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Dear Henry,
I recently mentioned my attempts to measure the index error
the way you describe,
by comparing Sun's lower limb with its upper limb:
(See my message on "star-to-star distances of
Wed Sep 29 2004 - 16:02:05 EDT, paragraph 6).
This was one of the three methods recommended by the manual
to my SNO-T sextant.

The other two were a) measuring the distance from a star
to itself and b) the horizon method.

After many repeated measurements I found the star-to-itself
method more precise than the Sun method. This is the case
with my sextant, my scopes and my specific observation
conditions, I do not generalize this to all situations.
(Unfortunately I have no possibility to use real
horizon at this time).

I can post the numerical data if this is interesting.

However I don't know of any other method (other than star-to-star
distances) of measuring what you call "centering error"
without special equipment.
This should answer your sentences:

> Unless I have misunderstood the intent,
> the purpose thereof,
> however, from a purely navigational point of view,
> appears somewhat
> elusive.

and

>Interstellar distances, to the best of my somewhat
>antiquated knowledge,
>were considered a second line method of determining centering error,

You say that the better method was:

> At one time, an instrument
> could be sent off for testing by a system of
> collimators,

But I don't have a collimator. And many other people don't.
Bougainville :-) who used the stars for a similar test,
as described in the recent messages:
Fri Oct 01 2004 - 00:42:16 EDT and
Fri Oct 01 2004 - 10:58:55 EDT

Alex.

   

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