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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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