# NavList:

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

**Re: Sextant Positions versus Map Datums?**

**From:**Dov Kruger

**Date:**2002 Jan 18, 10:06 AM

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. You can read all this in Meeus, and there is a program out there called Astrolabe which though not the best code, uses VSOP and is therefore one of the simplest to read. By contrast, Steve Moshier has some code out there that is supposedly even better (more accurate, using the currently most accurate model, DE404) but it is nearly impenetrable, filled with lots of different conflicting models, dead code left in, and just a more complex algorithm (I think!). If you are an astronomer, you care about accuracy to the arcsecond so you can point your telescope. If you are interested as a navigator, you can live with any of these models, they are more than accurate enough, particularly this century. All this so far has nothing to do with the ellipsoid or datums, except to contradict any claim that the position of bodies in the almanac is somehow corrected for any of that. It can't be -- the position is calculated based on the center of the Earth, and a wild and crazy equation for Heliocentric position of the other body based on the julian century T. The apparent position is then corrected for lightspeed and nutation. What you can measure is altitude, and as Trevor corrected me and explained to everyone, if there were to be a correction for the ellipsoid, you would do it as you convert between GP/AP and altitude and azimuth. If you are using the standard equations, it's obviously not happening. It could be in the 229, but if you look at the notes about how the tables are generated, clearly no such correction has been done. The equation they use is spherical, and the only note they make is that for angles that would result in bad roundoff error they use a different form that is analytically equivalent. That is an issue of computation, not changing the shape of the sphere. So perhaps Meeus merely mentions somewhere that when they record an observation, astronomers convert the other way to determine the position of the body? I assume that would have more to do with parallax. Anyway, I would also like to see the reference. >According to Jean Meeus, Astronomical Algorithms, 2nd edition, >astronomical observations (which are the basis for the Nautical >Almanac) use the IAU 1976 ellipsoid (International Astronomical Union >1976). Those values are