Re: Sextant Positions versus Map Datums?
Subject: Re: Sextant Positions versus Map Datums?
From: Hal Mueller (hal@XXX.XXX)
Date: Fri Jan 18 2002 - 13:06:26 EST
At 12:06 PM -0500 1/18/02, dkruger wrote:
>The astronomical positions of VSOP87, and all the derived variants are
>computed first in Heliocentric coordinates, thus the shape of the
>earth is irrelevant to that part of the computation.
>
>Then, it is converted to geocentric coordinates based on the position
>of the center of the earth. I don't remember seeing anything about
>the ellipsoid here, but Hal if you have a page reference in Meeus, I'd
>like to see it. What is much more relevant is nutation, because as
>the Earth "nods" up and down due to the gravitational effects of the
>moon on the equatorial bulge, the axis changes, and that changes the
>entire coordinate system. So the accurate conversion to geocentric
>coordinates takes into effect nutation, and for distant bodies (beyond
>Mars) the time it takes light to arrive. Those are the only
>corrections I have seen at this stage, and I believe Hal is wrong
>about the ellipse.
The conversion to earth-surface based coordinates happens after all
of this. The Meeus discussion is in chapter 11, "The Earth's Globe".
Page 82 has the IAU76 ellipsoid definition.
He also goes on (p. 83) to give a formula for the difference between
geocentric latitude (assuming a spherical earth) and geographic
latitude (using the local vertical; what is plotted on charts). It
reaches a maximum of 11'32" at 45 degrees latitude.
