NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunars for dummies like me
From: George Huxtable
Date: 2004 Sep 24, 00:00 +0100
From: George Huxtable
Date: 2004 Sep 24, 00:00 +0100
Jim Thompson wrote- >I am still crawling up the lunar distance learning curve. > >I understand most of the issues around how navigators estimated their >longitude using GMT from a lunar distance sight, and latitude from a noon >sun sight. But I am stupidly stuck on step 5. Did navigators of the day >set a ship's clock to GAT obtained from the lunar distance, and then >therefore know the difference in time between when they took their lunar >observation of GAT, and when they took their morning or afternoon time sight >of the sun? Or did they do this without any ship's timepiece at all? Jim seems to be getting up that curve rather well. To be honest, I don't know the answer to Jim's reasonable question. I think it might have changed somewhat as the years went by. Once a clock that could be used at sea (but not yet any sort of chronometer) had evolved, it would be useful for establishing the local apparent time (for setting the sequence of the four-hour watches, for example). No great accuracy was called for. It would bridge the intervals between sunny days, when a time-sight would be used to set it right. In those cloudy periods, clock adjustments might be made, perhaps each day, to allow for estimated DR Easting or Westing. If the ship's navigator didn't concern himself with longitude by lunar distances (and many vessels still relied on latitude sailing well into the 1800s) then the ship's clock didn't matter much. Jim asks whether they could have assessed their longitudes from lunars without any timepiece at all, and the answer is (in certain circumstances) yes. The special cirumstances were a Sun-Moon lunar, taken in the morning or afternoon at a time when the Sun's altitude would provide a useful time-sight. Then, because Local apparent Time was determined from the Sun time-sight, at the same moment that Greenwich Apparent Time was determined from the lunar distance, there was no need for an on-board clock or deck-watch to determine the time-interval between them. In general, however, (and especially using star lunars at night) the measurements of Local Apparent Time and Greenwich Apparent Time were separate events, measured at different times. To determine longitude from the difference between them, first they have to be put on the same time-scale. To do this, it's necessary to know the time-interval between them, and use it to advance or retard the Local Apparent Time from the time-sight, to become the LAT at the moment of the lunar. That's where the ship's timepiece comes in. And because it's only a time INTERVAL that's required, it doesn't matter a damn whether the clock that's used is following local time or Greenwich Time. All that's needed is (ideally) some knowledge of its rate (of gaining or losing, in seconds per day, with respect to GMT), and to be sure no adjustment has been made to it in the interval. No great accuracy is called for here. If the interval could be relied on to say half-a-minute of time, that would contribute no more than 7.5 arc-minutes to the resulting longitude: acceptable in a lunar context, which has low accuracy anyway. That half-minute error should be maintained over the interval between the two observations, which will normally be less than 12 hours or so, unless a long period of cloudy weather intervenes. So it's not a demanding requirement. Of course, if a proper chronometer was on board, that would be used in preference. I have omitted a small complication above. A clock can follow Mean Time, but not Apparent Time, which is irregular. To follow Apparent Time requires regular resetting to allow for the changing Equation of Time. So a clock's rate of gaining/losing on Apparent Time is deduced from its known rate on Mean Time after adjusting by a small correction that varies in a known way over the year. =================== >For example, is this correct?: > >1. Navigator took a lunar distance observation on the evening of day 1, >yielding GAT for that point in time and space. >2. Navigator continued careful dead reckoning. >3. Navigator determined latitude at LAN from a noon sun sight on day 2. >4. Navigator took a time sight of the sun late in the afternoon of day 2, >when the sun was nearly due west, yielding LHA of the sun based on careful >dead reckoning for latitude, and therefore obtained LAT. >5. Navigator advanced his estimate of GAT from the lunar sight to the >afternoon sight, using a local timepiece set to GAT at the time of the lunar >sight the day before, and then determined longtiude using the formula GAT = >LAT +WLo or -ELo. ================= I think this is fine until you get to 5. You can advance/retard either observation, by the time difference between them shown by the clock, to put it into step with the other, and then the resulting longitude will be the position at the moment of the "other". And there's no requirement for the local timepiece to have been "set to GAT", as long as its difference from GAT was established, the day before. I hope I've got that right. I still find that time is very slithery stuff; hard to get a firm grip on. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================