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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: On measuring the distance
From: Arthur Pearson
Date: 2002 Mar 22, 15:37 -0500

Gentlemen,

Having just come in from taking 11 distances over 15 minutes, I appreciate Bruce's comments. The longer I take on any one contact, the blurrier my eyes get, the more lost I seem to get in the sight. Faster seems to be better. I also find his suggestion that one twist the knob between sights to be a good way to keep yourself honest by making each observation independent of the one that came before. Unfortunately, the results can be humbling. My series is accented by jumps of 3' to 5' in distance over intervals of only 1 or 2 minutes of time. I also observed two instances of apparent retrograde motion, which is to say the gradually increasing distance decreases sharply before resuming its expected rise. Either I have made a shocking new discovery about the moon's motion, or my frozen fingers twitched at the wrong time, or my watery eyes read the vernier scale wrong (likely one of the  latter two).

Whatever the imperfections in my observations, I would appreciate some feedback on the method I am using to graphically average my series of distances. I have found it useful to average them graphically by plotting Ds against watch time and then plotting a "referencee line" that shows the slope of the line that connects the computed values for the hours before and after the observations (D1 and D2, the comparing distances). This line shows how my observations should be increasing or decreasing during the hour.  I throw out any garbage sights and use the reference line to make a best fit line to the reasonable sights (favoring sights I might trust more than others).  I select a convenient time during the observation period (usually a whole minute) and use the best fit line to pick the Ds that corresponds to my selected time. I solve for GMT using the the selected time and its corresponding Ds, even if I didn't make an actual measurement at that time! .  I got the idea for this from an article posted on www.starpath.com, I would be grateful for comments as to whether I am using it appropriately in this situation or if there are hidden problems with this technique.

I will post my observations and what I did with them this evening,

Regards,

Arthur

Arthur Pearson
arthurpearson---.com
----Original Message Follows----
From: Bruce Stark
Subject: On measuring the distance
Date: Fri, 22 Mar 2002 14:17:48 EST
Didn't get back to the computer as soon as expected!
Far and away the most critical aspect of lunars is the observer's skill in
measuring the distance. George has pointed out that a phenomena he's
discovered and given the name "Parallactic retardation" makes the measurement
twice as critical if the moon is near the meridian. That came as an
unpleasant shock, but it reinforces the importance of developing good sextant
technique.
One thing I noticed about Chuck's measurements of the distance is that he got
and recorded them in short order. That's the way it should be. Agonizing over
the perfection of each contact only wears you out.
Another thing I noticed is that, although the distance is increasing, his
measurement of it gets shorter each time. And I believe it is getting better
each time. My own experience is that it often takes one or two contacts to
warm up. Perhaps for his next lunars he'll take only one altitude of each
body before and after the distances and use the time saved to get more
distance contacts. Maybe five or six, and throw the first one or two away.
My own eyesight is poor, and when I bring the sextant up again the last
contact still looks OK. So I always give the knob a little twist one way or
the other so as to have to make a new adjustment. That way, five or six
contacts usually gets GMT within two minutes.
Bruce

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