The Planisphaeric Astrolabe- Maker's Notes

Anyone serious about constructing an astrolabe should try to get a copy of James E. Morrison's masterpiece on this instrument, its use and construction: The Astrolabe.* (This book is now out of print, its author and publisher unfortunately passed away.) My rather brief and early notes are provided below.

This instrument consists of a number of parts as seen in Figure 1 below:

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 (see Morrison, above).

Let's begin with the Plate. (1 in Figure 1) The Plate is a map of the sky (no objects) centered on the North pole. A different plate is needed for each latitude. It 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 this instrument (2) is fabricated from 14 G copper sheet. In many instruments the mater is a casting with a relatively deep recepticle to accommodate a number of plates. Dividing the edge into degrees can be accomplished using a variety of methods.**

After laying out the circles, webbing and star positions, the Rete (5 in Figure 1) is a test of one's patience and skill with a jewelers saw and files.

For this particular instrument I wanted the siting vanes on the alidade (3 in Figure 1) 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 (see image below) will be easy.

The alidade (1 above, 3 in Figure 1), the pin (2 on image above, 4 on Figure 1), the horse (2 on image above; 6 in Figure 1), and the rule (3 on image above; 7 in Figure 1) are quite straight-forward.

*Morrison, James, E. The Astrolabe. Janus, Rehoboth Beach, DE (2007). This is THE book for anyone wishing to build their own instrument or to understand and/or use an astrolabe. The author provides geometrical, mathematical and computer methods for laying out the scales, plots etc. on astrolabes of all types and quadrants. I used a preprint from the author in laying out an astrolabe in progress and found it extremely helpful - just the thing as I had lost my notes from my previous instrument of twenty years ago, and had a limited memory of my calculations.

**You can find instructions for dividing circles and arcs by going to this page on my Medieval Science site.

© R. Paselk 2013, Last modified 26 December 2020