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Lisa Fiene wrote-

>When I received my sextant from the Ukraine (I'm in Australia), the
>index error was 16' at only 200 metres!  So, it needed quite a bit of
>adjusting, let me tell you.  All's well now though.

It's unfortunate that a sextant should be supplied that's so far out of
adjustment. But it only involves adding or subtracting the measured index
error, using a distant reference, so it's no more than a matter of
preference whether it's reduced to nearly-zero, or not. In my opinion it's
a waste of time worrying away at the sextant's mirror-screws to exactly
zero the index error. In the end, some error will remain, or arise, so it's
only a question of allowing for a small number or a larger number

But more worrying still, in Lisa's reference to "at only 200 metres"! What,
I ask, has 200 metres to do with the question?

To adjust the index error, using a terrestrial object, then that object
needs to be MUCH further away than 200 metres. This is because through the
telescope your viewpoint is slightly shifted vertically from your viewpoint
through the mirror, by, at a guess depending on the sextant, about 80mm
(say, 3 inches).

A perfect sextant, which has had its index error adjusted to zero using an
object at 200 metres, will have a resulting index error, for objects at
infinity, of as much as 1.4 arc-minutes.

For most purposes, a (smooth) horizon is far enough away to use for
adjusting index error, but for those that wish to work to ultimate
accuracy, say for lunar distances, then a celestial body is marginally
better.

George.

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01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
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