# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Precomputed lunar distances
From: Bill B
Date: 2005 Apr 19, 17:01 -0500

```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.  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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