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    Re: Homemade octant
    From: Frank Reed CT
    Date: 2006 Mar 19, 21:30 EST

    Greg, you wrote:
    "It seems to be +/- 1 minute  of arc. I can't tell where the error comes from
    - but in the end it comes out  within +/- 1 min of my MAC."
    That's excellent. Honestly though, even your  M.A.C. may not be that accurate
    over its whole range. Have you tested your  octant for large angles? Say,
    above 70 degrees?
    You asked:
    "but lets  say on the avg, to the nearest min. My experiments with running
    the  calculations, shows varying the Ho or Lat by 1min can move the AP about
    4  miles. There are different combonations; Ho+ Lat=0, H0+ Lat+, Ho- Lat  -,
    etc., etc. some give better results; but 4 miles is a safe number  when
    speaking of errors.
    Was this as good as it got ( Talking in period c  1800 if using a octant or
    sextant with this resolution)? and didn't you need  to resolve down to 10"
    or  20" to do lunars?"
    A mile or two  accuracy in latitude was (and still is today) a reasonable
    expectation. Even a  perfect sextant with perfect input data is still limited by
    the vagaries of  terrestrial refraction which makes the horizon variable by
    about one minute of  arc. Note that sextants were developed specifically to
    measure large angles and  to measure them at an accuracy about ten times better
    than you can do with an  octant. Octants were used for measuring altitudes.
    Sextants were used for  measuring lunars (which are not affected by terrestrial
    refraction). After 1850  (-ish) sextants were used for more or less all
    42.0N 87.7W, or 41.4N  72.1W.

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