Planisphaeric Astrolabe

Construction Hints and Details

This instrument consists of a number of parts as seen in Figure to the left.  The numbers will be used in the discussion below.The Plate (1) and the Rete (5) are perhaps the most challenging parts of the Astrolabe to construct in the sense that all of the arcs, circles, and star positions must be carefully calculated and laid out in order to produce a functioning instrument. 

Let's begin with the Plate. This is generally a thin disk of brass (18-16 ga) with a notch or tab at the bottom to align it in its receptacle in the mater. Starting at the North pole and working out the circles correspond too: first the Tropic of Cancer, then the Equator, and finally at the edge, the Tropic of Capricorn. (This is of course for an astrolabe for use in the Northern hemisphere, for use south of the equator the center hole would correspond to the South pole, with the order of the circles reversed.)

The mater (2) or body of this instrument is fabricated from 14 G copper sheet, with the rim pinned and sweat soldered on.

After laying out the circles, webbing and star positions, the Rete is a test of one's patience and skill with a jewelers saw.

For this particular instrument I wanted the sighting vanes on the alidade to fold flat both to prevent their harm in transport, and to allow storage in a thin carrying case. This required the fabrication of a hinge for each vane. The vanes were fabricated from heavy copper stock (its what I had in my shop, brass would work as well or better) which would allow me to drill a hole through the body of the material.

If you have managed the rest of the instrument, the back will be easy. In my case I made an error in months circle, so I cut it out on a lathe, back-filled with solder and re-stamped it.