How Astrolabes Work

Getting Your Own Astrolabe

If you're an astronomy buff, you've probably already invested in a good telescope. Now you might want to add an astrolabe to your collection. The simplest thing is to buy an astrolabe that's ready to go right out of the box. You can find some antique astrolabes on eBay, although any pre-20th-century instrument will set you back some serious dough. A better option is to buy a replica, which will give you an authentic ancient-astrolabe experience without the hefty price tag. Many Web sites offer a variety of planispheric and maritime astrolabes.

Modern materials do offer some advantages over brass and pewter. A good blend of old and new schools comes from Janus, a Delaware-based company behind several popular astrolabe resources. If you really want to kick it old school, you'll want to build your own astrolabe from scratch. To get the full experience, start by reading "A Treatise on the Astrolabe," the first English-language manual on the instrument. James E. Morrison, the owner and creator of Janus and The Personal Astrolabe, has translated Chaucer's work out of Middle English into kinder, gentler language we all can understand (a PDF of the translation is available here).

To build an astrolabe you can use as you read the 14th-century treatise, your best bet is to start with pre-existing templates. In "The History and Practice of Ancient Astronomy" (Oxford University Press, 1998), author James Evans provides a set of complete patterns to make an astrolabe. You simply photocopy the patterns onto paper (or onto an acetate transparency in the case of the rete), glue them down to card stock, cut out the parts, punch a hole in the center and bind everything together using a bolt and nut. He specifically provides patterns for two altitude plates -- Seattle and Los Angeles -- but you can also find others in the body of the book.

Another great resource is a hands-on astrolabe activity developed by the Institute for Astronomy at the University of Hawaii. The Web site includes a program that calculates a complete set of astrolabe templates for any location. Once you specify your location, the program generates files you can save to your computer or print out. The site also offers complete assembly instructions and a primer on how to use the thing once it's built.

No matter which route you take, store-bought astrolabe or DIY, you'll possess your own version of the world's first analog computer. And with it, you'll have a greater understanding of the night sky -- and a deeper connection to the world of ancient astronomy.