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From: Kieran Kelly (no email)
Date: Sat May 08 2004 - 00:17:17 EDT
-----Original Message-----
From: Kieran Kelly [mailto:]
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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