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Dear Lu,
again I can only point to a web site in French

http://www.ens-lyon.fr/RELIE/Cadrans/activpedago/TextesCours/WebAstrolabe/As
trolabe.htm

Seems pretty thorough, but you have to know the language. At least there are
some nice drawings (a picture can say more than a thousand words).

Regards,
Wolfgang




-----Urspr?ngliche Nachricht-----
Von: Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]Im Auftrag von Lu Abel
Gesendet: Donnerstag, 29. Juni 2006 16:55
An: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Betreff: Re: AW: Astrolabe. was: [NAV-L] The point of it all


Thanks, Wolfgang.  At the same time, is anyone aware of perhaps an
on-line explanation?  (It's nice to cuddle with books, but it's quicker
and cheaper to hit a web site).

Lu Abel

Wolfgang K?berer wrote:
> The National Maritime Museum used to sell an inexpensive booklet with the
> title "The Planispheric Astrolabe" - first published in 1976 and reprinted
> many times - which contained an explanation of the several uses of the
> instrument. As I cannot find on their website I assume that you must look
> for it on the used book market.
>
> They also published a catalogue of their collection of astrolabes by
> Koenraad van Cleempoel "Astrolabes at Greenwich" recently which I haven't
> seen yet - it is rather expensive. Because I haven't seen it I can only
> guess that it contains an explanation of the uses of the astrolabe.
>
> Apart from that there is "d`Hollander, Raymond; L'astrolabe: Histoire,
> th?orie et pratique, Paris 1999" which gives a thorough treatment of the
> astrolabe. As far as I can see it is still in print.
>
> Regards,
> Wolfgang
>
> -----Urspr?ngliche Nachricht-----
> Von: Navigation Mailing List
> [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]Im Auftrag von Lu Abel
> Gesendet: Donnerstag, 29. Juni 2006 02:30
> An: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
> Betreff: Re: Astrolabe. was: [NAV-L] The point of it all
>
>
> Does anyone know of a simple explanation of how to use an astrolabe?
> I've always been fascinated by the devices but have no idea how they
> work or how a Muslim pilgrim might have used one to find Mecca's
> azimuth.
>
> As a side note, Muslims use true (great-circle) azimuth (maybe that's
> because there was no concept of Mercator projections and rhumb-line
> directions back when the astrolabe was invented.  When I was offered the
> opportunity to tour a mosque in the San Francisco Bay area I was
> surprised to see it was oriented to the northeast, not southeast as I
> first expected.  The tour guide confirmed that the direction was
> great-circle and not rhumb line.
>
> On the issue of the antipode for Mecca, what looks to be the great
> temple in Mecca on Google Earth is at 21d 25.34' N, 39d 49.61' E.
>
> That would make the antipode at 21d 25.34' S  140d 10.39' W, which is
> about 15 nm north of Tematagi (also apparently spelled Tematangi)
> Island, part of the Tuamotu Archipelago at the far limits of French
> Polynesia.   It's also about 120 nm WNW of Fangataufa and Moruroa, the
> two atolls where the French conducted their nuclear tests.
>
> Lu Abel
>
> George Huxtable wrote:
>
>>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?"
>>|
>>| If so, what happens to the souls of those using a rough instrument
>>to
>>| determine sunrise etc. and Mecca to pray? 
>>
>>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 george@huxtable.u-net.com
>>or at +44 1865 820222 (from UK, 01865 820222)
>>or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>>
>>
>
>
>

   

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