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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: Horizons, was Summary of Bowditch Table 15
From: Fred Hebard
Date: 2005 Jan 30, 14:49 -0500

This is called deflection from the vertical, where the vertical points
to the center of the earth.  It is not a significant factor until you
get next to the Andes or some other huge mountain close to the sea.  As
I recall, the  errors are on the order of 1' of arc or so, which would
make it more a problem for surveyors than navigators.

On Jan 30, 2005, at 2:25 PM, Jim Thompson wrote:

> Before Trevor has a chance to reply (if he wanted to) I have started
> uncovering some information about the relevance of the plumb line to
>
> Bowditch 2002 makes the importance of the plumb line clear:
> "Horizontal,
> adj. Parallel to the plane of the horizon; perpendicular to the
> direction of
> gravity."  But I have found no other reference to gravity and the
> plumb line
> in Bowditch's text, where it is merely stated that the center of the
> earth
> is used as the reference for the horizontal coordinate system,
> presumably as
> an approximation for the plumb line?
>
> I found one paper which said, "An essential element in celestial
> is the determination of the exact direction of the local gravity
> vector. In
> traditional, marine-sextant celestial navigation, the observed horizon
> is
> assumed to be a circle orthogonal to the local vertical (without
> measuring
> the local gravity vector)."
>
> But I have not learned the significance in CN of the difference between
> assuming the geometric center of the earth, and using the plumb line.
> Presumably the difference is not significant, given that we tend to
> work
> with precisions of about 1-2 NM at best?
>
> Jim
>
>>> -----Original Message-----
>>> From Jim Thompson
>>> Trevor wrote in reply,
>>>> The sensible horizon might be better understood as a plane,
>>>> perpendicular to the direction of gravity acting on the observer and
>>>> drawn through the observer's eye. It is parallel to the celestial
>>>> horizon because that too is a plane perpendicular to the direction
>>>> of
>>>> gravity acting on the observer but drawn through the centre of
>>>> the Earth.
>>
>> Jim wrote but meant to finish:
>>> I have not yet found an independant reference to this idea that the
>>> horizontal coordinate system's horizons are perpendicular to gravity.
>>
>> Sorry, Trevor, I meant to complete this thought before posting
>> that message,
>> but my trigger finger slipped.
>>
>> I have not yet found an independant reference to this idea that the
>> horizontal coordinate system's horizons are perpendicular to gravity.
>>  All
>> the definitions I have found so far refer to the center of the earth,
>> not
>> the direction of gravity.  You were challenging my comment that
>> the horizons
>> are perpendicular to a line drawn through the center of the earth to
>> the
>> observer's position on the surface of the earth.  I think what
>> you meant was
>> that this would only be true if the earth was a perfect sphere and if
>> gravity pointed to the center of the earth, but the earth is geoid,
>> and so
>> the direction of gravity is a more proper reference than the center
>> of the
>> earth.  Is that so?
>>
>> Jim Thompson
>

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