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

**Re: Lunars: altitude accuracy**

**From:**George Huxtable

**Date:**2004 Nov 1, 11:19 +0000

Frank Reed wrote- >I general, good (slightly approximate) expressions for the required accuracy >of the altitudes are: >AccuracyBody = 6' * sin(Distance) / cos(BodyAltitude) >AccuracyMoon = 6' * tan(Distance) / cos(MoonAltitude). > >I have never seen these expressions in print anywhere. Has anyone else >encountered them? No, and they don't "feel" correct to me. I would like to see the argument that justifies them. Do Frank and I agree, I wonder, that the geometry in which the lunar distance is MOST sensitive to changes in altitude, of the Moon or other-body, is when the two bodies lie somewhere in an arc that passes through his zenith, so they have azimuths that are identical or else 180 deg apart. In that case the lunar distance is either the difference or else (180-sum) of the two altitudes, depending on whether they are on the same side of the zenith or opposite sides. If there's any change, or error, in either altitude, then a corresponding change in lunar distance, of ?100% of that amount, results. This is just as true for large lunar distances as for small ones, as I see it. (For other geometries, the lunar distance is always less sensitive to such changes, so a smaller percentage than ?100% applies. For lunar distances of 90 degrees, as Frank points out in another posting, the sensitivity can be 0%, but only, I think, in certain geometries.) If, for the most sensitive geometry, the resulting error in lunar distance is indeed independent of the lunar distance itself, then the two expressions Frank quotes can not apply: the problem requires some further examination. Frank has said, on the same topic- >generally the required accuracy (for constant altitudes) is proportional to the >sine of the distance which means that for short distance lunars, the >altitudes have to be significantly more accurate. This is yet another >reason why short >distance lunars would not have been popular historically." and- >Usually, it's said that you can have 5 or 6 minute error in the measured >altitude of the Moon or the other body and it will make no difference in the >clearing of the observation. Try it. It's often true. But if you work the math, >you'll find that this is true only "on average". Lunars can be much more >sensitive to altitude accuracy in many cases. What is the geometry which gives rise to such a dramatic increase in sensitivity, I ask? And by how much is the sensitivity then increased? =============== On a rather different matter, Frank added- >Finally, I should note that these >expressions are "slightly approximate" because they assume that refraction is >insignificant. As as long both objects are above 10 degrees or so in altitude, >that's a reasonable assumption. I question that. By my reckoning, changes in refraction with altitude are actually GREATER than changes in parallax with altitude, at altitudes up to about 14 degrees, and don't become "insignificant" until the altitude increases significantly beyond 14 deg. It's often recommended that altitudes above 10 degrees are avoided. But that doesn't imply that refraction is insignificant above 10 deg. Corrections for mean refraction are well understood and tabulated, down to much lower angles. The reason for avoiding angles below 10 deg is because in real-life, atmospheric effects near the horizon can cause the actual refraction to differ significantly from its predicted mean value. George. ================================================================ contact George Huxtable by email at george---.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================