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On Jun 7, 2005, at 1:40 PM, Alexandre Eremenko wrote:
> BUT: On May 31 I observed the distance between Vega and
> Altair, from the same window, with the same sextant, and
> with the same self-confidence,
> and in reasonably good weather...
> and got a long and consistent series... which was 1.5' off!
> I used both my scopes.
> And this distance is not large... in the same range as my Sun
> measurements. I am at complete loss. Is there indeed some bug
> inside my sextant's trommel? The bug which sleeps in the morning
> hours and wakes up at night? Or should I ask an oculist to check
> my night vision?
>
> Alex.

Alex,

I'd say you're about in the same place I am at this point.  Took me
several years, but you've done it in 9 months or so.

On the star to star distances.  You want the star viewed through the
horizon mirror to be dead center in the scope, and where you can rotate
the sextant and the star stays put: the optical axis is pointing
straight at that star.  Then, if the arc angle (arch angel ;) is
correct, the other star will appear as you rotate the sextant to the
appropriate spot.

If the horizon-mirror star is not dead centered and doesn't stay put
when you rotate the sextant, then the reading should be too large.  Was
your observed distance larger than your calculated distance?

Finally, a trick of Frank Reeds is to alter the sextant angle by
several minutes between readings, so that each reading requires a new
adjustment back to the observed angle.

Using a star to get index error would be good practice for this.  The
index error should be the same regardless of whether it was determined
with star or sun.

I'll be very interested to hear the results of your observations on
your sailing voyage.

Fred

   

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