Medieval Astronomical Timekeeping Instruments (Spring 2001)

Introduction: For this workshop I have brought instructions and materials for you to make two different time keeping instruments typical of the 11th-13th centuries. Before we begin the projects I will show you some instruments I have made which illustrate the operation of these instruments in terms of astronomical observations and phenomena. I will also discuss briefly some of the techniques I have used in making them, and relate how my methods compare to techniques current in the late Medieval period. You will find additional details about these instruments and techniques on my web-site.

I have brought three instruments that I have made over the years: an armillary sphere (a Ptolomaic, or Earth-centered, model of the Universe), a planispheric astrolabe (a combination observational tool and 2-D model of the heavens), and a torquetum (a complex observational tool, which also illustrates different reference frames useful for astronomical observation). Detailed descriptions of these instruments are available via the instrument page on my web-site. All of these instruments are fabricated from copper and/or copper alloys.

Planispheric Astrolabe: This instrument is made of copper and brass, both metals commonly used for instruments in Medieval times. The blanks were cut from 14 gauge sheet with a power saber (or jig) saw, and then trued up to round with a file on a wood lathe. Careful, this can be quite dangerous, as copper alloys are "sticky" and can grab, making the file into a deadly projectile!. And, of course, the spinning blank can act as a blade and slice a hand if you slip. For graduating this instrument I used a rotary table (commonly found in machine shops, though mine was modified from a micrometer actuated transit) with a fixed rule held above it. The metal blank was then centered and fixed on the table and lines scribed with a knife held against the rule. This enables slightly cleaner and deeper graduations than the scriber. This method of graduation would not have been used until the late 18th century with the advent of dividing engines. To cut out the rete I used a jewelers saw and then cleaned up with needle files. Note that this is an unlikely scenario for early craftsmen. High quality steel required to make the fine blades of jeweler's saws was not available until near the 19th century. Thus the rete's of astrolabes were probably made by drilling holes and then cutting and shaping with files.
Armillary Sphere: The rings were graduated as above with the planispheric astrolabe. The meridian ring was cut from 3/8" brass plate on a metal lathe, the horizon ring was cut from a bronze plaque with a power jig saw, then trued up on a lathe, and the remaining rings were fabricated by bending and joining straight stock.
Torquetum: The various circles were cut from brass stock after lay-out with wing-dividers sharpened to round points - you don't want any "knife-edge" as they will then tend to pull instead of cutting a clean arc (trammel points will work equally well, but you need something that can be locked at a dimension). The dividers were then used to layout the radius of the circle to give arcs at 60° intervals (note that this is theoretically absolutely accurate, the precision attained in laying out these divisions is limited only by your tools and your skill!). Additional divisions at 30° intervals were then accomplished by bisection. Bisection of the 30° intervals will then give 15° intervals, however, at this point division by divider becomes more problematic. This method of division was the method from ancient times through the 18th century, and in fact was used for observatory instruments through the 19th century. I did the remaining division using my "dividing engine," an extension of the method described for the planispheric astrolabe above.

Projects. Two simple projects were constructed during this workshop. These projects are illustrated along with instructions for construction and sample calculations via the links below: