A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 May 5, 15:49 -0700
One little detail on those calculations, Norm. If you compare your sextant readings with the calculated Venus-Jupiter distance, you may find that they differ by a couple of minutes of arc or even more when Venus is low in the sky. You have to correct the measured distance for refraction, or alternatively you can use the apparent coordinates (with refraction included) to generate distances for a particular time of night.
Here's a pair of posts from 2004 where I talked about using star-star distances to test your sextant:
All of this applies directly to Jupiter-Venus sights except that you would use the GHA and Dec for each planet to the nearest hour of UT, and also you may want to add a small correction for the parallax of Venus (opposite in sign to the refraction correction). Refraction moves both Venus and Jupiter towards the zenith. Venus is lifted more through the end of June since it is at a lower altitude. That makes the observed distance somewhat shorter than the "pure" geocentric distance. Also with Venus and Jupiter, you have to decide whether to bring the two planets together edge-to-edge, in which case there's a small semi-diameter correction for each planet, or merge the planet images on top of each other. I think the latter is easier and probably preferable for this observation.