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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@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
>


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 Richard B. Langley                            E-mail: lang@unb.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
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     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
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