Since the article was illustrated with a Sloane astrolabe from the British Museum, I thought I’d draw attention to another great resource: The Astrolabe: An Online Resource. The website gives you access to every astrolabe in the History of Science Museum’s collection, which is the largest single collection of astrolabes in the world. Astrolabes in the collection range over a millennium and much of the Northern Hemisphere, from the late 9th-century Syria (one of the oldest surviving astrolabes) to the 20th-century England. Most of the instruments date from the pre-modern period. While perhaps not the most flashy of websites, it is probably the single best resource for studying astrolabes.
Browsing the collection always turns up something new. Just a few highlights:
astrolabe 37148, which has amazing engravings of the constellations on the back;
astrolabe 48213, which has an ingenious set of gears (the oldest complete gear train) that show the phases of the moon and motion of the sun and moon;
astrolabe 45359, which is a so-called “Chaucer astrolabe” because it resembles the illustrations in Chaucer’s Treatise on the Astrolabe, especially the cute little dog’s head star pointer at the bottom of the rete;
astrolabe 49687, which is the only surviving spherical astrolabe (there are various descriptions of spherical instruments, but only this instrument survives)
While I don’t agree with the Washington Post’s author’s conclusion—“a visit to Epact is a glimpse into a bygone world — one in which scientists dared to dream and discover”—I do agree with her earlier statement that spending a few minutes browsing the items on Epact or The Astrolabe: An Online Resource will “make you appreciate the artistry and intricacy of now-obsolete scientific tools or leave you starry-eyed over each instruments’ function and a role.”
The museum formerly known as the “Museum of the History of Science” ↩
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. 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, .
The rete on this astrolabe is a cage-like structure that would rotate around the globe inside it. 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.
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. ↩
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. ↩
The rete is the map of bright stars that rotated about the pole. For more information about astrolabes, see my Guide to the Astrolabe. ↩
In late 2003 the Coalition Provisional Authority governing Iraq introduced new banknotes and coins to replace the Hussein government’s currency. Celebrating the contributions Islamic scholars made to science, the new 250 dinar note was decorated with an engraving of an astrolabe on the obverse (and the Great Mosque of Samarra on the reverse).
It wasn’t the first time Iraqi currency had celebrated the astrolabe. A decade earlier, in 1993 the Hussein government had issued a 1/2 dinar note with the same engravings on both sides, the astrolabe and the Great Mosque of Samarra. And then in 2002, the government issued a 10,000 dinar note with an astrolabe on the reverse. The astrolabe on this later note is a different engraving.
In 2005 when I was working on the astrolabes at the Museum of the History of Science, I learned that the new 250 dinar note had an astrolabe on it. So I wrote to the Iraqi government asking how I could acquire one. They sent me eight, which I shared with my colleagues at the Museum. I still have three. ↩