A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Sean C
Date: 2016 Jun 15, 04:57 -0700
Sorry for the late response, but I was tied up all day yesterday.
What instrument and what scope magnification, if I may ask?
Astra IIIb w/ 3.5x scope.
You also wrote:
In your graph, I don't understand what the dashed line represents, and I don't understand the points you've marked with squares and keyed as calculated distances. How were these calculated?
I calculated the distances using the formula from your "easy lunars" page: cos-1(sin(Dec1)·sin(Dec2)+cos(Dec1)·cos(Dec2)·cos(GHA2-GHA1)), the same fomula used to calculate the distances for the whole hours preceeding and following the observations. I picked two times that would fall within my graph and I used a second Y-axis on the right to account for the difference in the arcminute range. (The difference doesn't really matter. It's the slope I'm after.) The dotted line represents the slope of the two calculated distances, adjusted for a best fit among the sextant distances. This method was discussed in David Burch's PDF on sight averaging linked to in Bob Goethe's post on Jul. 18th, 2015.
The idea is that, at any given time, the actual slope of a body's changing alitude (or, in this case, apparent distance from the moon) can be calculated and compared with one's observations. Hopefully, this would yield a better analysis of possible errors than simply drawing an arbitrary best-fit slope through the sights. However, this isn't the first time that I've noticed the difference between that slope and the slope I get by using the error values from your online clearing app. My question was whether or not this is due to the fact that I'm trying to fit a slope from calculated geocentric distances to "pre-cleared" topocentric observations. It seems, perhaps ironically, that I would have actually been better off using an arbitrary slope in this case. Do you think I would get a more accurate fit if I somehow worked parallax back into the calculated distances, or instead used cleared distances in the graph, as opposed to raw sextant distances? Or is this method simply not suited to analyzing lunars?
Hope that made sense. ;)