This instrument consists of a number of parts as seen in Figure 1 below:
1: The Plate is a map of the sky (no objects) centered on the North pole. A different plate is needed for each latitude.
The Plate and the Rete 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 to: 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 or body of the instrument (1) is fabricated from 14 G copper sheet
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 siting 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.
The alidade, the pin, the horse, and the rule are quite straight-forward.
|Instruments||Medieval Science & Scientific Instruments|