A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Jim Rives
Date: 2020 Jan 19, 05:08 -0800
Shot my second lunar, this time from Rye, NH, on 17 Jan 2020. Bitter cold day... 12F 20kts wind from NW. Clear. Horizon was not sharp, obvious effects of warmer water under cold air. Not that the horizon mattered for the lunar, of course. (Might have if I had taken altitudes rather than using calculated ones.)
Using 5 data points and a visually fitted line I chose my 1500 GMT LD to be 88d 45.7'. Using Frank's browser based Lunar Calculator I came up with a Cleared LD of 88d57.3.
So: True LD for 1500 was 88d 54.2'
Cleared LD was 88d 57.3'
for a difference of 3.1' in LD of predicted versus observed/cleared.
Using the Lunar Almanac on Reed Navigation I found the rate of closure of the moon on the sun between 1400Z and 1500Z to be .54167'/min of time.
Then, 3.1' of arc divided by .54167'/minute of time results in a time difference of 5min 42sec, early. GMT would be 1500Z minus 5min 42sec, or 14:54:17.
So, my question to everyone is have I thought through this process correctly? And, if so, isn't this a very large error? I have included an excel sheet of my sight plot.
As an aside: For this observation and for my first a couple of weeks ago I plotted my LD's per sextant on graph paper first and determined my LD for a whole hour from the line I fitted visually. Then I ran it in excel and used the statistical data pak to come up with some statistical characteristics. In both cases the LD I would have chosen by either method came up to within .1' of each other, which surprised me. I will probably not do the manual graph going forward much because, as fun as it is to plot, is a bit time consuming. And as I continue to do these sights I'm interested in analyzing my shooting ability, which I anticipate being "gradable" by the degree of variance in the regression line. I mean, afterall, it's not really about figuring out where I am, is it?!
Thanks in advance for any and all feedback. Navlist is such a great resource!