NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Index Error
From: Robert Eno
Date: 2004 May 4, 21:38 -0400
From: Robert Eno
Date: 2004 May 4, 21:38 -0400
Question for the list on some fundamentals and I
add that I am somewhat embarrassed to ask such a question.
A method I have long used for determining index
error when taking practice observations on land is to use the sun's limbs by
setting the sextant at 32' on arc, touching up the reflected and actual sun's
limbs, recording the number, then doing the same off arc. One half
the difference between the value on and off arc should give you your
index error; the sign being the greater of the two values. I have found this to
be a reasonably accurate method for determining index error; as long as the sun
is at least 30 or more degrees above the horizon.
Here's the problem: theoretically, if one adds
the reading off and on arc, subtracts the index error and divides by four, then
the result should equal the tabulated value for semi diameter in the Nautical
Almanac.
I have seldom found this to be true. The result
often differs from the tabulated value by up to 0.4' of arc. That's a
lot.
Does anyone have any explanation for why this might
be? I should add that my sextant, a Plath, has never suffered any grievous
insult so I do not believe that instrument error is the cause. Hundreds of
observations also indicate that the instrument is pretty darn
precise.
