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Re: October Lunar
From: George Huxtable
Date: 2008 Oct 6, 17:28 +0100
From: George Huxtable
Date: 2008 Oct 6, 17:28 +0100
Jeremy wrote- | I have repeated this experiment the next day with Venus, and then | again today (5 Oct) with the Sun. I am getting a consistant error of | 1 minute on of my averaged sights, so I am guessing I have an | alignment issue with my sextant. Fortunately, since the error is | constant, it is easily compensated for. ================= Presumably, with the second Venus observation, on the next night, the angle being measured on the sextant was significantly greater, by at least 10�. So that's two widely-spaced data points on a "calibration curve" of his sextant, that are both 1' out. And presumably the Sun lunat was well-spaced again, from those two, and was still 1' out (presumably, in the same direction). If all points on a sextant calibration are out by 1', that's not so much an error in the overall scale as an index error of some sort. It could be a built-in error, due to the zero-mark on the sextant's arc being a bit out of place, but that seems a bit unlikely. More likely is some difference between the way the index-error is being assessed, compared with the way the angles (lunar distances) are being measured. And this takes us into rather deeper waters. The discrepancy that Jeremy reports, though consistent, is only a small angle, of 1'. 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. The eye does its best with them, but it's beset with the problem of "irradiation", in which brighter objects such as the Moon disc, against a darker background, appear to be slightly enlarged. And this affects different observers' eyes to a different extent. It may be that after long practice, a lunar observer gets to know just how much overlap (or alternatively, gap) to allow for, to end up with just the right answer. I think this may be what Jeremy was getting at when in [6343] he referred to a "personal observation error". 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. 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 -~----------~----~----~----~------~----~------~--~---