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    Re: Round-the-globe almanac
    From: Herbert Prinz
    Date: 2004 Apr 1, 12:03 -0500

    Ron Roizen wrote:
    > [...] 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?
    You are quite right in being skeptical. But the reason why part of the story is
    implausible is not the almanac. The predictions of lunar eclipses in the
    almanacs of Regiomontanus were typically accurate within half an hour. This
    would have been good enough for Columbus's purpose to frighten the Indians. The
    diameter of the Earth has no impact on the ephemeris.
    However, in order to predict the local time of the event, one has to know one's
    longitude. This, Columbus did not have. On the contrary, he attempted to obtain
    his longitude from timing the eclipse. His wildly wrong result has been widely
    The diameter of the Earth certainly affects dead reckoning in that it changes
    the relation between angular and linear distances. But the longitude from dead
    reckoning after crossing the Atlantic must be highly questionable at any rate,
    no matter how good your idea about size and shape of the Earth are.
    All that was required to frighten the Indians was the announcement that an
    eclipse would happen on a particular day. Having the almanac, Columbus could be
    fairly sure of this. This much of the story is plausible. The rest is mostly
    adornation. Some stories on the net are just plain wrong. For instance, the
    begin of the eclipse happened before sunset and was neither visible to Columbus
    nor to the Indians.
    Zinner discusses the eclipse of 1504 in his biography of Regiomontanus. This
    work has been translated into English, but I have not seen the translation.
    There is an excellent discussion of the navigation of Columbus in general on
    the "Columbus Navigation Homepage" by Keith A. Pickering.
    This site also contains a bibliography.
    Herbert Prinz

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