# NavList:

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

**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 > celestial navigation. > > 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 >