NavList:

Hello Sean,

You wrote "Using MICA, I've noticed that the declination between geocentric and topocentric locations doesn't seem to change exactly by the amount of parallax in altitude".

Maybe - and maybe only -, one explanation to this might be that the "parallax in altitude computation" you are refering to is not comprehensive/accurate enough (I do not know which algoritm you are actually using for such computation).

For the Moon - a close body with an important semi-Diameter - if you require Parallax in Altitude accurate to better than 1" in all cases - which seems consistent in order to wipe out all systematic computation errors which may be so much magnified/expanded (up to 30 fold if I remember correctly since Moon moves at 0.5" per second of time, and during this time Earth has rotated 15") when converted into Time/Longitude determination through Lunars - you NEED to compute such parallax AND ALSO semi-diameter effects in 3D from the Ellipsoid.

If your actual parallax in altitude and SD computations are not performed in 3D from the Ellipsoid, which by the way REQUIRES to take in account the Moon azimut here - which is not the case in all usual Parallax/SD computations - this could explain the discrepancy you are observing.

I have extensively studied that in some [remote] past and was forced to study it all over again and quite thoroughly when studying Lunars a few years back. I should be able to retrieve real world cases in my archives if necessary which demonstrate that our "traditional" 2D Parallax and SD Computations may induce systematic errors exceeding 8" or even up to 9" in the Moon Parallax determination under extreme cases (including cases with high Moon absolute declinations e.g. +/- 28°) if I remember correctly.

Best Navigational and Lunarians Regards

Kermit