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Peter Smith wrote:

> Between datums, yes. The "horizontal difference" I was referring to
> was between the ellipsoids that underlie the various datums. When you
> project celestial LOP's onto either a sphere or one of these
> ellipsoids, you get pretty much the same result. It's the landmarks
> that appear to move when you compare charts built on different
> datums

I must be missing something here.

As I understand it, the various datums are different ellipsoids, each of
which is a mathematically-perfect surface which closely (but not
perfectly) matches the shape of the real globe, the early ellipsoids
differing in which part of the real globe they most closely matched.
Surely then it is not the landmarks which move (they, after all, are
rooted on bedrock) but the positions of latitude and longitude lines
which are defined relative to the particular ellipsoid?

If so, Jared's question seems a valid one. For which datum (i.e. which
ellipsoid) are the tabulated numbers in the various sight-reduction
tables calculated? Or are are they in fact (as I rather suspect)
actually calculated on an assumption that the Earth is a perfect sphere?
[Most of the numbers in the Almanac presumably relate the positions of
celestial bodies to the centre of the Earth and so are not affected by
the chosen ellipsoid. Or am I off-base on that?]


Trevor Kenchington


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Trevor J. Kenchington PhD                         Gadus@iStar.ca
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