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    Re: Point Venus, August 1773
    From: Frank Reed
    Date: 2007 May 02, 22:05 -0400

    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.
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