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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Name for sixty nautical miles
From: Magnus Sjoquist
Date: 2016 Mar 18, 11:24 +0100

I think students of navigation have to bite in the sour apple and accept that "degrees", "minutes" are angular/circular measures and not distances in terms of meters.
(As everybody knows,the nautical mile (NM or nmi) i defined as being exactly 1852 meters and surely in most practical cases in navigation this is very near to 1 minute of arc along the great circle when we consider the earth as being a perfect sphere.

They simply have to get used to the fact that a very small error is introduced when we say "1 minute of the great circle (earth) is 1 NM". They will meet the problem when we talk about meridional parts and similar effects when we do take into account the flattening of the earth, so they have to introduce this in their minds (and, for instance, avoid mixing the lat/long-scales on nautical charts or in calculations.

So, they will have to understand that angular/arc-measures are not the same as distances in terms of length-units like meters, feet or whatever even if we in sailings usually regard 1 minute of GC-arc  as 1 NM.

My opinion is that we should keep the present terminology and - especially when teaching/talking to students - use the words "Great Circle distance" to e.g. a geo-position of a star or "Great Circle distance" from point A to point B when doing distance calculations.
Leave alone that when we navigate real ships in real waters we express intercepts for "x NM towards or away" from AP even if they truly are minutes of GC-arcs or putting an equal sign between the two when talking distances between points on earth.

In my world related to, but slightly off topic:

Some years ago when I still was in active work (note 1) I compared the GC distance taken from a GPS with the cosine formula I noticed there was a difference. I assumed that the GPS was converting the GC minutes to NM.

A few minutes ago I checked the same phenomena using:
From lat S 20-00 / E 125-00
To     lat S 50-00 / W 127-00

Results:
Cosine:                                                           5140,7226 minutes GC
NavCom 48 for pocket calc. HP 48 GX              5140,7           "
gc-sail (program, US, official)                            5140,72235
Seems like all three methods fall back on our good old friend the cosine formula.

but:
with my handheld GPS (Garmin GPS 12)            5152 NM!

I suspect that the handheld GPS did convert the GC-minutes into NM, but I am not very good at contruction of satellite navigators. If anyone on the NavList feels like explaining this discrepancy I would be truly grateful.

Note 1:  (=seagoing 35 years. After I left the High Seas I "worked" as a senior lecturer (12 years) at Kalmar Maritime Academy in Sweden but that was sheer fun and not really work).

Best Regards to all
/Magnus
(ex seaman, ex teacher, keen reader of NavList posts, still alive)

Med vänlig hälsning
/Magnus Sjöquist

2016-03-18 9:48 GMT+01:00 Peter Monta :
- nautical darc (degree arc)

- nautical tristride (from the seven-league boots) (a little too cute perhaps)

For the first one, clearly we should revise all the textbooks, replacing "nautical mile" with "nautical marc".  More seriously, I sometimes find it handy to know that a nautical sarc is about 30 meters.

Frank, in some sense the metric angles did catch on a little, at least in the niche domain of European surveying instruments.  To this day, angles are reported in gon, centigon, and, if you can believe it, centi-centigon or cc.  For example, a good total station might have an accuracy of 1 cc or so (~0.3 arcsec).  My Wild/Leica T2002 (an electronic theodolite from the 1980s) can display in degrees, or gon/c/cc, or the even more obscure mil, which is (2*pi/6400) radian, close to one milliradian.

Cheers,
Peter

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