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