A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Mark Coady
Date: 2015 Aug 24, 10:28 -0700
I am a licensed captain who took up celestial as a nice backup and historical matter of pride. The skipper is always supposed to be able to find his way no matter what. This last year I became interested in lunar distances as a true test of absolute accuracy, and a nice backup for total disaster. I remember a sailing experience pre gps days with all power lost at sea...but the sails still worked..back then that reduced us to unverified timex watches and dead reckoning.
I am now 51, with excellent distance vision, but now require + 1.5 to + 2.0 reading glasses as I was farsighted when young and can no longer compensate.
Here is my background data and question:
1. Shooting off the dock (stationary), in humid summer conditions (just started doing lunars).
2. Doing index corrections prior to lunar sight. I used both distant object & horizon method, and the on/off scale sun diameter method.
3. Using the standard astra 3.5 x 40 sunscope (I just ordered this AM the 7 x prism scope to try it)
4. Seem to have trouble with visual resolution, verifying index correction to absolute reliability. One reading on sun diameter came out perfect on the checksum from the NA of 15.8, another was off by .3'-.4' Part of it is a slight haze, i am having trouble get sharp edges to touch. I see a gap...but can't quite tell if I just touched or am overlapping a wee bit.
My eye also seems to be sensitive to depth of field on the 3.5 x 40 I want to keep tweaking for best focus based on movement of the scope closer/farther to my eye.
Sum total is I seem to have a doubtful range of index correction confirmation correction with the 3.5 x 40 of about +/- .2' -.4', in the humid conditions. I can't satisfy my soul the index correction is .7...as sometimes I get .9...if you get my idea.... testing the same time, same place.
I do not feel it is the instrument at all. The instrumet is tight, feels very precise, and I use single direction turning on the drum mic. If i miss, i back up a good ways, then turn the same way again to minimize gear error.
Part of it is blaming it on the fuzziness created by our warm humid haze, part of it I blame on my own imprecise eye for exact conjunction. I am hoping the higher power prism scope eases my visual debates on edge contact a bit.
Does this experience match others findings.... have you found any neat tricks?
My first lunars got me to accuracies in best (4 minute), worst ( 6-8 minute range) and I have to confess these first were sitting on Block Island in a plastic chair with a drink at hand. I would guess there are rules agoianst drinking and navigating..LOL .but the island wasn't moving very fast) My most recent sights felt good (in decent conditions), and got me best (2-3 minute range), with a worst in the 4 -5 minute range.
I don't have a good backlog of data on the metal sextant. I learned on the davis plastic units, which had enough wobble to blame the sextant for small Hs reading variations thant maybe with the operators limitations.
I am using UTC of my GPS & atomic watch + DUT1 from the US naval Observatory bulletins I recieve for a check time. I stopwatch each lunar distance sight to check against the true time correct to GMT/UT1.
Curious what I should be demanding of my older eyes. I am hoping the higher power scope improves my edge reading.