NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: time of noon from the Sun.
From: George Huxtable
Date: 2006 Nov 24, 19:59 -0000
From: George Huxtable
Date: 2006 Nov 24, 19:59 -0000
Ken Muldrew wrote, (in [NavList 1739] Re: time of noon from the Sun) about using time from a pocket watch that had been set approximately from the Sun at noon, and not yet corrected by a later time-sight- | Let's agree that if you're using a mechanical pocket watch to navigate, | then you're also stuck somewhere about 200 years ago. I'm puzzled why Ken says that. Even navigating with a chronometer, you would still need some measure of local time, whether it's used to set a pocket watch or to compare with a chronometer reading, to get longitude. And the worst moment of the day to make that measurement is the moment of noon. Only if it's widely bracketed on either side of noon, and suitable corrections are made for North-South speed and declination change, will that setting of noon, at noon, be accurate. | This means that you | are not doing position line navigation but independent latitude and | longitude measurements. Agreed. | Since the noon sight has given you a good | latitude, and since the best you can hope for with longitude will come | from a lunar distance (at least as far as celestial observations go), | which is pretty innaccurate even with good sights, then I would say that I | would be perfectly happy to navigate using the watch that shows only | approximate time. | | My watch is showing local apparent time, so it is being used to measure | intervals for updating the dead reckoning; I wouldn't be trying to infer | longitude from the time. But if you HAVE taken a lunar distance, inaccurate though that may be in itself, to get Greenwich time to within a minute or two , then it's necessary to use a measure of local apparent time to infer the longitude. And even if you have a precise chronometer, giving you Greenwich time directly, again, you need local time for longitude. That's the demanding application for which a rather precise determination of local time (within a minute or so) is needed. For other purposes, such as ringing the ship's bell to end a watch, precise time isn't needed, of course. | If land is nearby, then surely I would be | navigating by latitude + observation + soundings and not by anything that | requires knowing the precise local apparent time. I'm not sure what I | might miss by having a watch that is, say, 10 minutes off. Well, if you don't care about the longitude, then of course, you don't need precise local time. But why would you wish to know latitude and not wish to know about longitude, with land nearby? If you were navigating with a watch that was 10 minutes off from local time, it would mean that any lunar-distance or chronometer determination of longitude would, quite unnecessarily, have incurred an error of 10 minutes of time, or 2.5 degrees of longitude, just from that error in the clock setting. That would far outweigh the errors in any lunar-distance measurement itself, and would ruin it; just as it would ruin a longitude-by-chronometer. For measurement of longitude, reasonably-precise local time is essential. 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 -~----------~----~----~----~------~----~------~--~---