A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2019 Jun 7, 12:34 -0700
"Should the Altitude, noted as "apparent" in the Stellarium data for each object agree precisely with my sextant reading (Hs)?"
No. You can get close with a little effort. The displayed altitudes are, first of all, the altitude of the body center. Also, there is no "dip" and no provision for setting height of eye.
Suppose you take the Sun altitude from Stellarium, and you want to compare it with a sextant observation. The altitudes in Stellarium include refraction and parallax, but they do not include dip or SD. For a quick fix for approximate comparison, subtract 12' from the displayed apparent altitude of the Sun in Stellarium.
You can also remove refraction from Stellarium altitudes by turning off the atmosphere. So for stars and planets, you can directly compare corrected sextant altitudes (Ho values) with the displayed altitudes in Stellarium with the atmosphere off.