Astrolabes and almagest

From: Steve Rennell <software_at_spis.co.nz>
Date: Thu, 4 Jun 1998 16:57:11 +1100


 "Hibbs, Philip" <philip.hibbs_at_tnt.co.uk> asks
> Has anyone got any ideas what an Astrolabe is, or what "Almagests" are?
> There they are, lurking in the starting equipment for a civilised Adept
> Sorceror ... but what the hell are they?

An astrolabe is a tool for measuring star positions with some reasonable accuracy. They were used as early as 130BC, and later on were used as navigational instruments as they allow you to figure out lattitude I think, at least on Earth. (but probably not relative to True North, more likely relative to wherever the astrolabe was made.

Generally it's a disc of brass shich could be suspended from a ring, with an observing bar that could be used to measure the angle to the star in question, and on the back were engraved charts that allowed one to determine the current position of the sun on the eclyptic. Could also be used to determine the current astological "sign".

More advanced versions (Islamic - not sure when) had off set circles to handle an observers local co-ordinates, so you built an astrolabe for a particular location and measured lattitude as offests from that point. If you were staying in one place for a long time (like in your observatory) you might have a personalised one built that you wouldn't need to calculate offests for, because you already knew your offset. This also allowed you to calculate a fairly accurate time with one observation of a known star at night, or the sun in the day time. (Or you could predict when the sun would rise on a particular day of the year by the same method).

Almagest was a treatise by Ptolemy (the guy that had better maths than Copernicus, but said "if the earth went around the sun then we'd all fall off, right? so obviously the earth is at the centre" so doesn't get any credit.) about stars and their positions, and the regular movements of the planets. I believe that prior to Copernicus, books of Ptolemy's tables were used by amateur astronomers, (was there any other sort?) and as such would make an approriate accompaniament to an astrolabe.

Steve


Powered by hypermail