A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Latitude + Longitude @ Noon
From: George Huxtable
Date: 2005 Jun 6, 11:42 +0100
From: George Huxtable
Date: 2005 Jun 6, 11:42 +0100
Henry Halboth wrote- >George, >Forwarded for your further comment is the following posting of 31 Jul >2004, on the subject. I must say that your failure to respond, and for >that matter the failure of this list to do so in general, was most >dissappointing. I try only to post on practical navigation matters that I >have tried or experimented with at sea and can only say that this list >appears disinterested in such matters. Thanks to Henry for giving us a second chance to examine these observations. I'm sorry their value went unrecognised last time, by me and by others. All I can say, in defence, is that it's only after delving into the numbers that one becomes aware that this is a remarkable example of the ultimate accuracy achievable from sextant observations, taken from on-land. It has given me cause to rethink my view of what's possible. Clearly, that posting called for much more than the hasty scan it was given, and should not have been binned. >On Sat, 31 Jul 2004 22:56:10 -0400 "Henry C. Halboth"
>writes: >> I have recently returned from a sojourn at the North Carolina >> Beaches, >> and there had the good fortune of staying literally on the beach, >> with an >> unobstructed view of the sea horizon from almost east to west >> thought >> south. This stay afforded the opportunity for a real "navigation >> holiday" >> - unfortunately, I was plagued with an almost constant "Gulf Stream >> horizon", i.e., hazy to an extent that impacted on the accuracy of >> my >> observed sextant altitudes. Regardless, an effort was made to "try >> out" a >> few of the old favorites sometimes here spoken about. First, let's >> take a >> look at Latitude + Longitude determination at noon by equal >> altitudes - >> actually determination of Longitude by equal altitude + Latitude by >> reduction to the meridian. In this example. a Plath vernier sextant >> was >> used; IC = 0, and height of eye = 20-Ft. >> >> On Tuesday, July 20, 2004 - Chronometer considered accurate > >> 1. For the Longitude >> AM obs @ Chro time = 17-09-30 GMT - Sun's LL @ 75-38-20 >> PM obs @ Chro time = 17-19-00 GMT - Sun's LL @ 75-38-20 >> Mean Chro time of obs = 17-14-15 GMT = time of LAN >> GHA by NA for 17-14-15 GMT = 76-58-12 W = Long @ LAN =============== Let's pause here. I've no doubt that any competent navigator could get a decent value for latitude, but it's the longitude that's of particular interest in the light of recent discussions. Let's concentrate on longitude. I've checked over Henry's working in a different way, and have no cause to question his deduced long. of 76deg 58' 12" W. We need to compare that with his long., taken from a map, which is quoted later as 77deg 00.097', or 77deg 00' 06". So that observation is within a couple of miles of the true longitude. Good going, indeed, particularly for an observation that spanned only 9 min 30 sec of time, and was taken with a Vernier sextant. It could, of course, have been a fluke. When there's a scatter in observations, now and again, just by chance, the occasional result will be spot-on the true value. So we need to check for consistency. Further down the page, we are told- "successive Latitudes obtained on surrounding dates all produced results of 34-38-40 N + Longitude 76-59-00.W." So, on surrounding dates, he has measured longitudes to be within a mile or so of the expected map value. Consistency was achieved. The observation of 20 July 04 wasn't a fluke, then. Henry provides no detailed observations for those "surrounding dates", and it would be interesting to see those details, though I have no reason to doubt his conclusions. ============== Henry doesn't give details of how he made his observation, but it seems likely to me that he used the following simple trick for equal-altitudes, or something like it. Perhaps he will confirm whether or not my guess is right. Having estimated what the maximum altitude of the Sun is going to be at noon, the sextant is deliberately clamped at a value that's roughly a couple of arc-minutes minutes short of that. From that moment, the sextant adjustments remain untouched. Then chronometer times are carefully noted, for the Sun's lower-limb appearing on the horizon, on the way up, and again, after noon, on the way down. No attempt can be made to measure the Sun's maximum altitude, because it would involve disturbing the sextant adjustment to do so. So, for longitude, the observer's task is simply to assess the two moments when the position of the Sun's limb on the horizon is exactly the same. It doesn't matter whether the image is affected by irradiation, it doesn't matter whether there's a bit of overlap, or not, between Sun and horizon, as long as the two pictures look EXACTLY THE SAME the result will be correct. The sextant reading doesn't enter into it, so as long as it's firmly locked during the measurement, a Vernier sextant is just as good for this purpose as a micrometer type. It's a matter of judgment and experience to click the watch at the right moments. It requires memorising exactly what the Sun-on-horizon picture was at the first time-click, and clicking again when it looks identical, after noon. From the accuracy of his resulting longitudes, I reckon that Henry must be capable of estimating the equality of those two pictures to within about 0.1 arc-minute, which is only 1 part in 300 of the Sun's diameter. Quite an achievement... Although he has complaned that "I was plagued with an almost constant "Gulf Stream horizon", i.e., hazy to an extent that impacted on the accuracy of my observed sextant altitudes.", I think his horizons, around those noons, must have been pretty sharp to allow such precision. Henry Halboth has told us before that he was a US Navy navigator in World War 2, which must put him into his eighties now, and he must be pleased, as we are, that his right eye is as clear, and his right arm as steady, and his mind as sharp, as ever. The precision he has been able to achieve has caused me to rethink my own estimates of what's possible from an on-land observation. Quite an eye-opener it has been, indeed. Although the accuracy of the sextant reading is irrelevant to the longitude determination, it's important for latitude. Because the peak altitude wasn't measured, latitude had to be calculated using the before-and-after observations, and Henry refers to Bowditch tables 29 and 30, designed for the purpose, for that small adjustment; a valid and accurate procedure. ============== But let's not get too carried away by all this. These measurements were made from on-land. As I wrote, in response to a posting by Lu Abel, "It's quite a lot harder, and less accurate, when you observe a Sun altitude in real-life, at sea, above a real sea-horizon." And I'm sure that Henry Halboth is very aware of that difference. I ask him to estimate, from his own considerable experience, how much his estimate of longitude would be degraded if it was measured from a small craft at sea, wedged against the mast, the vessel heaving up and down on the waves, the horizon-line made up of overlapping wavetops, the sextant needing to be tweaked about to keep the images in view, and perhaps a telescope with less magnification. Not storm conditions, just the ordinary seas that any offshore navigator expects to contend with. As for me, I would expect, under those circumstances, that instead of comparing two altitudes, before and after noon, to an accuracy of say 0.1 arc-minutes, I would find it difficult to claim to detect equality to within 2 or 3 arc-minutes from my own small boat. The time-span of the observation would need to be extended accordingly, along the lines suggested by Frank Reed. So, what's realistic, from a small craft at sea rather than from a hotel-room balcony? That's the question I ask of Henry. =============== Finally, a small detail. Henry acknowledges that measurements taken from a moving vessel would require correction for the Northerly component of velocity; a correction that doesn't apply to his observations from a fixed point. But he has neglected another correction, due not to the motion of the observer, but to the change in declination of the Sun, which is moving Southward, away from him, at about 0.4 knots at that date. That motion causes his centre-of-symmetry to be slightly earlier than the true moment of meridian passage, and by my reckoning the required correction would shift the position, calculated from his observations, a couple of miles further West (if I've got it right). See my recent posting "Northing correction to Noon longitudes". This would put his deduced longitudes even closer to the mapped values! George. ================================================================ contact George Huxtable by email at firstname.lastname@example.org, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================