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Re: Lunar trouble, need help
From: George Huxtable
Date: 2008 Jul 12, 00:08 +0100
From: George Huxtable
Date: 2008 Jul 12, 00:08 +0100
Kent and I seem to be disagreeing, somewhat, about the principles involved in correcting for refraction, and I had written-. " In practice, in the case we are considering, the difference in refraction between the Moon's centre and upper limb is pretty infinitesimal: no more than 0.4 arc-seconds. Presumably, Kent is keen on such details because he wants to be completely rigorous, and get the principles right. And it seems to me that he hasn't got the principle right here. I will try to convince him. He had written, and my comments are [interpolated]- "Consequently in my way to do the reduction I start with refraction on the UL ..." [all right so far... ] "and then I calculate the difference in refraction for the UL - SD for the moon." [that's the bit that's unnecessary, and indeed wrong]. ======================== Kent has replied- I am still of the opinion that the refraction correction used in altitude reduction must be referred to the geocentre of the moon. I find this correction by calculating the refraction both on the UL/LL and the geocentre, which means as in Jeremy�s case that I add a small value due measurement of the UL. George means that this approach is unnecessary because the correction (in Jeremy�s case) is so small. Even if George has tried to convince me I still keep my opinion." ========================= NO! I do NOT argue that "this approach is unnecessary because the correction (in Jeremy's case) is so small". I argue that it is wrong in principle, no matter how big or small the refraction might be! If the upper limb is being observed, then there is only one light-ray involved, that from the upper limb to the observer, and that is the ray which the atmosphere is refracting, and that is the ray to be corrected for that refraction. The refraction from the centre and from the lower limb play no part, because there's no light-ray from those directions being observed. Kent is welcome to keep his opinion, but unless we can somehow persuade each other into agreement on something so fundamental as this, we will not get any further forward. ======================= As a separate issue, Kent has written- "Furthermore, it seems that we do not yet agree on correction for clearing the LD. The corrections I use are similar to what can be found in Henry Raper, 1840, Table 45. This correction is for small deviations in refraction due to the angle between the distance and vertical (and when reducing altitudes this angle is 0d). Maybe this convince George that my way of treating these corrections are not completely off-road?" Actually, Kent and I agree about that correction, when working a lunar with maximum rigour. My edition of Raper dates from 1864, and in that, it's table 53, "Correction of the lunar distance for the contraction of the vertical semidiameter". Perhaps Kent will confirm that's what he is referring to. That's used, just as it says, to correct the measured distance for the apparent vertical shrinkage of the Moon, and that certainly does depend on the difference in refraction between the Moon's centre and limb. Indeed, I took a look at that table, and noted that for all Moon altitudes above 30�, it would be less that 1", so disregarded it. So in this case, (unlike for the corrections to altitude, above) that correction really was a matter of being right in principle but numerically trivial. But it isn't to be used when correcting for altitudes above the horizon. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---