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    Re: Round-the-globe almanac
    From: George Huxtable
    Date: 2004 Apr 1, 21:50 +0100

    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---.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.
    ================================================================
    
    
    

       
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