A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Wolfgang Köberer
Date: 2017 Nov 11, 08:17 -0800
it is highly unlikely that this is a mariner’s astrolabe:
1. All other known mariner’s astrolabes show the cutout of the mater first depicted in the 1517 Zorzi picture. They were necessary to reduce windage. Although it may be that between 1502 (terminus ante quem: Da Gama’s departure for his 2nd India voyage) and 1517 this refinement took place but – as astrolabes had been in use on Portuguese ships for at least 25 years before and their deficiencies in onboard use discussed – it is hardly likely that this development took place just between 1502 and 1517.
2. The object is obviously too thin: 1,5 mm according to the Esmeralda wreck website. Compare this to the thickness of other mariner’s astrolabes: 15 mm to 45 mm. That was necessary so the instrument would weigh enough to be steady while being used; this instrument would rather move like a leaf in the wind.
3. There is no example of a mariner’s astrolabe being marked with the coat of arms like this; not even a “normal” planispheric astrolabe. Normally one would take this as an indication that the instrument is fake. I would not go this far, but the video shown on the web site rather looks like the discovery of an object that has been planted there.
4. Where is the alhidade? Most mariner’s astrolabes that were found (Madre de Dios, Batavia, Lyme Bay, Padre islans, Concepción, even Santa Margerita) even if they are badly deteriorated still have the alhidade or at least the pin for it.
5. The claim that the markings on the edge – spaced about 5 degrees apart – indicate a use for navigation is ludicrous. A difference in reading of 5 degrees means a difference in latitude of 300 miles; even if you think that one could guess fractions of this (say a fifth, which is difficult regarding the size of the object, meaning 60 miles of difference in latitudes), I don’t think that anyone who knows a bit about navigation would consider this a useful tool for his work.
6. Finally: Nobody bothered to ask (Portuguese) experts about what they thought this object is: Antonio Estacio Dos Reis or José Malhao Pereira come to my mind. For newspapers (and the BBC) it is obviously easier to repeat exaggerated claims (the “best”, the “earliest” etc.) than to talk to people first who know more about the subject before repeating those claims.
This is therefore a circular object with a hole in it and markings on the rim bearing the arms of Manoel I, that has been found in an early 16th century wreck. No more.