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    1491 The year China discovered longitude
    From: Kieran Kelly
    Date: 2004 May 8, 14:17 +1000

    -----Original Message-----
    From: Kieran Kelly [mailto:kkelly---.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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