The most common type of astrolabe is the planispheric astrolabe, which works by projecting the sphere of the heavens onto a plane in a way that preserves angular distances and allows the user to carry out a wide variety of calculations.[1] The rarest form type of astrolabe is the spherical astrolabe. Although Latin and Arabic descriptions of such instruments exist, and other sources indicate that some mathematicians might have owned spherical astrolabes, only this single example survives. This astrolabe is about the size of a baseball, .[2]
The rete on this astrolabe is a cage-like structure that would rotate around the globe inside it.[3] As with the rete on a planispheric astrolabe, this one includes a number of bright stars, 20 to be precise, a zodiac, and a meridian line. Unlike a planispheric astrolabe, which has different plates engraved for each latitude to depict the portion of the sky above the horizon at that latitude, this spherical astrolabe as a single “plate”—the globe on which the heavens are depicted. To adjust it for each latitude there were a different holes in the globe. A pin was inserted through a hole the cage-like rete and into the hole in the globe, both fixing the portion of the sky visible from the latitude and providing a point around which the heavens would rotate.
The maker signed and dated the globe, “The work of Mūsa. Year 885.” Unfortunately, we know nothing more about the maker, whose name is rather common. Based on the decoration and calligraphic style (a Kufic script, if you care), it seems likely to have been produced in the Eastern Mediterranean somewhere, probably Damascus or Cairo.
If you want to see more, it is Museum of the History of Science, item #49687.
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The mariner’s astrolabe is not, as far as I care, an astrolabe. The mariner’s astrolabe is an instrument for observing the altitude of a celestial object, e.g., the sun, and from that determining latitude. For me, the key difference is the fact that mariner’s astrolabes cannot be used to carry out any of the various calculations that define the planispheric astrolabes. I realize that some people want to call the mariner’s astrolabe a type of astrolabe. So be it. I, however, will not. ↩
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For those more familiar with cricket, a cricket ball and baseball are nearly the same size: men’s cricket ball is 224–229mm in circumference; an official, professional baseball is 228–234mm in circumference. Field hockey your sport? A field hockey ball is about the same circumference, 224–235mm. Don’t like sports? Neither do I really. ↩
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The rete is the map of bright stars that rotated about the pole. For more information about astrolabes, see my Guide to the Astrolabe. ↩