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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Star - Star Distances
From: George Huxtable
Date: 2010 Apr 27, 09:20 +0100
From: George Huxtable
Date: 2010 Apr 27, 09:20 +0100
Richard Reed wrote, about correcting star-star distances- "John Karl's book has some more pairs than Bauer (12 I think, upto 113 degrees), with a refraction table for each, but no aberration tables." ============ A bit of care is needed, for those following that route, if using the first printing (2007) of John Karl's book "Celestial Navigation in the GPS age". Significant changes were made in that respect in the second printing of 2009 (which is still labelled as first edition). In that first printing, a wrong assumtion was made. Taking an observer at a known latitude and a star-pair A and B, it was assumed that if star A had a known altitude, then that would also define the altitude of star B at the same moment. The problem with that is that there are two distinct times of any night when star A has that altitude. These times are equally before and after the moment of culmination: when it's rising, somewhere to the East of the observer, and when it's falling, somewhere to his West. And, at those two times, star B will have two very different altitudes, so the assumption in those tables breaks down. As I understand it, John's first refraction correction table considered (without saying so) only the case when star A would be rising, so it could be used only when that star was somewhere to the East, not the West, of the observer. Then, it would give the right answers. John's later version, in the second printing, handles refraction quite differently, and gets around that problem. However, that leaves the question of star aberration, which can shift the angle between star-pairs significantly, by up to half an arc-minute, or so, either way, for the most-distant pairs, depending on the geometry of the observation and the time of year. John, in the first printing of his book, just accepted the value of aberration at one particular moment in the year (New Year), which meant that for some star-pairs the worst-case aberration could be doubled. More recently, he has changed this to ignore star aberration altogether: a better solution, which limits that worst-case error. John accepts, in his explanation, that for high-precision work, it's necessary to take another approach which allows for aberration, as Brad and Richard seem to be doing. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "Richard Reed"To: Sent: Monday, April 26, 2010 9:53 PM Subject: [NavList] Re: Star - Star Distances Thanks for all that work! John Karl's book has some more pairs than Bauer (12 I think, upto 113 degrees), with a refraction table for each, but no aberration tables. For just one pair so far (I really need a tripod!), I went the long way and got aberration-corrected Right Ascension and Declination from Solex 10.2. I then used Frank Reed's calculations here: http://www.fer3.com/arc/m2.aspx?i=015098&y=200404 Using Frank's distance formula on Regulus-Betelgeuse, 62 deg 26.6' in Karl, without refraction, I got 62 deg 26.94' from both the Solex data and the US Naval Observatory data. I then used the Bennett refraction formula as modified by George Huxtable and used Frank's refraction distance correction. I was quite happy to be within 0.2 minutes that time, but it's only one shot. Richard Reed ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ----------------------------------------------------------------