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Jeff,

You wrote:

> I have looked at Jim Thompson's new diagrams and the
> 2002 thread posts by Paul Hirose and others.


I hope that you have also looked at the more recent thread of a couple
of weeks ago.

> Below I
> have summarized what I think I'm reading.  Can you let
> me know if I have indeed understood?
>
> Due to the oblateness of Earth, the plumb line differs
> from the actual center point of Earth by a
> navigationally significant amount, up to 12 minutes of
> arc.


Fair enough.


> But this fact does not effect the practice of
> celestial navigation because the lat-lon grid itself
> and the Almanac's GP data are based on the plumb line.


Not quite.

Longitude doesn't come into it. The axis of any ellipsoid used to
describe the oblate Earth is the same as the axis that a theoretical
spherical Earth would have. Thus longitude is not affected by the
"oblateness". (Is that a word?)

The parallels of latitude are not based on the orientation of a plumb
line (which responds to local gravity) but on perpendiculars to the
ellipsoid chosen as a representation of the geoid. Those two directions
differ (as you note below) by the deflection. However, you are right
that the practice of CelNav is not affected, because deflection is
(usually) far too small to matter.

CelNav observations are related to the direction of local gravity, and
hence to the direction indicated by a plumb line.

However, I think you are wrong to say that the Almanac's data are
related to a plumb line. I think that all the tabulated data would be
correct for a spherical Earth. The major error in assuming sphericity is
the problem with latitude and that is taken care of by using charts
based on geoidal latitude. The remaining errors in the assumption of
sphericity (e.g. horizontal parallax of the Moon must differ depending
on whether you view it eastward from a point on the Equator or southward
from a point in high northern latitudes) seem all to be too small
to matter for navigational purposes. [I am a little unsure of the points
in this paragraph but I'm sure someone will correct my errors.]

> The local plumb line may differ by additional amounts
> as well, called deflections, due to surface anomalies,
> but not by a navigationally significant amount.

Yes.


Trevor Kenchington


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Trevor J. Kenchington PhD                         Gadus@iStar.ca
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