# NavList:

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

**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