# NavList:

## 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: Jim Thompson
Date: 2005 Jan 30, 15:25 -0400

```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 navigation
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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