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    Ancient Lunar Longitudes?
    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
    
    
    
    
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