Wolfgang and Frank, as is common in such debates,
the context is as important as the content. The words in my first posting
from "The correction for Polaris's circumpolarity ..... " are exactly from
Bedini at page 230. The context is the measurement of
distance.
A discussion of the mnemonic diagrams giving
circumpolarity is found in Bedini at page 512 in the context of navigational
instruments. There is a diagram of an example 'roda'.
( I understand the word 'regimentos' to apply to a table of the sun's
declination.)
A further discussion of the use of Polaris in
finding position is found on page 508. It does indeed show that Polaris
was well understood a hundred and more years earlier than Columbus.
However, that is not the same as saying that it was used to find latitude.
I quote the paragraph that follows on page 508.
"That there were people in Portugal capable of
adapting astronomy to the needs of navigation is shown by early tables, such as
the 1339 Almanac of Coimbra. It seems more likely however, that the
initiative in this innovation came from pilots, who would undoubtedly have
noticed the difference in altitude of the North Star at Lisbon and in
Guinea. Latitude can be defined by the elevation of the polestar, but the
procedures introduced by Portuguese pilots in the 1450s to check distance sailed
in a north-south direction between two observation of the polestar have nothing
to do with establishing the latitude, which indeed was not then marked on
charts. The difference in angle between the two observations was
multiplied by 16 2/3 to give the linear distance in leagues (a figure later
changed to 17 1/2, still an underestimate). Observation was made with the
seaman's quadrant, and in the early days of altura navigation, as it was called,
it became the practice to mark on the scale the names of the various Capes,
islands and landmarks whose star altitudes had been observed. At the
same time, astronomers at home were compiling lists of latitudes, which by 1473
had reached the equator. It should be emphasised however, that until
latitude navigation was introduced after 1485 (when Joao 2's mathematical
commission worked out the procedures), altura navigation did not involve the
concept of latitude."
There is an example of a seaman's quadrant with
these star altitudes marked upon it in the maritime museum in
Barcelona.
I concur that the claim made for Columbus is
surprising but now you will have to debate among
yourselves as I will be out of circulation for two months.
as aye, Keith
----- Original Message -----
Sent: Tuesday, January 31, 2012 3:56
AM
Subject: [NavList] Re: Nocturnals
Wolfgang, regarding Keith's comment suggesting that the circumpolarity of
the North Star was not known before Columbus, you wrote:
"Well, Bedini was
clearly wrong. The circumpolar motion was well known to Portuguese mariners
before Columbus."
I agree. And it's such a strange suggestion that I wonder if there might
have been some editing error or mis-translation. Does anyone have access to
Bedini's original statement here?
-FER
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