculated by some ancient (I think egyptian) using two vertical sticks. When the sun was overhead, he compared the lengths of shadows (they were some distance apart) which gave him the basics to have a good stab at the diameter and then the circumference.
He probably didn't care about nautical miles though, since the degree/minute system had yet to be invented!
Is this helping?

 

[Gordonmc]

Yes. (ish).
The actual Nautical Mile at the pole is 6108 feet while the same at the equator is 6046.
Knowing the shape of the Earth is irregular explains the need for a standard linear measurement, ie 6080 feet or 1852 meters or 1.1508 miles - the International Nautical Mile which is slightly smaller than the UK Nautucal Mile based on a minute in the English Channel, thankfully now dropped out of use for obvious reasons.
Early navigators could work out arcs of latutude using the stars and it would be relatively easy to measure the distance sailed through one minute and back calculate from that to arrive at the circumfrence of the globe.
As it happens metric measurement uses the same principle. A Kilometer is one-ten thousandth the length of a line running from the North Pole to the Equator through
Paris.

So there!

 

[Malcb]

I thought that in those days they thought the earth was flat.

 

[Superstrath]

Everyone except the librarians. See here.

 

[stephenh]

"How did they know that ?" possibly the wrong question...
Perhaps "How was the nm defined " would be better.
The Babylonians (or Iraqis !!!) were the worlds first mathematicians.
However it was early days ( 4,000 yrs ago) and they had no concept of either zero or the decimal point - these were yet to come (from India).
To satisfy their interest in numbers less than 1 they invented a number system based on 60 - I won't go into this , it will make the post too long - and from this , to this day, we have 360 degrees to the circle, 60 minutes to the hour etc.
The Egyptians wanted to know sea distances in the Med. and from this the size of the Earth.
They measured an angular distance from the Tropic of Cancer in southern Egypt ( midday midsummer, sun vertically overhead) to Cairo ( same time, Sun not vertical - measure angle - say 7.5 degrees, I forget ) then measured it on the ground and there you go. The earth's circumference was first calculated.
The measured length of one minute of circumferential arc over the years then gets refined and thus we have it today.
Hope this is of use.

 

[Fill]

First diameter was by Eratosthenes (or something - ancient Greek anyway) who took bearings of sun from a well and then moved south and tried same again and measured distance of shadows. Came close but not that accurate.

It was done more accurately some time around late 1700s when the great survey of India was carried out. They did this using very accurately made chains and "the great theodolite" - which apparently weighed over a ton - and doing triangulation all the way up the spine of India. They also found that the diameter had been miscalculated before and that mountains had a gravitational effect on the plumb bob they were using to get a vertical for the theodolite.

Dunno that that answers your question either, but the great survey certainly improved the accuracy of the measurement of the diameter (and circumference) of the earth.

However, consider that now we have GPS all the charts are having to be redrawn more accurately so who says that a nautical mile is exactly the measurement it was at say 1950?

 

[bobgosling]

There is a very good book called The Map-maker's Wife, by Robert Whitaker which describes in detail exactly how they measured the Earth's diameter and describes a French expedition to calculate the distance equating to a minute of arc at various latitudes to determine the shape of the globe. It is a rivetting read and explains the science in simple layman's terms. Also, Bill Bryson's A Short History of Nearly Everything contains details IIRC.

 

[dulcibella]

I seem to recall that the Bedford Level was the scene of an effort with surveying poles that allowed a good calculation of the Earth's diameter by the intersecting chords theorem. Sort of thing that we did in maths lessons in the late 1950s!

 

[john_morris_uk]

If I remember correctly, an Italian Mathematician used two deep wells and some stars to calculate the diameter of the earth and hence the other dimensions.... Can't remember the year -but a few hundred years ago.

 

[boatone]

[ QUOTE ]
Teaching idiots is easier than those with an enquiring mind!!


[/ QUOTE ]

Ah yes.....but it's those with an enquiring mind that keep you on your toes, bring you stimulation and debate and improve the quality and knowledge base of your teaching.

And you don't really need to 'teach' them - they are hungry to learn which makes the tutor's role so much more exciting and worthwhile.