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Alex, the index error of a bubble sextant can be determined from an unknown
position.

By taking sightings on bodies in opposition, the separation of the
resultant position lines will, on average, be twice the index error.

In my posting #2531, I advocated taking four sightings, a North-South pair
and an East-West pair, on bodies at the Cardinal points. The North-South
pair effectively give you your latitude and the East-West pair your
longitude. The position lines from the four sightings should form a square
box if index error is the dominant error. If the box is not substantially
square, there is a non-systematic error somewhere which is of the same
order or greater than the index error.

See http://www.pisces-press.com/desert/14thMarch.jpg for an example of what
I mean. In this example, the box is substantially square, giving us
confidence that the dominant error is indeed index error. The size of the
box is about 10' on a side, from which we deduce that the uncorrected index
error was 5'. The position lines are all 'away from' the body in question,
so the sextant altitudes are 5' too small. I need to add 5' to my altitudes
to correct the index error.

A subsequent round of sights at a different location a few days later show
the results when this 5' is factored in. See
http://www.pisces-press.com/desert/19thMarch.jpg

Geoffrey Kolbe


At 20:24 06/04/2007, you wrote:



>Geoffrey,
>
> > It is easy to determine the Index Error of a marine
> > sextant, but not of bubble sextants.
>
>Easy or not, but I infer from your message that you
>can find the IC of your bubble sextant SOMEHOW.
>
>I don't see any way to do this at all,
>except taking an altitude from a known position.
>
>This can be done once in a while, but certainly not
>before or after an observation, if the purpose of
>the observation is to find this position.
>
>Alex.
>
>
>
>

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