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From: Bill (no email)
Date: Tue Apr 19 2005 - 18:01:02 EDT
Bill wrote, regarding angular refraction corrections:
> They do not seem to reflect refraction moving along a straight line to me,
> where I might expect the corrections to be similar to a curve derived from
> refraction values at those altitudes."
Frank responded:
>
> I take it that you're suspicious of these results because it seems as if the
> correction is just about 0.6 minutes of arc across a wide range of
> altitudes. Strange, huh? Strange but true... in this case where both stars
> have the same altitude (and as long as the altitudes are above about 12
degrees).
In fact that is not what raised a red flag for me. I had drilled down too
far and done a scatter graph with Excel, so every or hundredth or thousandth
was magnified. Now that you mention it, I recall a discussion about bodies
of equal observed altitude and the 0.6' minute figure.
I apologize to the you and the list for rehashing a subject that was
apparently covered in October. At that point I was still working with a
cardboard sextant and H0229, playing with various sailings, Bowditch tables,
and constructing Mercator plotting sheets. I did not anticipate the journey
would take me this far, and did not pay close enough attention to postings
that seemed above me at the time.
Bill you wrote:
> "Yes, that is my image. A two-dimensional representation of three
> dimensions. What a camera would see."
Frank responded:
> OK. And this can be useful so long as you remember that the sides are
> actually straight as an arrow.
So is a wire-thin hula hoop when viewed from the correct angle. <G> In
another special case, a circle. The rest of the time, and ellipse ;-)
Regarding my question, "Another hypothetical scenario. If I take the same
two stars, calculate true separation of 34d 27.7', they have identical Hc's
of 1d 36.8', and hypothetical refraction is -88d, what separation might I
expect to measure with a sextant?"
I asked if for two reasons:
1. As a sanity check to determine if my mental model was workable. I would
expect that there would be almost no observed separation given the above
scenario. Is that correct?
2. When I ran the above scenario through the separation refraction
correction formula, the correction was only a matter of a degree and a half.
If above paragraph/assumption is correct, I would expect separation
corrections in the -33 degree range. Perhaps extreme refraction is out of
the intended scope of the formula, or one of those lovely rules like "If cos
X is <0 and it is a Tuesday with an odd date subtract 90" needs to be
applied.
Bill
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