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Re: 1491 The year China discovered longitude
From: George Huxtable
Date: 2004 May 8, 16:01 +0100
From: George Huxtable
Date: 2004 May 8, 16:01 +0100
1491 The year China discovered longitude. I agree with Kieran in being suspicious of such claims about early Chinese navigation, although I have not read the book in question and know little about Chinese navigation and astronomy. Being in general a sceptic about almost everything, I have some diagreements both with Menzies and with Kieran. Kieran quotes from Menzies- >"Solar and lunar eclipses occur when the sun, moon and earth are in line >with one another..." All right so far >...and when the moon's orbit around the earth is in the same >plane as the earth's orbit around the sun." The Moon's orbit round the Earth is never, ever, in the same plane as the Earth's orbit around the Sun. There is a rather-constant tilt between them of about 5 degrees. What Menzies should have said was that when the Earth-Moon line crosses the plane of the Earth's orbit round the Sun, that's when a lunar eclipse can occur (if the Sun is at the right place then). Further on Menzies states- "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." There is no way to time a lunar eclipse with "absolute precision", as anyone who has watched one will know. The whole event is fuzzy and indefinite. If the Sun was a point-source of light, then the edge of the Earth would show a sharp shadow, which we could time as it crossed the centre of the Moon's disc, or as it covered the Noon entirely. Then (with a timekeeper of some sort) the re-emergence could be timed in the same way, and the midpoint between them would give what was wanted. Because the Sun subtends half-a-degree, however, the Earth's shadow has a VERY indefinite extent (the penumbra); the moment of the Moon entering completely into full shadow is indeterminate, and the moment of mid-eclipse can be estimated only within 5 or 10 minutes by human eye. It's made rather worse because even in full umbra, light escapes round the Earth's disc through refraction in its atmosphere, so the Moon never gets quite dark. Kieran should remember that what was proposed in Menzies' text was not longitudec at sea for navigation, but longitude on land for geographers, by lunar eclipses. This was first proposed by Hipparchus about 160 BC, and used by the Greeks for determining some longitudes, so the Chinese were not 300 years ahead of Europe, but 1700 years behind. Ptolemy's famous map of 2nd century AD relied, to some extent, on longitudes determined that way. It's true, however, that the displacement of scientific learning by religion had then set such European knowledge back to near-zero in the period Menzies refers to. But even in that period the Portuguese and Spanish explorers were starting to try out eclipse observations for longitude. Even that navigational incompetent Columbus was doing so, though making his usual hash of it. Once on land, it's easy to establish the direction of true North from a few star-azimuth observations from the shore, then knock in a couple of posts to mark a meridian line, and observe what stars were crossing that line at the start, and end, of full umbral eclipse. Without decent predictions of the Moon's position, which were not available in Europe until the 18th century, Moon eclipse observations would have to be referred back to records of simultaneous Moon measurements made back at the base city (Cadiz or Beijing) before a longitude difference could be deduced. That's one reason why they were useful for exploring geographers and chart makers, but of little direct use for navigation. Menzies tells us that the key event that had to be timed (with a star), was what he calls U3, at the first signs of emergence from full umbra. This seems surprising, as U2, the entry into full umbra, would have been equally useful, and the combination of the two more useful still. Menzies' account makes me wonder if there is some misunderstanding on his part, in that of a translator, in that of the Chinese scribes that wrote the account, or in that of the Chinese astronomers who developed the technique. Indeed, later, Menzies contradicts the earlier statements, in pointing to the advantages of averaging four eclipse-moments. ====================== Kieran says- >Ummm. I think this is a load of old cobblers for the following reasons: Not complete "old cobblers", I suggest, but I think there's quite a bit of cobbling in it. >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. Not a problem from an on-land observation, given a night's star-observing to set up a true North-South line. >The author >suggested they used a clepsydras (water clock). Would this have been >accurate enough? Yes, I think so, on land. >Simply recording maximum altitude would not have told them >the time of meridian passage. True but irrelevant. >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. But not if you had marked a meridian line with posts beforehand. >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. If that could be done, it would provide enough information for longitude to be deduced, back in Beijing. >2) What instrument did they use to make a sufficiently accurate >celestial observation of a star to determine its meridian passage? Two posts in the ground: see above. >Certainly >not a sextant! Did they have telescopes to determine the exact moment of U3. >I don't think so. I don't think they did. As explained above, the different moments within a Lunar eclipse are so fuzzy and ill-defined that a telescope would be no better than the naked eye. A modern photometer would have been extremely useful, however! >3) Could this observation have been made without a very accurate set >of tables such as a Nautical Almanac? Yes, the observation could, and then compared with observations back home. But it would have been useful to know at which full Moon a lunar eclipse was likely, and that doesn't need precise positional prediction. The explorer would be aware that an eclipse happens (if it does happen) at the very moment of full Moon, so he could work out which night to keep his eyes open for it. >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. Well, the Chinese were certainly an inventive lot. They could have made a sketch of a constellation which spanned the meridian line, indicating just where that line crossed it. > >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. Kieran keeps placing the observer on board ship, but I don't think any claim is being made that this technique was for use at sea. The magnetic compass isn't involved. >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. That assertion is quite true and has been known for over 2000 years. The difference in longitude between two locations can be defined as the difference in Local Apparent Times of a simultaneous event observed in both. No rewriting of history is called for. =================== Note that one reason why lunar eclipses are so little use to mariners is that they occur so infrequently, and presumably, by Sod's Law, never when needed. From what I have read (with some scepticism) about Chinese exploration, their travels, around South-East Asia, were rather leisurely ones. The geography of the area implied that theirs were mainly coasting passages, not great ocean travels. Here and there they would stop, to plant, grow, and reap a crop of rice to keep them going for the next stage. So there was plenty of time on-land to observe lunar eclipses. Note also that in the passage quoted, Menzies makes no claim for what accuracy the Chinese obtained for their longitudes, except for his unjustified statements about the precise timing of an eclipse. The eclipse method required none of the extreme precision demanded by lunar distances, and I suggest that a careful observer might have managed to get longitudes to a couple of degrees. It all depended on how well he could time that fuzzy eclipse. Thanks to Kieran for introducing an interesting topic, likely to bring out other views than mine. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================