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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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