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    Re: Round-the-globe almanac
    From: Herbert Prinz
    Date: 2004 Apr 1, 18:03 -0500

    George Huxtable wrote:
    > 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.
    > "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?
    The calendar in question seems to be the Venice reprint (first one from 1478)
    of  the Latin version of the calendar that first came out in a German and Latin
    version in Nuernberg in 1474. Since they are "perpetual" calendars, they have
    been reprinted many times until the end of their useful lifetime in 1530. I only
    have a facsimile of the German version of 1474, where the time of mid-eclipse is
    given as 13h 36m. There were certainly printing errors in some copies, but you
    may want to check the above date again. At any rate, Columbus did not use these
    calendars. He used the ephemerides (year books), which gave detailed information
    on the positions of all the planets. The eclipse data and the diagram should be
    the same. I don't have that available, so I cannot check.
    Reference meridian is Nuernberg. Zinner argues that Columbus was not aware of
    this and used Cadiz. This explains only part of  the difference between the
    actual longitude and that of Columbus.
    The complete data for the eclipse of 1504 can be found at
    Note that it is recorded as eclipse of March 01, as geocentric conjunction was
    at 0:15 UT on that day.
    Herbert Prinz

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