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Within a great deal of sense, Trevor Kenchington recently said-

>Any sextant has some (though usually very
>little) slop in its micrometer screw thread, else that thread would lock
>in the one cut in the arc. That slop will produce a random error in
>observed altitudes.

I wish to quibble about that (and only that) minor part of his mailing.

What he says would be the case if the worm, and the rack, of a micrometer
sextant both had straight-cut teeth, like castellations. But I doubt if any
do. Perhaps others can advise about that. Even my plastic sextant has
pointed teeth, with faces angled at about 45 degrees, on both the rack and
the worm. The two components are pressed firmly together by a spring, so
both faces are forced to kiss, in intimate contact. If implemented well,
the design virtually eliminates slop from that source. I've always thought
that all modern sextants were like that.

What remains, however, is the possibility of a slight movement, along its
own axis, of the worm, with respect to the index arm as its direction of
movement is reversed. Its pivot is designed with end-bearings that have as
little slop as possible, to minimise that lost-motion. Also, a spring is
usually fitted, to bias the worm in a favoured direction and press it
against one of those bearings, which will work well enough unless any
stiffness in the index-arm movement grows to rival the strength of that
spring.

However, I don't claim to be any sort of expert on modern sextants: perhaps
others can confirm (or indeed contradict) what I've said above.

After all that, I expect every navigator worth his salt to adjust his
sextant with the final touch being always in the same direction.

===================

On that point, Jared Sherman added-
Bearing in mind, if the user always turned the screw the same way (i.e.,
>past then back, >past then back, or opposed to "this way" or "that way")
>then there should be less slop >and the errors would fall to one side of
>the possible range, rather than spread to both >sides of it, no?

Well, not as it appears to me, no it doesn't.

True, adopting such a policy certainly reduces the range of such errors.
It's the right thing to do, without a doubt.

However, the resulting altitude from a sextant measurement is the
difference between the best-estimate scale reading corresponding to the
observed altitude of the body, less the best-estimate scale reading
corresponding to the index error. If both those measurements have been made
as well as possible, this difference will scatter equally either side of
the true average, with no preference for one side or another. Adopting
Jared's measurement method, the amount of that scatter will be less.

George.


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