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    Re: 1491 The year China discovered longitude
    From: Richard B. Langley
    Date: 2004 May 8, 11:44 -0300

    Correct title of book is 1421: the Year China Discovered the World (except in
    the U.S. where it was published with the title 1421: the Year China
    Discovered America -- dare I comment on the change of title?).
    Menzies general thesis needs to be taken with a large grain of salt. See,
    e.g., .
    -- Richard Langley
    
    On Sat, 8 May 2004, Trevor J. Kenchington wrote:
    
    >Kieran,
    >
    >I wouldn't want to give any credibility to the claims in Menzies' book
    >(though I haven't read it, so I shouldn't detract from it either) but I
    >don't see the problems with the suggested technique that you do.
    >
    >> "After landing in an unknown territory, Chinese navigators and astronomers
    >> would have been instructed to observe the lunar eclipse
    >
    >
    >This, I would suggest, is both the key element to the method and its
    >greatest weakness. Lunar eclipses are not that frequent so, unless
    >chance brought a landfall close in time to the next eclipse, the
    >hypothetical explorers would need to stop on their new-found land for
    >some extended time. That makes the method not very practical but it also
    >gives ample opportunity to prepare for the great day when the eclipse
    >observation will be made.
    >
    >And let us spare some sympathy for the survey team which, after such
    >long preparation, endured thick cloud cover obscuring their eclipse!
    >
    >> "When the astronomer returned from his voyage, he and his colleagues in
    >> Beijing compared their data. Using their time keeping device, calibrated
    >> from the gnomon, they timed the interval between the transits of the star
    >> observed in the new territory at the time of the eclipse and the star seen
    >> by the astronomers in Beijing at the same moment.
    >
    >
    >That seems a rather unnecessary step. All they needed was a star
    >catalogue, with angular measures equivalent to SHA or Right Ascension --
    >the sort of thing that the astronomers back home should have been
    >working on anyway.
    >
    >> 1)       How did they determine what star was crossing their local meridian
    >> at the time of U3? To do this they would have needed an accurate clock and
    >> done a double altitude shot both ante and post meridian.
    >
    >
    >Not at all. All they needed was a sighting line set up on their meridian
    >-- a low-tech version of a meridian telescope.
    >
    >I don't know how the medieval astronomers prepared such things but one
    >could erect a vertical rod or a standing stone and observe the Sun's
    >shadow, picking two points where (on the same day) that shadow has equal
    >length, and then using terrestrial survey methods to bisect the angle
    >defined by the rod and the two selected end points of the shadow.
    >
    >A nice tall vertical rod and an "observatory" many metres across would
    >allow for quite precise definition of the meridian, particularly since
    >the observer would have many days of observations during which to refine
    >the alignment while waiting for the next eclipse.
    >
    >> 2)       What instrument did they use to make a sufficiently accurate
    >> celestial observation of a star to determine its meridian passage? Certainly
    >> not a sextant! Did they have telescopes to determine the exact moment of U3.
    >> I don't think so.
    >
    >
    >I'd take a guess that the "instrument" was two bronze rods, set firmly
    >in the ground, using a plum bob to ensure that they were vertical. The
    >observer then aligned his eye with the sides of both rods and noted
    >which star lay on that line. He would not have had the advantage of a
    >telescope but astronomers managed without those for millennia.
    >
    >> 3)       Could this observation have been made without a very accurate set
    >> of tables such as a Nautical Almanac?
    >
    >
    >Of course. What possible use would an almanac have been, other than for
    >the prediction of the timing of the eclipse -- which likely was nowhere
    >near precise enough for use in determining longitude?
    >
    >> 4)       What happened if no star was crossing the meridian at the time of
    >> U3 or was so faint that it could not be observed? As suggested above they
    >> may have picked a star and determined the time interval between its meridian
    >> passage and U3.
    >
    >
    >That seems likely, though it would have required them to have a
    >sufficiently-precise measure of time.
    >
    >The other option, on a clear dark night, would be to pick a fainter
    >star. (Given a dark enough night and good enough eyesight, some visible
    >star should be crossing your meridian every minute or so.) That would
    >justify the return to home before determining the SHA of the star in
    >question, though it would mean making a pretty detailed sketch map of
    >just which star was selected, so that it could be uniquely identified later.
    >
    >> 5)       The technique requires knowledge of local magnetic variation
    >
    >
    >Only if one intended to find the meridian using a compass and nobody who
    >understood variation would try that.
    >
    >> I would appreciate any input from list
    >> members because if this assertion is true it requires a complete rewriting
    >> of history.
    >
    >Why so? The history of terrestrial determinations of longitude, perhaps
    >(though as Antonio has pointed out, the method was hardly new in 1491).
    >But just because the Chinese of the 15th century ("of the Common Era" as
    >politically-correct U.S. TV stations insist on noting) could have, or
    >even did, determine longitudes from eclipses would not itself
    >demonstrate that they fixed the locations of anywhere outside the
    >well-known range of Chinese voyaging in the decades before the Ming Ban.
    >That would need a quite different kind of evidence.
    >
    >
    >Were Chinese eclipse predictions at the time accurate enough for people
    >to know which Full Moon would have an eclipse or did our hypothetical
    >explorers have to sit around, month after month, waiting against the day
    >when the Earth's shadow would cross the Moon's disc?
    >
    >
    >Trevor Kenchington
    >
    >
    >--
    >Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    >Gadus Associates,                                 Office(902) 889-9250
    >R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    >Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    >
    >                     Science Serving the Fisheries
    >                      http://home.istar.ca/~gadus
    >
    
    
    ===============================================================================
     Richard B. Langley                            E-mail: lang---.ca
     Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
     Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
     University of New Brunswick                   Fax:      +1 506 453-4943
     Fredericton, N.B., Canada  E3B 5A3
         Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
    ===============================================================================
    
    
    

       
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