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Re: Stereographic Devices
From: George Huxtable
Date: 2010 Apr 20, 09:41 +0100
From: George Huxtable
Date: 2010 Apr 20, 09:41 +0100
About devices using a stereographic projection I had written "Yes. The oldest instrument of all; the astrolabe uses a stereographic projection." and Wolfgang responded, on 15 April-- "That's true. But the astrolabe was never used to solve the navigational triangle. And the Mariner's Astrolabe - which was used in navigation, indeed - is not engraved with the stereographic projection." =============== I would like Wolfgang to explain a bit further, please. We agree that it's the "conventional" astrolabe that is being discussed, not the simplified mariners' version. Such a conventional astrolabe has a serious limitation; that it works for only a single latitude. It isn't, then, a navigational instrument, and was not used for that purpose, and therefore, as Wolfgang states, "was never used to solve the nevigational triangle". But within that limitation, once time and date have been set, the pointer corresponding to a marked star, such as Sirius, say, is to be found in position against a curved grid which allows its altitude and azimuth to be read off directly, within a degree or so anyway. Isn't that the equivalent to "solving the navigational triangle"? That was fine for an astronomer-observer, always at a particular spot, but not for a traveller. To overcome the latitude problem, some astrolabes were provided with a set of interchangeable plates, each engraved for a particular latitude. This rather unwieldy solution was replaced, from the 16th century, by the universal astrolabe, which could handle a wide range of latitudes; some of which, but not all, used a stereographic projection. But this is taking me into areas about which I can't speak with knowledge. There's rather more in "Astrolabes at Greenwich", van Cleempoel, 2005. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
