Re: Datums & Geoids

On Fri, 12 Aug 2005 17:08:50 +0100, Nick <do.not@xxxxxxxx> wrote:

>I'm dragging up little used knowledge on this one, but thought that
>astro position fixing is transferring the positions of stars (or moon)
>from the (virtual) celestial sphere to position circles (or points if
>directly overhead) on the terrestial sphere. The angle between the star
>and the horizon enables the calculation of the radius of the circle
>centred on the the point on the earth's surface directly below the star.
> The datum used for that point must be the same as the datum of the
>position circle and ultimately of the fix.
>

You make my point. You are measuring the angle between a celestial
body and the horizon - that is, the angle between local horizontal and
an object whose location is fixed in a non earth referenced system (RA
and declination are angles from fixed points in the heavens, one of
which happens to coincide with the projection of the Earth's spin axis
onto the celestial sphere). The point is that latitude and longitude
can be thought of as being the orientation of local horizontal with
respect to a reference plane. Most of the rest of what you mention is
for computational convenience, and is not terribly meaningful in terms
of what is actually being measured and computed. The only measurement
you have - the angle between a celestial body and the horizon - can
only be used to determine the orientation of the local horizontal WRT
to a reference plane.

Paul


.

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