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Ken,
Thanks! That's a very interesting suggestion:
to try to make a correction table where the correction
depends on the minute reading, rather than degree.
On the next sunny (and Moony:-) day I will try to do this.
(I find that in general my Sun-Moon measurements are much more
precise than the measurements at night, which is not surprising).

Alex.

On Tue, 29 Nov 2005, Ken Gebhart wrote:

> On 11/28/05 1:09 PM, "Fred Hebard"  wrote:
>
> > On Nov 28, 2005, at 1:00 PM, Alexandre E Eremenko wrote:
> >
> >>> For instance, the bump at 16
> >>> degrees wasn't
> >>> reproducible between November 13 and November 7.
> >>
> >> That's the main problem. If they were reproducible, I'd
> >> make a table of arc correction. But they are not, and I am stilll
> >> at loss whether this has something to do with my observations
> >> or just the sextant is flimsy.
> >>
> >> Alex.
> >
> >
> > Alex,
> >
> > My guess would be observational proficiency.  It took me about two
> > years to achieve proficiency in altitude shots with the artificial
> > horizon; I quickly got within 0.5' of arc, but getting well under
> > 0.2' took much longer.  You seem to be pretty close.  Perhaps Frank
> > could comment on how long it took him.
> >
> > Fred
> >
> Gentlemen,
>
> I have been paying occasional attention to your posts (due to travels), and
> it seems to me that you are neglecting the effect of drum eccentricity.  The
> readings of the micrometer drum itself can be in error by as much as 20 or
> 30" of arc all by itself. This is caused by any microscopic deformation of
> the drum shaft, or errors in machining (or damage to) the worm gear. This
> can be added (plus or minus) to any errors determined on the arc.  This will
> pretty much make a mess of any attempts to calibrate the arc unless all
> measurements are made at the same drum reading (not likely when measuring
> stellar distances).
>
> It seems to me that a way to get a handle on your own drum eccentricity
> might be to measure a moon- star distance over a period of about 2 hours.
> This should give a series of readings over a complete revolution of the
> drum, that could be plotted of graph paper to see what is happening.
> Ken
>

   

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