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    Re: Astrolabe. was: [NAV-L] The point of it all
    From: Lu Abel
    Date: 2006 Jun 29, 07:54 -0700

    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
    > 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.
    >>| the astrolabe and nocturnal (or some derivation) fall under" rough?"
    >>| If so, what happens to the souls of those using a rough instrument
    >>| 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
    >>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?
    >>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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