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Re: Round-the-globe almanac
From: Ron Roizen
Date: 2004 Apr 1, 13:28 -0800
From: Ron Roizen
Date: 2004 Apr 1, 13:28 -0800
The two responses to my question are both wonderfully edifying and kind. Thank you very much! Ron Roizen P.S. The eclipse could be seen wherever the moon could be seen -- of course! What a dunce I am! Also, it was Columbus's fourth voyage, not his third, as I said. By the way, my other favorite piece of Columbus trickery was the way he kept two logs of distance covered, one true and the other to encourage his crew that good speed was being made. Ironically, the crew-bucking-up log turned out to be closer to the truth than the real one! -- or so one of my authors reported. ---------- From: George HuxtableTo: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM Subject: Re: Round-the-globe almanac Date: Thursday, April 01, 2004 12:50 PM Ron Roizen asked- > >If I remember this correctly, Columbus and most of his crew became marooned >for months on Jamaica on the great explorer's third voyage. Before long, >all were starving. Columbus, as the story goes, had an almanac with him. >He used it one night, with local indians present, to trick the indians into >bringing food to his men. He told them that if they did not provision his >men, then he would take away the moon -- using, as the story goes, the >precious almanac to predict the eclipse's time of occurrence. The eclipse >happened on schedule and the terrified indians did indeed begin supplying >food. > >Question: Is this story plausible? Would an almanac constructed in a time >when the true diameter of the globe was imperfectly known have allowed >Columbus to bring off this clever stunt? ============================ >From George- Eminently plausible. Unlike an eclipse of the Sun, which occurs over a small part of the Earth, an eclipse of the Moon occurs ON the Moon, so it can be seen, at the same momemt, from anywhere on Earth where the Moon is above the horizon. For this reason, it's a fair way to establish longitude, if you can compare the local time of its occurrence with the local time of the observatory at which it was predicted. Only a fair way, not a good way, because the shadowing is a rather gradual process, which it's impossible to time precisely. The best account, in my view, is that of Samuel E Morison, "Admiral of the Ocean Sea", (Little, Brown, 1942). He says (page 653) - "Among the few books on board ship was a Regiomontanus "Ephemerides", printed at Nuremberg before the end of the century, but containing predictions of eclipses for 30 years ahead. In three days time, on the night of February 29, 150, Regiomontanus predicted a total eclipse of the Moon". Morison describes the dire warnings about the Moon that Columbus gave the local Indians, and translates the account as follows- "...the eclipse beginning at the rising of the Moon, and augmenting as she ascended...", and describes the resulting panic that ensued. At this time Columbus retired to his cabin... "And when the Admiral observed that the totality of the eclipse was finished, and the Moon would soon shine forth, he issued from his cabin..." Columbus then told them a load of stories about how they had been pardoned, as long as they provided a steady food supply (which they did). Morison goes on to say- When he retired to his cabin the Admiral was not putting in his time praying, but measuring with his ampollieta or half-hour glass the duration of the eclipse, in order that he might compute the longitude of Jamaica. The result is recorded in his Book of Prophesies- "Thursday 29 February 1504, I being in the Indies on the Island of Jamaica in the harbour called Santa Gloria which is almost in the middle of the island on the North side, there was an eclipse of the Moon, and as the beginning theof was before the Sun set, I could only note the end of it, when the moon had just returned to its light, , and this was certainly two hours and a half after the night [fell], five ampollietas most certainly. The difference between the middle of the island of Jamaica and the island of Cadiz in Spain is seven hours and fifteen minutes, so that in Cadiz the Sun sets seven hours and fifteen minutes earlier than in Jamaica (see almanac)." That last sentence is, of course, complete nonsense, as Columbus should have been able to deduce from the timing of that eclipse [and others, earlier]. The difference between the longitudes of Cadiz and mid-Jamaica is only 70deg or so, or 4hours 20min in time. I don't know what that Regiomontanus ephemeris predicts as the time of that eclipse. It would be interesting to find out, if some listmember has access to it. Lunar theory was highly inaccurate then, so predictions several years ahead would not be precise. But if he had been a good geographer, Columbus would have measured the time-of-day of that eclipse as well is he could (with his sandglass) and brought the information back to Cadiz, where it could have been compared with an actual time-observation there, on-land, of that same eclipse. Doing things in that way, errors in the prediction were immaterial. If anyone has a computer program that can go back that far in time, it would be interesting to learn the actual local time of sunset in Jamaica on the evening of 29 Feb 1504, and the intervals between start and end of the eclipse. It may be worth pointing out that Columbus will have been using the Julian calendar, and if your computer insists on using the Gregorian calendar, the date will be 10 days later, on 10 March 1504. "Columbus and the age of discovery". by Zvi Dor-Ner, (Harper Collins 1991) is one of those books related to a TV series, with all the advantages and drawbacks of its kind. It shows on page 295 a useful picture taken from a "contemporay manual of astronomy", credited as follows- "Joannes Regiomantanus, calendrium, Venice, 1507, by permission of the Houghton Library, Harvard University." The extract shows a fully-eclipsed Moon, captioned as follows- 1504 Eclipsis Lunae 29 13 26 Februarii Dimidia duratio 1 46 which I take to mean "February 29 1504, lunar eclipse [centred on?] a local apparent time of 13h 26m [astronomical time; i.e.1.26 am on the following morning?], duration 1h 46m. But if, according to the credit, that was published in Venice in 1507, it was not a prediction but was an after-the-event record, and can not have been the document that Columbus took to sea with him. Another question that arises is this: what observatory were these predictions made for? It was long before the days of Greenwich and a standard longitude. Yhe extract in the illustration doesn't tell us, but no doubt the complete document would. If it was made for Nuremberg, or Venice (both having similar longitudes) both would have a time difference between the prediction and an observation in Jamaica of nearly 6 hours (still well short of Columbus' stated "seven hours and fifteen minutes"). We have to bear in mind that throughout his time in the Americas, Columbus contiued to claim that he had reached the Indies, or Cathay (China). Any evidence that indicated his Westerly longitude was less than he was claiming, Columbus simply didn't want to know about. In my opinion, he was about the worst navigator that ever returned successfully to base. As for discovering America, we Europeans find it hard to forgive that. 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. ================================================================