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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.


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