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Herbert Prinz wrote-

>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
>
>http://sunearth.gsfc.nasa.gov/eclipse/LEhistory/LEplot/LE1504Mar01T.gif
>
>Note that it is recorded as eclipse of March 01, as geocentric conjunction was
>at 0:15 UT on that day.

===================

Response from George.

Herbert is right, and I was wrong (not for the first time!)

I carelessly mistranscribed the time of mid-eclipse as 13h 26m, where it
should have been 13h 36m.

Thanks to Herbert for putting me right, and adding other useful information.

======================
======================

Doug Royer wrote-

>Sometimes it's better to be lucky then to be good or competant in anything.

I agree with Doug, to a large extent. Even the most cautious and competent
navigator (and here I am thinking particularly of Cook) had to sail
uncharted seas, night and day, in a vessel that was incapable of turning
round and going back the way it had come. All that was needed was in
invisible rock somewhere to put an end to the voyage. When that happened to
Cook (in the Barrier reef) he was in reasonably sheltered waters, and there
happened to be a good harbour close at hand: how lucky can you get? Mind
you, the subsequent saving of the ship can be put down to Cook's
overwhelming nautical competence. But nobody could continue such voyaging,
for three circumnavigations, without a certain amount of luck on his side.

Columbus, being such a useless navigator, required more than his due share
of good fortune.

>Wasn't America named after Amerigo Vespuci?

Yes. Vespucci, captained by Ojeda, explored the South American coast from
Trinidad to the Gulf of Venezuela, and back through the Caribbean, in
1499-1500. He covered ground that had been visited already by Columbus, to
a large extent. He didn't touch the North American continent, and neither
did Columbus. For that, you have to choose between John Cabot, 1497 and
1498, or else much-earlier Scandinavian voyagers. Perhaps the North
American continent should really have been named "Cabotia".

>And during the time of the Spanish/Portugese naval and discovery period of
>Europian history wasn't Cadiz the prime meridian used?

Well, that's what the Spanish used. The Portuguese would have used Lisbon.
A mariner would start to reckon his departure (Westing) from the port he
set off from. Before Greenwich became the astronomical centre, English
mariners would "take their departure" from Plymouth, or Falmouth, or the
Lizard, and reckon longitudes from there. In the days of chronometers, they
would check their chronometer at the last port they visited that kept a
time-standard. There was no agreed prime meridian until the 19th century,
when charts started to become international. The French, of course, were
the last to conform. Even in recent years, the Michelin motoring maps were
taken from the longitude of Paris (and measured in grads rather than
degrees).

>I've read that the respective engagement positions between English and
>Spanish vessels of the 16th and 17th centuries were not the same or even
>close because of this practice.Is there truth to this also?

Not sure what Doug means by "engagement positions". If they had fought
their sea-battles at appointed longitudes they would never have got within
hundreds of miles of each other. At sea, it was common for friendly ships
to exchange their position-estimates when they met, and these differing
standards must have caused much confusion, even if they understood each
others' languages.

George.

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