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I Forge Iron

16th century wood and steel astrolabe

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The astrolabe was an instrument used by astronomers, astrologers, scientists, and others from the late 9th century until the 17th century or so.  It consists of a stereographic projection of the dome of the night sky, over which is a projection of the major stars in the sky and the ecliptic (path of the sun in the sky).  The back has a number of scales including calendars.

Using the astrolabe, you can determine the local solar time based on the position of the sun or stars, the date based on the sky, the altitude of various stars and the sun, the height of buildings, direction, etc.  According to the Islamic writer al-Sufi (d. 987) it had 1000 uses, and Geoffrey Chaucer wrote the English language's first tech manual on the astrolabe.  They were frequently made from gilt brass and were status symbols as well as scientific instruments.  Many, though, were made of wood and even parchment and paper. 

I've taught using the astrolabe in classes and fascinated by the things.  I've made paper ones, but wanted something more substantial. 

This is a wooden astrolabe made by Phillip Danfrie in 1584.  It's wood and pasteboard with printed paper and some copper alloy elements.  The Museum of the History of Science in Oxford has excellent photos, including of it disassembled, which I was able to use to make my own. 


I used plywood, and heavy cardstock for the main body.  The back is solid, and the front is a ring (to hold several climate plates, which are different depending on your latitude.)


The "rete" (pronounced reet) was made from really heavy cardstock and I cut it out with a jeweler's saw.


I made the metal parts from steel and brass brushed them.  The central bolt was made from a carriage bolt that I filed down and rethreaded.  The nut was made from another carriage bolt head.  The sighting vanes on the alidade (the front rule), used to sight the elevation of the sun and stars, were tenoned and brazed.


I drilled through everything and assembled.  It's a little smaller in diameter than the original, but not much.  The whole thing is sealed.


Five climate plates for five different latitudes.  The little tabs fit in a slot in the front to keep them oriented.


I've used it and it's reasonably accurate.  A good astrolabe is accurate +/- 2 minutes, but the rete is a little deformed on the original (and hence the repro), plus stellar precession.  A very good astrolabe can be made for any latitude using a generator at astrolabeproject.com, which also has a good user manual if you want to learn how to use them.  Sure, you can tell the time by your iphone, but isn't it cooler to be able to tell the time by the altitude of Deneb over the horizon?


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Wow! looks like much to intricate and delicate work than I could do! great job!


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Thanks, all!  Frank, I didn't do the calligraphy, that was Danfrie, who was a printer as well as instrument-maker.  Instead of individually made, engraved brass astrolabes, paper and wood ones could be made from a single block and were a lot cheaper (though sacrificing durability).  There are a couple other astrolabes in museums that are made from the same printing blocks.

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Most impressive! I have seen these, but had no idea it was a time piece. I put together a 3000 piece puzzle last winter, (I glued a backer behind it, once was enough.), the puzzle is an old stylized world map with 2 of these pictured in the center field. That is just fascinating, thanks for posting!!! :)

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On Wednesday, January 04, 2017 at 7:48 PM, Nick said:

Sure, you can tell the time by your iphone, but isn't it cooler to be able to tell the time by the altitude of Deneb over the horizon?

Pass along your knowledge to whoever you can ; this is a nice accomplishment you have here. Thanks for posting this.

Way off topic , but I can't help but think...wasn't there a rapper who wore huge clocks around his neck? He needs one of these...

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