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Mr Royer,

I was suggesting measuring the moon's semidiameter to check whether you
were sighting properly on the edge of the moon.  It was a desperate
attempt to find a reason for the 5' of arc gap.  However, since you
measure the sun's semi-diameter whenever possible, it would seem that
you should be able to do likewise with the moon.  But certainly if your
moon semidiameters were too large, that would suggest improper sighting
was contributing to the gap.

Although there was some scatter in your observations, usable distances
could still be plucked from the data; if things had been right, you
should have been able to determine the time to within 30-40" from those
data, in my opinion.

It may well be that atmospheric conditions affect lunar distances.
Certainly they would if refraction were abnormal.  Although Snell's law
seems to hold most of the time, it may be that a cloud layer could
introduce some departures from it.  This is mere speculation on my
part; I am blissfully ignorant of most all aspects of atmospheric
physics.

I have been trying for some time to bring my observations of altitudes
under the last half mile.  Lately, they seem to be getting pretty close
(|Hc - Ho| < 0.2') fairly often.  However, I can offer no reason why
this should be so, other than I am becoming more proficient in the use
of the sextant and at judging the proper time of a sight.  Perhaps also
you might find that your lunars become more accurate with time.
Certainly as one became used to the contortions required, one's
proficiency might increase.

Accurate timing of a lunar is not as critical as accurate timing of an
altitude.  Knowing this might help.  Additionally, you cannot wait for
a gap to close, but must adjust the sextant to close the gap.  For
altitudes, I usually wait for a gap to close or begin to open.  The
technique is different for a lunar.

At this point I am hoping that I become proficient enough to assess
whether atmospheric conditions affect my altitudes by correlating
conditions with departures of |Hc - Ho| from < 0.2'.  However, a nice
several-week-long stint when almost all observations come in at less
that 0.2' must intervene first!

Yours Sincerely,

Fred Hebard


On Monday, May 19, 2003, at 20:05 US/Eastern, Royer, Doug wrote:

> O.K.,let me explain better what I was asking earlier.I wasn't
> referring to
> takeing altitudes to the visible horizon.That is not the concern
> here.The
> concern is about the distance angle between the Moon and another
> body.Everyday at sea I check the Ie against the semi-diameter of the
> Sun.All
> other errors are checked periodically as the Tamaya's error has been
> consistantly 0.1'- 0.2' off the arc for eons under many differant
> conditions.Checking the Moon's semi-diameter accomplishes the same
> thing.I
> check for the true horizon as noted below during every observation.When
> takeing the distance measurements of the bodies I swung the arc.When
> Jupiter
> or the star touch the Moon's edge I took the reading.In the awkward
> position
> the sextant was in to get the readings maybe the objects were off
> center in
> the scope.I don't know if that would give a consistant error or
> not.5'of
> error is huge.I'll be on the alert for it from now on.My question about
> under what atmospheric conditions you guys observe Lunars remains
> unanswered.Is this a fair weather(clear skies) proceedure or have
> others
> accomplished it under adverse conditions,not that the conditions I had
> were
> all that adverse?
>
> -----Original Message-----
> From: Philip Ouvry [mailto:ranch.flamingo@INFONIE.FR]
> Sent: Monday, May 19, 2003 12:55
> To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
> Subject: Re: Technique question
>
>
> Many books warn of a possible false horizon under the moon.   However
> by
> panning left and right it is usually possible to determine the
> position of
> the true horizon.   With experience if you can see the edges of the
> moon
> then it should be feasible to take sights.
>
> HTH
>
> Philip Ouvry
>
>
------------------------------------------------------------------------
Frederick V. Hebard, PhD                      Email: mailto:Fred@acf.org
Staff Pathologist, Meadowview Research Farms  Web: http://www.acf.org
American Chestnut Foundation                  Phone: (276) 944-4631
14005 Glenbrook Ave.                          Fax: (276) 944-0934
Meadowview, VA 24361

   

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