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    Re: Point Venus, August 1773
    From: Alexandre Eremenko
    Date: 2007 May 2, 23:35 -0400

    I also noticed all these convenient IC adjustments
    on the very old sextants/octants.
    But still, I think there should be some reason
    for re-adjusting the IC in the middle of
    a series of shots. I could inderstand if the IC
    after the adjustemnt became smaller.
    But in this case it became bigger...
    So I suspect that this was due to some other adjustment
    that they found necessary.
    For example, even a very small adjustment of
    the big mirror changes IC very much.
    Do you have any conjectures WHY they prefered
    frequent adjustment approach to the modern no-touch
    approach? Once I asked on this list why the adjustments
    are necessary at all. Why not to fix both mirrors rigidly
    in a factory. (As they do nowadays with colimation).
    The answer was that an adjustment is necessary when you
    replace mirrors. Sounds reasonable.
    But why frequent adjustments?
    By the way, the old sextant I have (late XIX century)
    is surprisingly non-rigid. When I switch the hand
    and hold it with my left hand by the frame
    (this is sometimes convenient when doing Lunars)
    the IC changes substantially.
    I just see how well aligned stars become non-aligned
    when I switch the hand. Nothing like this happens
    with SNO.
    On Wed, 2 May 2007 navlist@historicalatlas.net wrote:
    > Hello Alex. I haven't had little time recently to reply to your interesting
    > messages in this thread, but this caught my eye:
    > "And the thing I find most puzzling is
    > that IC of their sextant used to take the Lunars
    > changed from +1' to -3'42" (This is what they
    > recorded !!!)
    > Was it dropped??
    > More likely is that the observer noticed a substantial
    > side error/lack of parallelism or whatever while
    > shooting the first series. So he adjusted the sextant
    > and re-checked the IC."
    > I have a distinct impression from reading old navigation manuals and
    > journals that IC was regularly changed and re-measured in this period,
    > rather than simply treated as a nearly fixed quantity that had to be
    > measured. There is also evidence in the design of the instruments. Old
    > sextants and octants especially (like the one I showed you) had large,
    > easily-manipulated knobs whose only purpose was to change the IC. One day, I
    > think it would be possible to contruct a history of the phrase "don't try
    > this at home" just by measuring the size of screws and knobs! I'm not being
    > entirely serious here, but clearly very small screws with small, unusual
    > heads are intended for rare adjustment by knowledgeable users, while large
    > "user-friendly" knobs that fit easily between thumb and forefinger are
    > intended for frequent use by average users.
    > I also notice something here that seems to be common from this period --a
    > lack of understanding of significant digits. Or perhaps more likely, no
    > means to distinguish varying levels of significance. So, for example,
    > calculations for altitude corrections were often worked out to down to the
    > second of arc when the observer surely knew that the observations were only
    > accurate to the nearest minute of arc.
    >  -FER
    > >
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