NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Clive Sutherland
Date: 2006 Aug 18, 05:35 -0500
As a first step you need to distinguish between faulty Observations and faulty Calculations.
Take about 10 or more sights of the moon as close together as possible in time and to plot the result. If this plot is a straight line, you could compare the slope of the plot with the rate of change of the Moons altitude. If this is OK and the statistics give a reasonably precise set then the fault is not with the observation and you need to examine the Procedure, the Data or the Maths of the calculation. Examine your procedure to see if the average of the Time set really corresponds with the average of the Altitude set, if you use this method.
By using a computer program such as Excel you could reduce every sight individually to an intercept and analyse the spread of the values you get. If they are not all exactly the same then how wide is the scatter compared with you star sights.
Could the corrections from the almanac have been applied in the wrong order?
If all is copacetic<G>, then the explanation eventually may be with amount of Augmentation or Irradiation experienced both of which are controversial subjects, and have been discussed before on this list. I do not know of any physical reason that clouds would affect the measured altitude.This can be easily checked by comparing the view of the moon through cloud patches with the view against clear sky.
Ice blink might affect the horizon or the actual value of dip might be suspect,especially at high latitudes.
Clive.
----- Original Message -----From: Robert EnoTo: navlistSent: Friday, August 18, 2006 3:19 AMSubject: [NavList 1077] The *&^%$#@ MoonGentlemenI'd like to throw this one out to the wolves.......er' list members, and see what comes back.Many navigation texts discuss what was apparently an "old sailor's tale" about the inaccuracy of moon observations. The story goes that in the old days, many navigators avoided moon shots because the believed that moon observations yield inaccurate LOPs; a belief which persists to this day in some circles. The texts go on to say that moon shots are perfectly accurate; no less so than any other body that one can observe; that with current correction tables, moon observations should yield results every bit as accurate as any other observations.Notwithstanding the pronouncements of navigators who have forgotten more than I will ever know, it has been my experience -- after thousands of observations of stars, planets, the sun and the moon, that moon shots do tend to yield results that are simply not on the money. I have tried to ignore this and embrace what the textbooks say about how moon shots are accurate, but the theory just doesn't pan out for me personally. I am one of those navigators who only uses the moon as a last resort because I fully expect (and am seldom wrong) that I will be wonky LOPs.Case in point: the other night, I took the opportunity to take some star and moon shots. Haven't seen the former since last April and while the return of the stars also signals the imminent return of old man winter in my part of the world, I nevertheless welcome the return of my old friends.To carry on, I took four observations of the moon and four of Vega (from a known position) using my Plath Bubble attachment. As is my practice, when observing from a static platform, I averaged out the results of the sights, all of which were taken within minutes of each other.The average result for the moon shots was 3.9 minutes of arc error, while the results from Vega were only 1.3 minutes of arc error. To add insult to injury, the Vega shots were extremely difficult to take because the illumination system in the Plath, even at its dimmest, tends to all but wash out stars. The moon shot was a breeze but the results were pathetic. I should add that clouds were moving in at the time. Could clouds have an effect on refraction?Ok, Ok, I know that a bubble attachment is simply not as reliable as a true horizon and that 8 observations do not constitute a statistically-correct defence to support my assertion about the inaccuracy of moon shots. I merely presented these data to illustrate what has been my experience since I first picked up a sextant 23 years ago. It is more often than not, the same for me: to borrow from the modern pop culture vernacular: moon shots simply suck. At least for me.So what gives? Have I been cultivating and reinforcing an as yet unknown bad habit vis a vis moon observations all of these years? If so, why would this not be the case for the sun, stars and planets?Is there anyone else out there besides me who simply does not get good results from moon observations?Is this all in my head??Robert
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