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Dear George,

>Any error due to wear on the worm would be expected to repeat at
>one-degree intervals, so could affect readings close

Sounds very convincing!
So it is the arc. Or...the arm pivot.

> Why not really misadjust the index mirror, in a big way,

> Ideally, one might wish to shift the zero by
> perhaps two and a half degrees, to use an

While I am experimenting, I found the following alternative.
Instead of determining my IC
by the usual procedure, using one star,
I use TWO stars at a small distance.

Yesterday I used Castor-Pollux distance for this purpose,
which is 4d 30'.3
(Nautical almanac does not have Castor;
I had to use ISNO Astronomical almanach to compute this).

I will post the details in a separate message,
but from this Castor-Pollux procedure I found the index
correction -0'.7. Such correction would make my all
other star distances taken in the last week almost perfect!

> It's a potential problem, that the older,
> Vernier sextants didn't suffer from.
> Another disadvantage of progress, perhaps?

I also think so. And I hope my next sextant will be
one of those old ones with Vernier and microscope
and no teeth to worry about! If I find a good one.

Alex.

   

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