A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Astrolabe. was: [NAV-L] The point of it all
From: Lu Abel
Date: 2006 Jun 29, 07:54 -0700
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 > 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 email@example.com >>or at +44 1865 820222 (from UK, 01865 820222) >>or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. >> >> > > >