DIY History | Transcribe | Scholarship at Iowa | Eccentricity of the Sextant by Frederic Furbish, 1893 | Eccentricity of the Sextant by Frederic Furbish, 1893, Page 66

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Eccentricity of the Sextant by Frederic Furbish, 1893

Eccentricity of the Sextant by Frederic Furbish, 1893, Page 66

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     The series of observations to determine index correction should consist of, at least, 5 sets of 2 observations each. Beginning with an "on arc" observation the last should of course be "off arc".
     In observations on the sun it is customary with most observers to ascertain the index correction at the beginning and end of the work as the heating of the instrument changes the value.
     The formula for obtaining index correction is :-

     a = reading on arc.
     I = index correction
     a' = " " [ditto for reading] off " [ditto for arc]
     S = sun's diameter
     S = a + I
     ? a + I = a' - I
     S = a' - I
     2I = a'-a' [?]
     I = ((a' - a)/2).
A sample set of observations is given here. - 
     On arc.          Off arc
          31'                 359°26'
          32                        27
          30                        25
          32                        25
          30                        27
    -------                   ----------
mean 31' mean 359°26' or -34'.

I = (1/2)(-3') = -1'30"

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[page][illegible][/page] The series of observations to determine index correction should consist of, at least, 5 sets of 2 observations each. Beginning with an "on arc" observation the last should of course be "off arc". In observations on the sun it is customary with most observers to ascertain the index correction at the beginning and end of the work as the heating of the instrument changes the value. The formula for obtaining index correction is :- a = reading on arc. I = index correction a' = " " [ditto for reading] off " [ditto for arc] S = sun's diameter S = a + I ? a + I = a' - I S = a' - I 2I = a'-a' [?] I = ((a' - a)/2). A sample set of observations is given here. - On arc. Off arc 31' 359°26' 32 27 30 25 32 25 30 27 ------- ---------- mean 31' mean 359°26' or -34'. I = (1/2)(-3') = -1'30"