A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Sean C
Date: 2018 Nov 7, 13:58 -0800
"How do you know the expected rate of change of a lunar distance versus time, if not derived from the observed data?"
You calculate the lunar distances for the time period in question or use a table of pre-calculated values. And you would certainly have one of these methods available as you would already be using them to compare to your observation.