NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Ancient Lunar Longitudes?
From: Frank Reed
Date: 2009 May 14, 09:04 -0700
From: Frank Reed
Date: 2009 May 14, 09:04 -0700
Probably too late to help much, I have changed the thread title. Marcel, you wrote: "Regarding the logistics problem: Having seen on TV a "documentary" on the various expeditions which were undertaken to observe the Venus/Sun transits of 1761 and 1769 at various places of the earth let me question whether their would have been much earlier an easy solution to "think of a way around that"." Right, but the whole idea that the Piri Reis map represents some ancient seafaring culture (worth considering, even if it's not likely) depends on some lost ability to sail great distances. One of the key technologies that enabled those voyages by Cook and others in the 18th century was just good diet. Scurvy was as much of a problem for those early northern European voyagers as uncertain longitude, maybe more so. Now imagine an ancient seafaring culture that has by chance avoided the problem of scurvy. Maybe they eat lots of oranges on a regular basis. Such a simple thing, not at all improbable, would have changed everyting. It is at least conceivable that they might have undertaken planned, intentional, long-distance voyages with some degree of success even three or four thousand years ago. So who knows... maybe that logistical problem is just a matter of funding! The Britain that sent Cook on his journeys was an impoverished place in many ways when compared to the ancient and classical civilizations of the Mediterranean. Anyway, back to NavList's main topic. How might they have navigated? And could they have used the Moon for longitude? You wrote: "If the Saros cycle was known, it could possibly also be used for predictive calculations such as for setting up a sort of ephemerides." The Saros cycle was definitely known, and it is a remarkable testament to the state power that supported such studies millennia ago. While the Saros cycle helps a great deal, especially when predicting eclipses, it doesn't tell us anywhere near enough to be able to use the Moon's altitude for longitude. But predicting eclipses is certainly an "enabling technology" for the Ptolemaic plan of observing a lunar eclipse from scattered points around the globe. It could be done! Yesterday or the day before, I posted something that was incorrect. I said that in the Roman period, they could predict the position of the Moon within ten minutes of arc. That was too generous for predictions by a factor of six or so. They could predict the Moon's position within a degree or so. Now since the Moon takes two hours to move a degree relative to the Sun and other celestial objects, a one degree error in the Moon's position means a two hour error in absolute time or a thirty degree error in longitude. So the available prediction algorithms would have been essentially useless for longitude. And yet even without a proper model of the Moon's motion, if we simply have an observatory --well-funded, well-staffed-- recording the Moon's position every hour of every day (while it's above the horizon naturally) then we can compare our position information with similar position information gathered at distant locations. And in that case, they could have probably counted on accuracies of about six to ten minutes of arc in the net comparison, which corresponds to 3 to 5 degrees error in longitude, and that wouldn't be entirely useless. You wrote: "if we haven't found such a document doesn't mean that it didn't exist." Without a doubt. I would say that it's considered "canonical" that huge quantities of astronomical documents existed two thousand years ago which have now been lost. We need a time machine and a library card to the library at Alexandria. They're in there, for sure. :-) And: "Don't misunderstand me. I don't want to "proof" here something. It just was an effort to sketch how a possible method might have been for measuring longitude without the means which became available later." Yes, I understand you. No problem. Neither the lunar distance method for finding longitude nor the method of finding longitude by lunar altitudes (closely allied) required any great leap in science to understand. The concept is simple enough, and Ptolemy et al. would surely have recognized it as an extension of the eclipse method. There's no "rocket science" in it. But again, it all hinges on tables of the Moon's position much more accurate than those known to have existed, OR it requires one or more serious observatories making continuous observations of the Moon's position, and it requires an instrument like the modern sextant. These things could have existed and been washed away. That's certainly possible. From my perspective, if they did exist, I would expect more evidence of their existence... not just a single map with a vague coastline of South America, but other accurate maps of points with much greater importance nearer to home, wherever that might have been. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---