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Re: 1491 The year China discovered longitude
From: Ant�nio Canas
Date: 2004 May 8, 11:07 +0100
From: Ant�nio Canas
Date: 2004 May 8, 11:07 +0100
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 > >