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Just try to read something from Claudius Ptolemy who lived from 85 to 165 a
few centuries before "The year China discovered longitude"

You can find that he explains how to use eclipses to find the longitude of
places

Antonio Canas - Portugal



----- Original Message -----
From: "Kieran Kelly" 
To: 
Sent: Saturday, May 08, 2004 5:17 AM
Subject: 1491 The year China discovered longitude


> -----Original Message-----
> From: Kieran Kelly [mailto:kkelly@bigpond.net.au]
> Sent: Saturday, 8 May 2004 2:12 PM
> To: Kieran Kelly
> Subject: 1491 The year China discovered longitude
>
> I have recently completed reading Gavin Menzies controversial "1421 The
Year
> China discovered the World". The book makes many extraordinary claims
> including an assertion that Chinese mariners discovered how to calculate
> longitude at sea more than 300 years before their colleagues in Europe.
The
> technique using lunar eclipses is as follows and is reproduced verbatim
from
> the book:
>
> "Solar and lunar eclipses occur when the sun, moon and earth are in line
> with one another and when the moon's orbit around the earth is in the same
> plane as the earth's orbit around the sun. In a solar eclipse, the moon's
> shadow blots out the sun over a small portion of the earth and it becomes
> night for a very short period. The spot of darkness, the umbra, travels
> across the earth as the moon rotates around the earth, and the earth
itself
> rotates."
>
> "Observers in different locations see the solar eclipse at different
times.
> In a lunar eclipse, the earth is between sun and moon, and because the
earth
> is so much bigger than the moon, its shadow obscures the moon. The great
> difference for astronomical observations is that observers may see the
event
> simultaneously across half the earth, whereas in a solar eclipse the event
> occurs only above a very small part of the earth at any one time. The
> ability to time a lunar eclipse with absolute precision and the fact that
> the same event could be seen simultaneously from different parts of the
> globe were to prove the vital steps in Chinese attempts to find a method
of
> calculating longitude."
>
> "The keys to using a lunar eclipse to determine longitude are, first that
> the event is seen across half the world simultaneously, and secondly,
while
> the eclipse is taking place, the earth's rotation makes the stars appear
to
> move across the sky.  There are distinguishable events during an eclipse:
> U1 - first contact, when the moon enters the dark umbral shadow; U2 -
second
> contact, when the moon has just fully entered the umbra and is totally
> covered; U3 - third contact, when the moon first starts to emerge; and
U4 -
> fourth contact, when the moon has just fully emerged. The Chinese
> concentrated on U3 and used it as the basis of their calculations.
>
> "After landing in an unknown territory, Chinese navigators and astronomers
> would have been instructed to observe the lunar eclipse, wait until the
> moment when the third event (U3) occurred, then determine what star was
just
> crossing the local meridian in the night sky. The local meridian was the
> imaginary longitudinal line, starting on the horizon directly north of the
> observer, passing over his head and ending at the horizon due south of
him.
> The known star crossing that line at the time of the third event of the
> eclipse was the key sighting for the observers in the new territory, and
for
> those back in Beijing.
>
> "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. The earth rotates 360?
in
> twenty-four hours. If the elapsed time between the two transits was six
> hours, a quarter of the time it takes the earth to rotate, the difference
in
> longitude between Beijing and the new territory would be a quarter of the
> total longitude around the world - 90? - one quarter of 360?. Errors could
> be reduced by timing each of the four events of the eclipse, U1, U2, U3
and
> U4 then averaging the results. By observing the same event at different
> locations around the globe and fixing the exact time at which this event
> took place, the Chinese could then compare their results. By determining
the
> differences in the time when the event took place, as observed from the
> separate locations they could then calculate the difference in longitude."
>
> Ummm. I think this is a load of old cobblers for the following reasons:
> 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. The author
> suggested they used a clepsydras (water clock). Would this have been
> accurate enough? Simply recording maximum altitude would not have told
them
> the time of meridian passage.
>
> As an experiment I went outside with a compass and tried to visually
> ascertain true North and which star was crossing my local meridian at a
> point in time. Impossible.
>
> One technique they could have used was to pick a particular star and
observe
> its meridian passage (with an unknown instrument) and determine the
elapsed
> time either before or after the U3 phase of the lunar eclipse. The time
> before or after the eclipse could then be compared to that back in Beijing
> at the end of the voyage. But what does that tell you? Nothing I think.
>
> 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.
>
> 3)       Could this observation have been made without a very accurate set
> of tables such as a Nautical Almanac?
>
> 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.
>
> 5)       The technique requires knowledge of local magnetic variation i.e.
> the observer is trying to find out when the star crosses his local true
> meridian. The Chinese knew the difference between magnetic north and true
> north by reference to Polaris visible at Beijing. Not so once the sailed
> down into the southern latitudes. Something like a shadow stick is a
> possibility I suppose.
>
> The technique described in the book, as I understand it, would give the
> Local Apparent Time of a Lunar Eclipse in a distant part of the world
which,
> some time later, could be compared to the Local Apparent Time of a Lunar
> Eclipse in Beijing on the same day. I would appreciate any input from list
> members because if this assertion is true it requires a complete rewriting
> of history.
>
> Kieran Kelly
> Sydney
> Australia
>
>

   

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