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This is to post Arthur's message and reply.

Arthur,

I agree that a timing error would not be critical in a lunar, because
of the slow change in distance with time, as you point out.

In my imagination, I can see that it would be easier to detect a gap
in contact than an overlap as two bodies rushed past each other in
the sextant eyepiece on a pitching, rolling, and perhaps yawing sail
boat.  This might bias the observation to an overlap.  Perhaps if you
tried to achieve a slight gap it would reduce the systematic error.

Another motive I had for starting this thread was to point out that
it was the disappearance of the systematic error in my altitude shots
under certain conditions, ie at meridian passage, that led me to
detect its cause.  Perhaps you could check planet or star lunars,
where it might be easier to judge contact more accurately.  According
to one of my navigation books, one can see planets during the day if
one points one's telescope to the right part of the sky.  That is for
altitude sights.  I don't know that daytime planetary lunars would be
feasible.

Fred


>Fred and Herbert,
>The list has not been posting my emails for the last 24 hours, so I
>will send two emails directly to you. You have been most active in
>this thread which I don't want to end prematurely. Feel free to post
>this and your reply (if any) to the list, that was my intent but it
>just won't work for me today.
>Arthur
>
>
>Fred,
>
>This is an interesting source of error to examine. I did some quick
>calcuations with CelestNav gauge the magnitude of the effect.  A sun
>sight this morning (Feb.3) at 12:00:00 GMT taken at low latitudes (12�N,
>61�W) changes at about 0.5' every 2 seconds. That is a pretty low sun
>(~20�) that should be rising relatively fast (Az=112�). Mid latitude
>suns would be rising more slowly, I get about 0.3' per 2 seconds for a
>morning sun in my home waters of Maine in mid-June.  I observe alone and
>use CelestNav to mark the time, so I usually count out 5 seconds after
>the sight and then hit the button that captures the time assuming a 5
>second delay. It is easy for me to believe I could have a systematic
>error of a couple of seconds. This would explain errors in altitude, but
>I don't think it would explain the systematic error in my lunars.  An
>error in time recording of 15 secs was needed to move my estimate of
>distance error by 0.1', so I think my systematic lunar error seems to
>lies elsewhere.
>
>Thanks,
>Arthur
>
>
>----Original Message Follows----
>From: Fred Hebard 
>Reply-To: Navigation Mailing List 
>To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
>Subject: Re: Timing Error in Sights
>Date: Mon, 3 Feb 2003 06:34:15 -0500
>
>Thanks Jared,
>
>It wasn't just the mechanics of recording the time.  It was also
>making an accurate decision as to when coincidence was achieved.  I
>resorted to the trick of waiting for the sun to just touch itself in
>the artificial horizon.  Against a natural horizon, one would wait
>for the sun to rise above it or settle to it, when shooting the lower
>limb.
>
>I still haven't gotten very good with stars and planets in this
>regard; those might be a bit easier with the advent of spring!
>
>Fred

   

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