NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar distance accuracy
From: Alexandre Eremenko
Date: 2007 Nov 4, 23:18 -0500
From: Alexandre Eremenko
Date: 2007 Nov 4, 23:18 -0500
Frank, > "So you're saying these observations > exhibit excess kurtosis " Frank. I really don't want to discuss theoretical questions of statistics on this list. I think the plots of distributions of errors tell EVERYTHING to anyone who cares to look at them. While trying to characterize the error with a single number (error of the average, average error, sigma, most probable error, median error, whatever) is somewhat misleading. > Alex, your messages are not getting "lost". Good. So you saw the plots of distributions. These plots tell you everyhting I have to say (and everything I know) about Lunars accuracy. If you disagree with this, then tell me what exactly is the point of disagreement. a) You think these plots are somehow not typical and DO NOT reflect the real accuracy of the lunar distance observations (from sea and land, with a hand held sextant, by an observer with experience)? Or b) you agree that these distributions are sort of typical, and really show what can be practically achieved? And in this last case (b), the only disagreement is how to derive a SINGLE number from these distributions, the number which characterizes the accuracy? If we are in situation (a) please give me a reference for better data. If we are in situation (b), let me state two rough conclusions from these plots: 1. Observations at sea a substantially less accurate than those on land. 2. Even on land, under good conditions, errors of up to 0'5 in a single distance shot are unavoidable, and occur in about 1/10 of all shots. These are the conclusions I make from these graphs. > For a long time, I thought that you personally, Alex, > were shooting lunars because > you wanted to determine the arc error of your > instrument. For this I was shooting star distances. I have to say that I failed to determine any arc error of the instrument. Freiberger and Cassnes Plath independently issued certificates that say that my instrument has no perceptible error. So in my Lunars I assume this (as White did:-) > "game"-- then mounting the instrument on a stand Lunars is a poor method of determining arc error. Because there are only few distances that you can shoot on any given night. And for arc error, it is desirable that all shots are made under the same conditions, with index error unchanged etc. Years ago, one list member (George perhaps?) challenged us by asking whether anyone succeeded in determining the arc error of his (modern) sextant himself, by distances or any other observations. No one responded to this challenge. I tried and failed. But perhaps my sextant really does not have any perceptible arc error. Alex. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---