NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: October Lunar
From: George Huxtable
Date: 2008 Oct 13, 00:24 +0100
From: George Huxtable
Date: 2008 Oct 13, 00:24 +0100
Frank Reed found some points to agree with in my comments on Jeremy's lunar, bur argued with others. Some may be worth pursuing. I had written- "And this takes us into rather deeper waters. The discrepancy that Jeremy reports, though consistent, is only a small angle, of 1'." To which he replied- "Through a sight tube or a low-power scope, that would be a fair statement, but with a 7x scope, which Jeremy has, an angular difference of a minute of arc is plainly visible. " Well, it's still only a small angle of 1', that discrepancy, just as I wrote. Indeed, such an angle may well be plainly visible in a 7x scope, but the aim is not just to see it, but to measure it, systematically. And it's near, but not (I agree) at, the limits of such measurement. Jeremy appears able to produce repeatability of better than that amount, but something appears to be failing in a systematic way. To my comment- "To measure a planet against the Moon's limb calls for a bit of judgment, because the image of the star is not a precise point, and the image of the limb is not an infinitely-sharp boundary." Reply- "Yes, and because the plains and mountain ranges on the Moon vary the outline (at average sextant scope resolutions) by anywhere from one to two seconds of arc, that is the absolute limit on any historical lunars (a modern computation could include limb effects)." Errors of a second or two are quite irrelevant to the present discussion; a red-herring.. I wrote- The eye does its best with them, but it's beset with the problem of "irradiation" " Frank- "This is irrelevant to this Moon-Venus lunar. Except under extreme circumstances, irradiation is a minor issue." That's an assertion; can Frank back it up? And is irradiation irrelevant when assessing index-error using the Sun, the measurement that may be at the root of the problem we're discussing? Irradiation is a real effect. It's been dropped from upper-limb predictions, in the Almanac, not because it's negligible, or unreal, but because it's so variable between different observers. To my conclusion- "Whenever we near the limits of what we can perceive, some such degree of personal judgment comes in. It sets a limit to the inherent accuracy available in an observation, and being systematic rather than random, repetition and averaging aren't going to help." Frank responded- "That's a good point in general terms. It's fine rhetoric. But it does not apply here." Dismissing it as "fine rhetoric", and claiming, on dubious grounds that it doesn't apply, don't allow that matter to be avoided, particularly the question of systematic error, which I will return to later. Frank continued "The "limits" of what we can perceive are well-known. The resolution of the human visual system is about one minute of arc for standard optical resolution tests at unit magnification (with corrective lenses or adjusted focus as necessary --in other words, when wearing eyeglasses or contacts-- and assuming no exotic uncorrected eye defects)." That's a gross over-simplification: that everyone's eye is the same. There's a wide spectrum of visual perception. I would put, at one end of the scale, the astronomer Hevelius, from Danzig, in the late 1600s, who amazed his contemporaries by the precision of his star catalogue, obtained without use of the new-fangled telescopes, or Copernicus' mother, who, shown Venus through such a telescope, asked why it was upside-down. Out of contention, at the other end of the scale, is my own eyesight, raddled by age and retinal lasering. All we can say about the eyesight of the observer in question is that it's good enough for a watchkeeper's certificate (which must mean reasonably good). But we're discussing, not perception (whether or not you can perceive something) but whether you can measure it, and that demands judgment, in placing one image with respect to another, if they are not completely sharp (and no optical system is completely sharp). Frank continues, in familiar vein- "It has been my experience that observers with well-adjusted sextants equipped with 7x scopes, with reasonably good conditions (on land, no clouds, no haze) can observe lunar distances with an accuracy of about 0.25 minutes of arc (standard deviation) on each individual sight, and when four are averaged, the results are generally twice as good (0.13' s.d.). These are typical results for Sun and planet lunars. My results with stars have been worse and seemingly directly dependent on the magnitude of the star. Your mileage may vary." These are claims we have heard from Frank before, is just such anecdotal terms. Who ARE these observers, then? This question has been asked again and again on this list. Nobody, other than Frank Reed, has put his hand up and claimed such ability. As for averaging four observations, and getting twice-as-good results, that only applies (as Frank must know well) when any errors are random ones, varying unpredictably from one observation to another, and when all systematic errors are exactly known and corrected for, to an extent that makes their sum negligible compared with 0.125 arc minutes.. George. contact George Huxtable, now at george@hux.me.uk (switched from 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 -~----------~----~----~----~------~----~------~--~---