NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: lunars with and without altitudes
From: George Huxtable
Date: 2006 Nov 24, 10:31 -0000
From: George Huxtable
Date: 2006 Nov 24, 10:31 -0000
Frank Reed has offered more suggestions about his proposed method of position finding without a horizon, based on Moon parallax. It's a clever, interesting, and amusing concept, but we would be wise not to take it too far, or too seriously, as a practical tool. He has claimed that a skilled observer can measure lunar distances reliably to 0.1 arc-minutes; claims which have been treated with scepticism by some contributors, me included. Now he has put forward some rather unlikely scenarios, in which his proposed method might come into play. | Suppose you're sailing in the Bermuda race in June, 2007 and you decide to enter | under the celestial navigation rules (apparently you get a 2% time advantage | if you do so). It is not particularly uncommon in the Atlantic between New | England and Bermuda to find that the stars and Sun and Moon are visible but | the horizon is lost in haze (and in addition, this area is prone to extreme, | unpredictable refraction). So shoot a set of lunar distances, and work up your | position from them. Don't worry about the calculational work; the rules | permit computing devices for working any and all sights. You get a true position | fix, and you get bragging rights to spare. As long as the Moon is in the sky, | you can get a line of position and combine it with whatever other positional | information you may have. Well, what precision does Frank expect to achieve, in measuring his lunar distances, not this time from on land, but from a yacht bucketing about in the Gulf Stream? Has he taken lunars under similar conditions, and if so, with what resulting accuracy? | Or perhaps you're playing modern-day Arctic explorer sailing up above the | Arctic Circle next summer, near Spitzbergen perhaps, among ice flows and sea | fog and weird refraction --no place to measure altitudes unless you have no | other option. You were smart enough to bring a spare GPS to replace the primary, | which has failed, but not quite smart enough to bring batteries for the | spare. Luckily, one member of your group is a Navigation List lunarian and has | brought along a sextant. The horizon is a mess, so you shoot a lunar distance | between the Sun and the Moon (at known GMT), wait four hours, and shoot | another. Cross the lines of position and you've got a position fix (accurate to | about +/- 6 nautical miles *if* you're skilled enough with your sextant to | measure angles to +/- 0.1 minutes of arc). [note that you can try this in a | hypothetical case to verify that it works]. Of course, if you have a bubble | sextant, you would almost certainly prefer to use that. Like anything in celestial | navigation, there is a time and a place for every trick. Frank has acknowledged that a high Moon is needed to extract worthwhile accuracy from such a procedure. How high can the Moon be, from Spitzbergen? Once every 18 years (as it happens, this year) the Moon can reach an altitude of 40 degrees from Spitzbergen at some short period in each month. Most of the time, it's going to be much lower than that. So how does measuring a lunar distance angle to 0.1 arc-minutes, if that can be done, result in a position to +/- 6 nautical miles? We keep on asking Frank to provide a proper procedure for reducing such an observation, with a numerical example. He keeps insisting that he has done so, but the limited information that has been supplied is as yet insufficient for his steps to be followed. For example, I have pointed out how celestial positions, calculated from the Nautical Almanac, are insufficiently precise to provide the claimed precision, yet he has still not explained what the observer needs to have aboard to provide what's needed, and how it is to be used. 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. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---