How Astrolabes Work - Electronics | HowStuffWorks

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Once you learn how to use it, that brass astrolabe has a lot of information to impart. iStockphoto/Thinkstock

Long before the Texas Instruments LED calculator, or even the slide rule, there was another type of computer: the astrolabe. The device took its name, ultimately, from the Greek word astrolabos, or "star-taking." Astrolabes were used primarily to make astronomical measurements, typically of the altitudes of celestial bodies, but astute philosophers, astrologers and sailors devised hundreds of uses for the instrument. The astrolabe was, without a doubt, the slide rule of the Middle Ages.

Today, although computers and other technologies have replaced them in practical astronomical and maritime applications, astrolabes continue to fascinate technophiles, science historians and amateur sky watchers. Many educators use the tool in their classrooms to teach about the celestial sphere and how to plot and predict a variety of astronomical phenomena, including sunrises/sunsets, moonrises/moonsets, star transits, retrograde motions and much more.

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So what exactly is this astronomical instrument that compelled Geoffrey Chaucer to write about its structure and function in a 14th century treatise and then — nearly 620 years later — inspired Autodesk Fellow and software engineer Tom Wujec to demo a working replica for TEDGlobal? On the next few pages, we'll explore both the art and science — as well as the rise and fall — of astrolabes.

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