# NavList:

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

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Re: FW: Re: Chronometer Suggestions
From: Nicol�s de Hilster
Date: 2009 Jan 15, 21:30 +0100

```While were busy doing some math:
on Clarke 1866 one arc minute in latitude equals
at 0 degrees 1842.786m or 6045.886ft
at 30 degrees 1847.472m or 6061.260ft
at 60 degrees 1856.910m or 6092.224ft
at 90 degrees 1861.656m or 6107.795ft

in longitude this is
at 0 degrees 1855.344m or 6087.087ft
at 30 degrees 1608.138m or 5276.043ft
at 60 degrees 930.036m or 3051.299ft
and zero at the pole.

Nicol�s

glapook@PACBELL.NET wrote:
> The nautical mile used to be defined, in the U.S., as one minute of
> arc on a sphere having the same area as the earth as defined on the
> Clarke spheroid of 1866 and was 6,080.2 feet. The length of one minute
> of latitude varies from 6,046 feet at the equator to 6,108 feet at the
> poles on this spheroid (I don't know what it is on WGS84.) The length
> of the geographical mile, one minute of longitude on the equator, is
> 6,087 feet. (Bowditch, 1977)
>
> Also see:
>
>
> gl
>
> On Jan 15, 10:55 am, Lu Abel  wrote:
>
>> Curiosity question:
>>
>> It's well known that the diameter of the earth across the equator is
>> about 1/300th greater than the diameter across the poles.
>>
>> I would intuitively expect, therefore, that the size of a minute of
>> latitude to change by a like amount.  But looking at this graph, there
>> seems to be a 1/60 difference in the size of a minute at the poles vs at
>> the equator.  Is there an explanation that this technically competent,
>> but ignorant of the math of the oblate spheroid, person could understand?
>>
>> Also, I assume this graph is for geodetic latitude and not geocentric or
>> parametric latitude?
>> (For people curious about these terms, geodetic latitude is what you get
>> by drawing a line perpendicular to the surface of the earth down to its
>> axis.  Due to the flattening of the earth, this line will intersect the
>> earth's axis on the other side of the equator from the observer's
>> position.  The other two latitudes are what you get when you draw a line
>> out from the earth's center. This line is not perpendicular to the
>> earth's surface except that the poles and equator)
>>
>> Lu Abel
>>
>> Nicol�s de Hilster wrote:
>>
>>> On NavList 7052 Irv Haworth wrote:
>>>
>>>> "I think it's well known that 1' of arc varies in length as a function
>>>> (cos)
>>>> of the latitude."
>>>>
>>> On which Gary LaPook replied in NavList 7053:
>>>
>>>> That is true for one minute of longitude because parallels of latitude are small circles.
>>>> This is not true for one minute latitude of for any
>>>> other great circle. (Technically these also vary slightly due to the
>>>> oblateness of the earth but these small variations are ignored for
>>>>
>>> For those who want to know how much exactly that variation is I posted
>>> attached graph of it in NavList 4750 on 24/03/2008.
>>>
>>> ------------------------------------------------------------------------
>>>
>>
> >
>
>
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