NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Antoine Couëtte
Date: 2020 May 24, 13:34 -0700
Dear Dave,
If the Earth were a perfect sphere both geocentric and topocentric Azimuths of a finite distance body (here Lady Moon) would be identical since the vertical line drawn from your position would exactly cross the Earth Center.
On an ellipsoid, which is our case on Earth, the vertical line from your position does not cross the Earth Center except from the Poles and the Equator. Hence, when aiming at the Moon, there is a little "offset" or change of perspective between the Earth Center and the Earth Surface.
If we assume that Z topo = Z geoc + Parallax in Azimuth, then for your example : Z topocentric Moon = Z geocentric Moon - 12.986 " .
In other words, and with the definition here-above : Parallax in Azimuth = - 12.986"
If I remember correctly, and by comparison with the geocentric Azimuth, the effect of ellipticity tilts the geocentric finite distance topocentric Body Azimuth towards the elevated pole .
In extreme cases, Parallax in Azimuth can reach 180°, as this has already been adressed here.
Best Friendly Regards,
⚓ Kermit - antoine.m.couette[at]club-internet.fr ✈
