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From: George Huxtable (no email)
Date: Wed Jun 28 2006 - 18:21:41 EDT
Bill wrote-
| Still, I'll
| make a leap that "traditional navigation" is not limited to water.
Would
| the astrolabe and nocturnal (or some derivation) fall under" rough?"
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| If so, what happens to the souls of those using a rough instrument
to
| determine sunrise etc. and Mecca to pray? <g>
I have wondered about the existence of an anti-Mecca, at the antipode
of Mecca itself, and whether there might be some marker placed at that
spot, around which devout Muslims might gather, facing carefully away
from it. Alas, such a spot would be in the sea, in a Pacific island
group, and not on land. But you can imagine the problems that might
face a devout Muslim, working on an inter-island ferry within that
group, in working out which way to face when praying on his
journeying.
However, I doubt whether the praying direction called for exact
science. Bill asked about an astrolabe and a nocturnal, two very
different instruments. From either, you could get local time, by the
stars; the astrolabe will supply much additional information. If you
can see a clear sky at night, you can estimate North using Polaris,
corrected, according to the time, for its offset from the true pole,
much greater in the past than now. If you know the azimuth of Mecca
from your present position, the rest is easy. With an astrolabe,
knowing the height of the Sun, and the time of year, you can get the
Sun's azimuth, and continue from there.
Bill regarded an astrolabe as a "rough" instrument, and so it is,
though a very subtle one; the astronomer's astrolabe, that is, not the
mariner's astrolabe. With care, you can probably read it to half a
degree or so, if the maker has also engraved it with corresponding
care. It isn't really a traveller's instrument, in that it contained a
plate, engraved for a particular latitude, and some information was
precise at that latitude only. Often, there was a choice of such
exchangeable plates available, suitable for different latitudes,
perhaps 5 degrees apart. Star positions were shown, but were only
usable with accuracy for 50 to 100 years, before precession shifted
them out of place. Often, astrolabe makers would copy old instruments,
without updating those star positions, in which case they would be
inaccurate right from the start.
In a mariners' astrolabe, all those sophisticated scales had been
swept away, and it was used only as an instrument to measure altitude
of a star or the Sun., using the astrolabe itself as its own pendulum
to obtain the vertical. That must have been tricky in rough weather.
Here is an astrolabe question that has long puzzled me. Up to the late
1400s, mariners found their latitudes from the height of Polaris; not
from the Sun, because the Sun's changes of declination were not
predicted nor understood. This presented problems when Portuguese
navigators ventured near to the Equator, and Polaris vanished into the
horizon. King John II of Portugal commissioned the Jewish astronomer
Zacuto to produce a set of Sun declination tables around 1484 (not
many years before Columbus' voyaging). And yet, that solar information
was clearly available on the traditional astronomers' astrolabe, and
had been so for many years, having been worked out in the first place
by the Greeks, and preserved by the Arabs, though lost within
Christendom. Even Chaucer, presenting an astrolabe to his son for his
10th birthday in 1397, explains to him how to use that declination
scale, together with the height of the Sun, to determine his latitude.
So, if that information was available then to Chaucer (and even his
son), why was it not used by mariners until 90 years later?
George.
contact George Huxtable at
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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